The Citronelle Formation (Pliocene) and Its Flora (Matson and Berry 1916)

8/13/2019 The Citronelle Formation (Pliocene) and Its Flora (Matson and Berry 1916)

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DEPARTMENT OF THE INTERIORFRANKLIN K. LANE Secretary

UNITED STATES GEOLOGICAL SURVEYGEORGE OTIS SMITH Director

PROFESSIONAL PAPER 98-L

THE PLIOCENE CITRONELLE FORMATION OF THEGULF COASTAL PLAIN AND ITS FLORA

PAPERS BY

GEORGE CHARLTON MATSONAND

EDWARD WILBER BERRY

Publlahed September 11, 1818

Shorter contributions to l eneral aeoloay, 1916Pa1o.1 167- 208

WASHINGTONG O V E RN M E N T PR IN T IN G O FFIO

1916


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IV CONTENTS.

Page.The flom of the Citronelle formation, by E. W. Berry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193Introduction.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193Botanic charac ter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193Physical conditions indicated by the flora . . . . . . . . . . . . . . . . . . . . . 194Age indica ted by the flora. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195The flom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195

ILLUSTRATIONS.

PLATE XXXII. Sections of the Citronelle formation and profiles of the terracesXX.XIII. A Citronelle formation (?) overlain by Pleistocene sand and loam on the west bank of Escambia River south of 1\Iolino, Fla.; B Another view near the same locality, showing un-conformable contact . . .XXXIV. A Clay pit of the Southern Bri


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THE PLIOCENE CITRONELLE FORMATION OF THE GULF COASTALPLAIN.By GEORGE CHARLTON MATSON.

PRESENT INVESTIGATION.In the spring of 1910 the writer workingunder the direction of T . Wayland Vaughangeologist in charge of Coastal Plain investigations undertook a study of the later Tertiaryformations of the Gulf Coastal Plain . According to the plans outlined before the work wasbegun the beds that had formerly been

grouped undr the names Lafayette formationand Grand Gulf formation were to be studiedwith a view to their possible separation intomore satisfactory stratigraphic u n ~ thatmight be correlated with other formationswhich on the basis of their fossils had beenassigned to their proper positions in the geologictime scale. The original plan included a studyof the post-Vicksburgian Tertiary depositsfrom western Florida to Mississippi River andcorrelations with formations previously recognized in Florida southern Alabama and Louiiana This plan was subsequently modifiedto extend the investigation as far west as SabineRiver. The :field work was interrupted andthe office work was delayed by calls for geologicwork in other areas so that the prepamtionof the reports could not be begun until thespring of 1914.

SCOPE OF THIS REPORT.n this paper it is proposed to discuss thegeneral character and relations of beds ofPliocene age and to leave for subsequentpublications the details of lithologic character

and distribution of the Pliocene beds recognized. 4Jthough not abundantly fos siliferousthese Pliocene deposits contain some fossilplants and at two locJtlitie near Lambertsnorth of Mobile and at Red Bluff on PerdidoBay i t was i l e to obtain collectionsadequate for correlation. The fossil plantsfrom these places are described elsewhere inthis report by Mr Berry who also discussestheir geologic age.

EARLY INVESTIGATIONS.The area considered in this report bas beenstudied by a number of geologists though fewof them had adequate opportunity to examinethe formatiolli1 in detail. The major portionof the time at their command was naturallydevoted to the fossiliferous formatiom andthe nonfossiliferous beds were assigned to

groups that by some investigators were probably regarded as separable into formationsprovided enough time could be given to theirstudy. The resul t of this method of classification was to group together beds differing in ageand having very extensive geographic distribution. The beds of each of these groupswere composed of materials of similar lithologic character that appeared to be nonfossiliferous. The lithologic character of the ma-terials naturally depended on two factors-the source of the material and the processes towhich the sediments had been subjected sincetheir removal from their original position.The sediments of the Coastal Plain weretransported to approximately their presentposition by streams that existed when deposition began and in any broad drainage basinthe material for the different formations fromthe older to the younger was derived eitherwholly or in part from the older lands to thenorth even though shore currents shiftedsome sediments from one place to another anderoded and redeposited materials along thestrand line. n some places however noterrigenous deposits were laid down and theformations are made up either wholly or inpart of organic or chemically precipitatedmaterial.As the detrital deposits came from a commonsource they vacy only in accordance with thecharacter of the changea to which they havebeen subjected since they were eroded from theolder land surface. The processes of weathering that have affected the formations of the

167


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168 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY1 1916.Coastal,Plain since their deposition have beencomparatively uniform, and for this reason thesediments of different ages in the Coastal Plainthat are now available for examination arevery similar. In the later part of Tertiarytime, except in Florida, there was a generalabsence of strictly marine conditions, andorganic deposits of that .age are either thin or

0 F

both because of lack of time and because oflack of funds for their prosecution.FORMATION NAME.

The name Citronelle formation is applied tosediments of Pliocene age, chiefly nonmarine,that occur near the seaward margin of the GulfCoastal Plain, extending from a short distance

~ a)_ ett.esanct5tone aA ~ ' 8 = ~ ~ ~ ~ ~ ~ : a ~ s ~ : ; n e ; ;under-lymg ormations0 ~

cFIGURE 15.-Map or Louisiana, Mississippi, and Al lhama showing approxim ately areas covered by Citronelle formation and" Fayettesandstone and boundary between Cat:J.houla sandstone and underlying rorm l.tionB. BectlonB along lines A-A , B-B , C-C , D-D , E-Eare given1n Plate XXXIT; sectionB along lines F-G, G-H, and H I in figure 20 (p. 219).

entirely lacking. The Coastu.l Plain forma- east of the western boundary of Florida westtions in gener.al are composed of clays and ward to Texas. (See fig. 15.) Citronelle, asands derived f.rom the older land, transported town on the Mobile Ohio Railroad, in theto their present position by the streams and in northern part of Mobile County, Ala., wasplaces by marine cmrents and waves and sub- chosen as the type locality because of the exjected since their deposition to similar pro- cellent exposures of the formation in its vicincesses of weathering. As a result the deposits ity, especially to the north along the railroadare so si.mi .lar that they can be divided into dis- for a distance of 3 or4miles. The best collectinct formations only by laborious investigation tion of fossils was obtained from a clay bed awith caref.ul attention to detail. Such studies few miles south of the type locality, near a stawere beyond the re'ach of early investigators, tion called Lamberts, where a flora sufficiently


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PLIOCENE CITRONELLE FORMATION OF GULF COASTAL PLAIN. 169well preserved to permit correlation of the bedswith the Pliocene was found.

SYNONYMYThe Citronelle formation is the equivalent ofa portion of the deposits formerly classified as

Drift, Orange sand, and Lafayette.The name can not be regarded as a synonym forany of the older terms because in all the earlierdescriptions the old names were made to include not only the Citronelle formation, butoverlying alluvial and colluvial sands andgravels and extensive areas of sand and gravellying farther inland and belonging to a numberof different terranes. In addition, the earlierapplications of the old names were such as toinclude beds of Pleistocene age, forming a fringebetween the Pliocene beds and the coast andextending into the river valleys. The development and use of the different names will bementioned briefly in discussing the earlier pub-lications.The Lafayette of some writers may haveincluded not only the Citronelle but portionsof the older formations, together with thealluvial material forming the flood plains andterraces along the streams and the colluvialsands, gravels, and clays distributed on theslopes. In some places portions of the unweathered Miocene and Oligocene beds werecalled Lafayette, though in most localitiesonly the weathered portions were included.The use of the term Lafayette was so elastic that exposures on the valley slopes mightall have been included in the formation, eventhough the outcrops of the older beds wereunweathered, or the unaltered beds mighthave been called Grand Gulf, the weathereddeposits Lafayette, and the relations of thedeposits explained by assuming that the contact between the Lafayette and GrandGulf was irregular.

PREVIOUS DESCRIPTIONSOne of the earliest reports on the geologyof the Gulf Coastal Plain contains a descrip

tion of what is called DiluviUfll or northerndrift. 1 The areal distribution of the mate-rials described indicates that the gravels ofglacial origin borderine the Mississippi areincluded under that term, together with thealluvial deposits on some of the smaller streamsand also, it is thought, some of the beas' ofI Walles, B. L C., Report on the agriculture and geology or Mfsslslippi, pp. 245-253 Mississippi Geol. Survey, 1854.

Pliocene age. The siliceous pebble.;; mentionedin this report, especially the o s s i l i f e r o u ~ chertsare similar to those found in some of the grav:els of the Citronelle formation, though this canhardly be regarded as conclusive evidence forcorrelating these depo.;;its with the Citronelle)because such materials may be expected inother formations derived from the same sources.T h ~ occurrence of the porphyry mentioned byWailes would probably be limited to gravels ofglacial origin, because it is unlikely that suchmaterials would be derived from the sedimentary formations occurring in the drainage b s i ~of the Miasissippi and its tributaries.In a report by Harper/ published three yearsafter that of Wailes, the name ''Orange sandis used to designate materials that had beencalled Diluvium in the earlier publication.This name, which was taken from a report by

S a f f o r d ~ continued in use for many years,though 1t was necessary to redefine it in orderto harmonize the application by different writers. t is clear from the following quotationthat Harper 8 intended to apply this term to apart of the deposits herein named Citronelleformation:

This cover is thickest where the Eocene meets the Miocene. t decreases southward towards the s ~ o a s t andthe State of Louisiana; but nevertheless hills of the Orangesand, with the characteristic pebble stratum, are foundhere and there even very near the seacoast.

The application of the terms Eocene and Miocene by this wri ter is so unusual and so different from the present usage that for the benefitof thdse who may not have access to Harper'sgeologic map, it seems advisable to say that heincluded in the Eocene the entire area south ofthe outcrop of the Jackson formation. I t is inthis area that his Orange sand includes the Citronelle formation. A discussion of the otherbeds classed as Orange sand by Harper may beomitted because they lie outside of the areaincluded in this report.

The reports cited above were based on briefreconnaissances in which observations weremade at widely separated localities. Althoughsome of the later investigations were of similarcharacter, a tendency toward more detailedstudy is shown in the character of the reports.The deposits included in the Citronelle forma-

1 Harper, L., Prellmlnary report on the geology and agriculture of theState or Mississippi, p . 162 Mlsslssippi Gaol. Survey, 1857.

1 Betrord, J. M., Geological reconnaissance or Tennessee pn . 148 162' ' ' ' Harper, L ., op. cit., p. 162.


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170 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1916.t i o ~ w ~ r e first ~ y s t e m a t i ? a l l y ? i s c ~ s e d on t ~ e f r ~ m which they are derived. The report conbasiS of extensive field mvestigations by Hil- tams some speculation as to the means of trans

g ~ d t who i n c l u ~ e d .beds belonging to the portation of these pebbles to their presentCitronelle formation m the Orange sand. positions.He said: .After the publication of the papers cited

But the. formation which gives character to the surface there was a revision of the nomenclature beconformation of the State-whose presence is the rule, cause of the confusion resul tin f m thand whose absence the exception requiring special men- d.iff f g ro etion; which forms the main body of most ridges and, to a e r e ~ c e s 0 ~ a g e ~ different areas. Thevery great extent, their surface that which has necessity for this reVIsion was emphasized whenbeen very appropriately designated by Prof Safford, the McGee 1 correlated his Appomattox formationState geologist of Tennessee, as the Orange sand formation. with at least a part of the OranO e sand of The term Orange sand, as ed by Hilgard, Hilgard and other southern geologists."includes some of the deposits herein named In 1891 the name Orange sand was reCitronelle formation and also portions of older p l a c ~ d by Lafayette formntion," the typeand younger deposits correlated with them on locality for the new name being in Lafayettethe basis of lithologic character . County, Miss.,2 where, according to Berry,3 the. In a discussion of the Ornnge sand Hop- "O.range sand is of Eocene (Wilcox) age.kins 2 correlated some of the mnterirus in ThiS change was made at a conference in which

L o u i ~ a n a with the Orange snnd of Hilgard. ~ ~ g a r d , McGee, L e C o n ~ e ; and Loughridge parIn his section acro.ss the State the name tiCipated, and the demswn reached by themPrairie diluvium" includes the beds now was subsequent ly indorsed by Safford.4 Mccnlled the Citronelle formation, as well as both Gee was the first geologist to npply the termolder and younger beds. A subsequent report Lafayette to deposits distributed over the entireby Hopkins 3 contains a more comprehensive area of the Coastru Plain and extending acrossdiscllSsion of the drift," in which he included a the edges of the older formations betweenlarge amount of material varying in age from Washington ~ the Rio Grande.5 Althoughthe oldest beds exposed in Louisiana to the he regarded h1s Lafayette as somewhat morePleistocene. Among these beds are the de- rest ricted than Hilgard's Orange sand, it isposits here named Citronelle formation, whi< h clear that he included in the Lafayette the bedsHopkins mentioned in connection with the now known as the Citronelle formation. Thisdistribution of the drift in some of the parishes is shown by numerous references to the localisouth of the outcrop o the beds later desig- ties where this f o r m a t ~ o n is at the surface, bynated Catahoula sandstone. The geologic map the maps ~ c c o m p a n y m g the report, and byaccompanying Hopkins's report shows the numerous diagrams showing the relations o thearea occupied by the Citronelle formation as formations exposed on the Gulf Coastal Plain.covered by what he cnlls drift. A third The accompanying sections (figs. 16 and 17)publication by this author discussed the illllStrate the relations of the Lafayette to

drift in more detail, giving speciru attention o l ~ e r and to younger formations, and if theto its constitution and origin.4 The identifi- thiCkened portion of the Lafayette asable fossils occurring in pebbles are listed but represented in th ese sections is separated fromunfortunately some of the localities where these a port ion of the thin layer on the upland it w ll

p e b b l ~ s were collected were not given, though r ~ p r e s e n t approximately th e Citronelle formaHopkins stated the age of the geolooic forma- twn. As the present report does not den withtions that supplied these pebbles and the pos- the major portion of the deposits coating thesible geographic distribution of the localities surface of the upland, it has seemed best toomit them from the discussion.1 Hllgard, E. W., Report on the geology and agriculture of the Stateof Mississippi, pp. 4-29, Mississippi Geo l. Survey, 1860.

Hopkins, F. V., Louisiana Geol. Survey First Ann. Rept., for 1869(Included in the annunl report or the Louisiana State Seminary ofLearn-ing and Military Academy), p. 78, 18i0.1 Hopkine, F. V , Louisiana Geol. Survey Second Ann. Rept., pp.

21-26, 1871. Hopkins, F. V , Lo uisiana Geol. Survey Third Ann. Rept. (includedin lllliluel report of D. F. Boyd, superintendent Louisiana State University, for 1871), pp. 190-203, 1872.

1 McGee, W J, Three formntions of the middle Atlantic slope: Am.Jour. Sci., 3d ser., vol. 35, p. 330, 1888. Hllgard, E. W., Orange sand, Lagrange, and Appomattox: Am.Geologist, vol. 8, pp. 129-131, 1891.

1 Berry, E. W., The Lafayette formation: Jour. Geology, vol. 19, pp.249-256, 1911. Hllgard, E. W ., op. cit. , p. 131. McGee, W J, 'Jlhe Lafayette formation: U. S. Geol. Survey TwellthAnn. Rept., pt. 1, pp. 430-501, 1891.


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GEOLOGICAL. SURVEY

IllQ ()() Ill Q. : c f L 0 uell " c: L 0c. i i . " La. > Qlel c 0 I l l + . v0 >. E X '0 +. 0c Q) iiJ J: c (Qf) (Q x 0 0( . ) 0 +l t E 0.... . J I f ) E (QIf r1--......._ Q c::c 41a~ ~ ~ ~ : ~ ~ : = = : : : : : : : = = = = = = = = = = = = ~ = = ~ = = = = = = ~Generalized profile across southwestern Mississippi and southeastern Louisiana

FEETGOO

0

c"~ N .I l l>I l lA .r IIX

FEET400 e uccz_Q)0coL.L.0Ej ; j

Q0 tJ(Q 0LL EL.>-41 CX0 cJ: 0Et E0 Ill a: ::c

Profile of surface from Neame, La., o Gulf of MexicoSupplied by the chief engineer of he KBnsas City SouthernRailway

Q0QL. UL. je]Qllc:.If H

s. atJV -Ill

...cJuQa::

PROFESSIONAL. PAPER ga

(The land s u ~ e is a little above sea e v ~ l

AFEI;T400_0Q0 Sealevel

~ ~ ~C . L i 8B I s L L e ~ : . .BQ. I C5. > sNW. s >- :g . S ~ SE.1: -i E :E 0 ~ . .

FEET tj J _3 t E FEETz o o ~ ~ F z o o0 . -OSealevelProfile Qf surface from e m m e ~ to Mobile,Aia.Compiled from a profile_urnished by H21rry L.Fisher;chief n ~ i n e e r of Mobile County, Ala. and a contourm ~ p ol' Mobile by Nicoll 6: Bond,civil engineer.-.cs.FEE600

800 ~ ~ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ~ 8 0 0I

~ 1 0 cIJ ;OitJ 0..N. coS tJj 5111) >Rlt lo ..cVI): 1 .0 00FEET I -u CXl400zoo0zoo

Geologic section from Crystal Springs, Miss. to Hammond, La.

.r:: uc ul ;ii.ill ; u0...lU( / )

Geologic section from Washington-Mobile county line, Ala., o the Gulf of MexicoHORIZONTAL SCALE5 10 20 MILESE L ~ ~ c = = = = = ~ = = = = ~ = = = = ~ ~ = = 15

.IJ84).jJ

f.EET

.EN

Geol'ogic section from Lucedale to Laine, Miss.

SECTIONS OF THE CITRONELLE FORMATION AND PROFILES OF THE TERRACES.


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PLIOCEN E CITRONELLE, FORMATION OF GULF COASTAL PLAIN. 171After the appearance of the report by McGeethe name Lafayette was adopted by writerson Constal Plain geology and soon came intogeneral use, in an area extending from Maryland to Rio Grande. The present report,

however, is restricted to the area from westernFlorida to eastern Texas and does not dealwith the Atlantic border of the Coastal Plain

though some of the superficial materials on theolder Tertiary and the Cretaceous formationsare not included in the Citronelle. In dealingwith the thin coatings of weathered sand orclay resting on the older beds it is in manyplaces difficult to obtain enough physiographicevidence to warrant correlations, because ofthe lack of topographic maps.

FIGURE G.- Section through the Co ll9tnl Pl:Un in weste.""JJ Alabama; generoll.:cd in part from sections constructed lDd by E. A .Smith and L. C. Johnson. (After McGee.)

or with the major portion of the region westof Sabine River.Clendennin discussed the Lafayette formation as developed in Louisiana, and thegeographic distribution as he described it ismuch the same as that of the Citronelle, butthis author followed McGee by including ol


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\

172 SHORTER CONTRffiUTIONS TO GENERAL GEOLOGY, 1916.Mississippi is somewhat less complete than thatabout the area east of the river, but specialmention is made of the gravel pit in the southern part of Rapides Parish. Another paper bythe same author was published in one of thesubsequent reports of the Louisiana GeologicalSurvey.1 This paper is devoted very largelyto a discussion of the theories of the originof the Lafayette formation," with some mention of the distribution of the beds referred toit. The localities noted in the area covered bythe Citronelle formation are Neame, where thebeds are over 275feet above sea level; Sulphur,where the same deposits are reported to occurin a well extending to a depth of more than 400feet below sea level; and Morgan City, wherethey extend to a depth of more than 500 feet.The relations of these deposits are shown graphically in a diagram of the country from a pointnorth of Jackson, Miss., southward throughTangipa.hoa, Amite, and Hammond to theGulf.The reports by r i d e r ~ afford little additional information concerning the Citronelledeposits. This author included in the La -fayette formation the beds in the southernpart of Mis 'issippi belonging to the Citronelleformation and stated that they were reportedto attain a thickness of over 200 feet.Veatch 8 dealt in considerable detail withthe " Lafayette formation in Louisiana, including in this formation beds belonging to theCitronelle, as shown by his diagram and by hisstatement of the detailed distribution of theformation. He said:

South of the Catahoula and Fleming formations thesesands and gravels form the surface for miles and then passsouthward beneath the more recent clays of the Quaternary, forming there the water-bearing beds which furnisha portion of the waters used in the irrigation of ~ region.I t is possible that in addition to the bedsdescribed in this quotation some of the deposits classed as Lafayette lying north of thearea described are outliers of the Citronelleformation.

1 Harris, G. D., Geology and agriculture of Louisiana: LouisianaGeol. Survey , pt. 6, pp. 32-, 36, 1902. Crider, A. F., and Johnson, L. c., Summary of the undergroundwater resources of Mississippi: U.S. Geol. Survey Water-Supply Paper159, pp. 12-13, 1906. Crider, A. F., Geology and mineral resources ofMississippi: U. B. Geol. Surve y Bull. 283, pp. 44-46, 1906.a Veatch, A. c., Geology and underground water resources of northern Louisiana: Louisiana Geol. Survey Bull. 4, pp. 43-45, 1906.

In 1907 Smith 1 discussed the Lafayette formation," which he made to include the areaunderlain by the Citronelle formation. Theterm Lafayette formation" as used in: thisreport is not synonymous with Citronelle formation, but some of the beds covering a portion of the Grand Gulf" and lying south ofthe northern margin of that formation maybelong to the Citronelle.MatRon and Clapp discuss the Lafayetteformation" in Florida in a general way. TheCitronelle formation comprises only a minorpart of the area assigned to the " Lafayettein Florida and is not included in any of thedetailed sections given. In this report thename Lafayette is used for the weatheredportions of deposits of Oligocene, Miocene, andPliocene age and for surface deposits of yariousages.

STRATIGRAPIDC RELATIONS.The Citronelle formation rests upon thePascagoula clay (Miocene) and overlaps theOligocene formations. Wherever the contactwith the underlying formations has been observed it is marked by an unconformity, theolder formations having been eroded before thedeposition of the Citronelle formation. (SeePl. XXXII.) The seaward margin of theCitronelle formation is overlain by beds ofPleistocene age, and contacts between thePliocene and younger beds can be seen at manyplaces. They show a marked unconformity,the Citronelle formation having been erodedand the materials derived from it having beenincorporated with materials from other sourcesto form the deposits of Pleistocene age. (SeePl. XXXIII.) The unconformity may alsobe inferred from the fact that the Pleistocenedeposits form terraces along the seaward margin of the Citronelle formation and extend intothe valleys of the major streams, reaching entirely across the Citronelle and resting uponthe still older formations. The relation of theCitronelle to the other formations of the GulfCoastal Plain is shown in the accompanyingtable.The Citronelle formation differs from boththe older and the younger formations in being

1 Smith, E. A., The underground water resources of Alabama, pp.24-25, 302-316, Alabama Geol. Survey, 1907. Matson, G. C., and Clapp, F. G., A prellmlnary report on the geologyof Florida, with special reference to the stratigraphy: Florida Geol.Survey Second Ann. Rept., pp. 141-145, 1909.


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Age.

Pleistocene.

Eastern Texas.

Pensacola terrace.rra.mmond terrace.Port Hickey terrace.St. Elmo terrace.

Louisiana.

Pensacola terrace.Hammond terrace.Port Hickey terrace.St. Elmo t errace.

Pleistocene and Tertiary formations of th ulf Coastal Plain

Mississippi.

Pensacola terrace.Hammond terrace.Port Hickey terrace.St. Elmo tenace.

Alabama.

Pensacola terrace.Hammond terrace.Port Hickey terrace.St. Elmo terrace.

Western.Pensacola terrace.Tsala Apopka terrace.Newberry terrace.

Florida.Peninsula

Unconformity- - - I I 1 1

Pliocene.Citronelle formation. N o n m a r in echiefly, including high-level terraces.Yellow and red sands aud clay, locallygray where unweathered. Gravel nearthe landward margin. Thickneoo, 50-

2GO feet.

Citronelle formation . N o n m a r in e'chiefly, including high-level terraces.Yellow and red sands and clays, locallygray where unweathered. Muchgravel near the landward margin,especially in the valley of Red River.Thickness, 50-400 feet.

Citronelle formation. N o n m a r i n echiefly, including high-level terraces.Yellow a.nd red sands and clays, locallygray where unweathered. Muchgravel ncar the landwa.rd margin andin the valleys of the principal streams.Thickness, 50-4.00 feet.

Citronell e formation. N o n m a r i n eclriefly, including high-level tenaces,marine fossiliferous equivalents inwells. Yellow and red sands andclays; locally gray where unweathered.Gravel near the landward margins contains fossil plants. Thickness, 50-340feet.

Citronelle formation. Non-marine chiefly, includinghigh-level terraces; marinefossiliferous equivalents inwells. Yellow and redsands and clays, some gravel.Thickness, 50-150 feet.

CaloosahatchiNashua Marine.

Unconformity I I I

Jvliocene.Pascagoula clay. Marine in part. Blue,green, and gray clay, locally calcareousand fossiliferous. Some large calcareous concretions and many smallnodules. Some layers of sand andsandstone. Thickness, 250-300 feet.

Pascagoula clay. Marine in part. Blue,green, and gray clays, locally calcareousand fossiliferous. Some layers of sand;locally a sandstone near the op. Manynodules of calcium carbonate n somelayers of the clay. Thickness, 250-450 feet.

Pascagoula clay. Marine in part. Blueand gray clays and sands; some thinbeds of sa,udstono; locally graYols andconglomcratt>s at the base. Fossiliferous mads in places and some clayswith nodules of calcium carbonate.'fhickness, 50-


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PLIOCENE CITRONELLE FORMATION OF GULF COASTAL PLAIN. 173predominantly sandy, with many lenses andscattered pebbles of chert gravel. To understand fully the application of the name it isnecessary to know the mode of deposition ofthe formation. .After the Miocene strata hadbeen laid down the Coastal Plain was erodedinto broad, shallow valleys having approximately the same positions as those of the present streams. These valleys were filled by thedeposition of Pliocene alluvial sands and gravels, and near the coast the deposits were extended across the interstream areas. This:filling formed a broad plain, which was later. partly eroded, while at the same time themargin of the formation was pushed fartherseaward and three successively lower plainswere built, their sediments resting on the olderdeposits. The original deposit formed a seaward-sloping plain, and each successive addition formed a new plain, which was representedalso in the stream valleys. See Pl. XXXII.Some portions of the deposits were doubtlessreworked by the waves, especially in the interstream areas, and this accounts for the morecomplete rounding of the pebbles in portionsof the seaward areas of the formation than inthe stream valleys and the development offlat plains with shallow ponds in the interstream areas. The closing stage of the deposition of the successive plains is marked by the:fine sandy silts laid down at flood stages whilethe streams were eroding their beds to lowerlevels. The deposition was pushed far seaward in some places, as shown }>y sands withPliocene fossils at Pensacola and Mobile andby a leaf-bearing clay at Red Bluff, on PerdidoBay, Ala.

The sediments included in the Citronelleformation have sometimes been referred to thePleistocene becfl use of their physiographic resemblance to the terraced deposits of Pleistocene age. TheJ differ from the Pleistocenedeposits of the Gulf coast, how.ever, by beingmore sandy and containing more gravel. Theevidence of their greater age is shown by theirmature dissection, which in general exceeds thatof the coast Pleistocene and glacial deposits, andby the weathered condition of the pebbles, many of which are composed of chert so completelydecomposed as to break or even crumbleeasily in the hand. The-Pliocene age is furthershown by the presence of the Pliocene fossilplants, those atRedBluffshowingthatthedepo-

sition was originally extended some distancebeyond the southern margin of the generalarea shown in :figure 15 p. 168), and that thePleistocene material was later deposited uponthe eroded surface of the Citronelle. The linebetween the Pliocene and Pleistocene plainsis drawn between the 150-foot and 200-footterraces. The Pleistocene age of the plains atlower levels is shown by their slight dissectionand by the presence of crystalline pebbles inthe fluviatile portion of the 150-foot terraceat Natchez, Miss.

The 150-foot terrace rises gradual ly landwardto 170 feet or more, and its representatives inthe river valleys are still higher, attaining elevations of somewhat more than 200 feet a shortdistance from the coast. Deposits of this terrace contain the pebbles of crystalline rocks atNatchez-the Natchez formation.The Pliocene plains are more eroded than thePleistocene plains and when compared in thesame area hey may be distinguished by t isdifference. The Pliocene material was considerably eroded before the formation of theuppermost Pleistocene terrace, and relativelywide valleys were developed even on some ofthe small streams. The formation of thePleistocene terraces was begun by a submergence that affected the drainage some distancefrom the coast and resulted in the formation offlat, swampy areas along most of the streams.These readjustments in drainage conditions arestill marked by swamps in many of the smallstream valleys in southern Alabama, Mississippi, Louisiana, and southeastern Texas, theflat-bottomed valleys being overgrown byswamp vegetation through which the streamsmeander in very poorly defined channels.

LITHOLOGY.The Citronelle formation is predominantly

sandy but contains varying amounts of clay inthe form of thin layers or lenses. See Pl.XXXIV. The relative proportion of materials of different kinds varies from place toplace, and in any one section it varies from topto bottom, though in general sand is everywhere abundant see Pis. XXXV and XXXVI),and at many places there is some gravel. Theproportion of sand is greatest near the base,and the formation contains more sandy material in the vicinity of the principal drainagelines than in the interstream spaces. There is


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74 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1916.also a larger percentage of sand and gravel nearthe landward than the seaward margin of theformation (see Pis. XXXVII and XXXVIII),though in making comparisons it is necessary toconsider large masses and to choose localitiesbearing about the same relations to the principal lines of drainage, because more sand andgravel will be found in the vicinity of largestreams near the coast than on some of the interstream spaces farther inland..The sands of the Citronelle formation are sufficiently rounded to indicate that they havebeen subjected to extensive attrition. Thepebbles vary greatly in degree of rounding;many of them are much waterworn and resemble pebbles found on the beach . others are onlyslightly rounded, and still others are subangular.The sand is predominantly quartz, and the pebbles in the Mississippi embayment are mostlychert, with a somewhat smaller percentage ofcrystalline quartz and quartzite. 1\fany of thechert pebbles contain cavities lined with chalcedony in the form of agates or geodes, and alarge number of them are fossiliferous, containing fragments of corals, crinoid stem, and otherorganic remains such as are common in thePaleozoic limestones and chert. In Alabamathere are many pebbles of quartz, with a subordinate percentage of quartzite, but in someplaces chert pebbles are numerous. In westernLouisiana the pebbles are mostly quartz withsome admixture of chert.

Certain peculiarities in the distribution ofmaterials in the Citronelle formation are worthyof mention. In :Mississippi the outliers are ingeneral composed of coarse subangular chertgravel, with a varying percentage of small,well-rounded quartz pebbles. In Louisianaand Alabama quartz gravel is common. Thecoarse pebbles have in a measure the samecomposition and evidently came from a common source, and the same is true of the finepebbles. The coarse subangular fragmentsare mostly chert and have been subjected tofluviatile wear with very little subsequentrounding by waves, but many of the finer, wellrounded pebbles are quartz and have doubtlessbeen shaped by prolonged subaqueous erosion.Probably the subangular materinl was broughtdirectly from its original source, and the fine,well-rounded materiol was subjected to waveerosion by being deposited in some one or

more of the older formations of the CoastalPlain, from which it was obtained by Pliocenestreams. This conclusion seems warrantedbecause the gravel is a poorly assorted aggregate of materinl of all sizes from sand to pebbles an inch or two in diameter . (See Pis.XXXIX, XL, and XLI. f the gravel hadbeen subjected to wave action during Pliocenetime it would have been assorted, and it istherefore inferred that the perfect rounding ofthe small pebbles was accomplished by waveaction during earlier geologic periods.The clays vary greatly in character, some ofthe beds being relatively pure and others distinctly sandy. On the whole the sandy clayspredominate, and in many places thin layersof sand are found in the clay beds.The Citronelle formation has a very widerange in color. Doubtless when fresh someof the materials included in the fmmation wereeither blue or gray with small percentages ofred and yellow, but nearly all the exposuresavailable for observation at the present timehave been subjected to so much weatheringthat the original colors have been partly orwholly obliterated. The sands in most of thesections are either orange or red, though nearthe surface some of them are yellow. Thispredominance of the orange and red colors onweathered surfaces led to the designation ofthe materials m this formation, either whollyor in part, as Orange sand. The clays varyin color according to the degree of weatheringto which they have been subjected, beds thathave been buried to sufficient depth to protectthem from the oxidizing effect-of the atmosphere or surface water being either pale blueor gray. Where the oxidation has been slightthey are mottled, the first change producingspots or blotches of a light-red or peculiar purplish color. As the process of weathering continues the purple color disappears and a deepred color predominates, and still further weathering changes the red color to pale yellow.The iron in the clay, which presumably causesthe coloring, changes during the process ofoxidation to hydrous oxide, and on completeweathering of the clay to a yellow color theiron becomes more or less aggregated intonodules of varying sizes. Somewhat similaraggregations of iron hydroxide occur in thesands in the form of concretions and geodes,


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U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 96 PLATE XXXIII

A CITRONELLE FORMATION 1) OVERLAIN BY PLEISTOCENE SAND AND LOAM ON THEWEST BANK OF ESCAMBIA RIVER SOUTH OF MOLINO, FLA.

B ANOTHER VIEW NEAR THE SAME LOCALITY SHOWING UNCONFORMABLE CONTACT.


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U. S GEOLOGICAL SURVEY PROFESSIONAL PAPER 98 PLATE XXXIV

A CLAY PIT OF THE SOUTHERN BRICK CO. 1 MILE NORTH OFGOULDING FLA.

B SANDSTONE AND CLAY PIT OF THE SOUTHERN BRICK CO . 1 MILENORTH OF GOULDING FLA.


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PLIOCENE CITRONELLE FORMATION OF GULF COASTAL PLAIN. 175or as shells inclosing cores of clay. In otherplaces the iron accumulates in the form ofplates cementing thin layers of sand or as afilling for more or less definite cracks thatcross the strata at high angles. Where theiron hydroxide is disseminated among the sandgrains or pebbles it transforms the bed into asandstone or conglomerate. This cemento.tionapparently takes place near the surface and asa rule does not extend for any great distancealong a bed, so that in most places the sandstone and conglomerate are in the form of

l ~ c k s of vruying sizes. (See Pl. XXXVIII,A. The irregularity in size and shape of theseblocks usually prevents them from being utilized in const1 uction, even where they are fumlycemented,. but south of Citronelle, at the 28-mile siding on the Mobile Ohio Railroad, ahard sandstone belonging to this formationhas been quarried to a depth of over 5 feet, andthe rock obtained at this locality has been usedin the construction of a church.The texture of the formation in any particular locality depends entirely on the arrangement and relative percentages of the differentkinds of materials. In general, the sands formmore or less continuous beds containing lensesof pebbles or clay that in some places haveconsiderable horizontal extent and in othersare limited to only a few feet. Many clay lensesare only a few feet to a few rods in extent,though in places they interlock oth er lenseslying above and below them, and in tho.t waypresent the appearance of a continuous bed.unless it is possible to exo.mine the section indetail. In the sands cross-bedding and crosslamination are the rule rather than the exception, the layers of clo.y and pebbles in manyplaces being inclined at high angles. Smooth,even bedding is comparatively rare except inth:e clay lenses.

In this formation, as in many others, pebblesof clay are common, occurring at many strati-graphic horizons and having a wide geographicdistr ibution. Most of them are more or lessperfectly rounded balls of clay embedded insand in such a way as to form a lenticular clayconglomerate. In places there are more or lessrounded blocks of clay in which the originallamination is still preserved. Here and thE\l etlus lamination coincides in direction with thatof the sands, but as a rule it differs. Doubtless

many of the pebbles were eroded from thesubjacent clay by the currents that transportedthe sand. Some of the pebbles and probablymost of the angular fragments were derivedfrom the undercutting of small cliffs developedby waves or currents. (See Pl. XLII. Th:isexplanation for blocks of clay embedded inso.nd was first suggested by T. WaylandVaughan 1 to account for similar clay fragments found in some of the sands o the Wilcoxformation, but it is also applicable to such clayfragments in the Citronelle formation.

STRUCT ulm.In the Citronelle formation the arrangementof the sediments in lenticular beds makes itimpossible to determine the structure by observations on the elevation of beds (see Pls.:XXXIX, A, and XL, B except at a few localities where the conditions are unusually favorable. The prevalence of cross lamination andcross-bedding renders determinations of dipwith a clinometer valueless because the inclinations observed may not agree in direction

with the dip of the beds, and they may varyin direction or be in opposite directions in asingle section. Under such unfavorable conditions the recognition of local structural features is everywhere difficult and in most placesimpossible.The inclination of the base of the formationcan be measured in many places, especiallynear the landward margin. The results of suchmeasurements show that the dip is not uniform,and in addition to irregularities resulting fromdeposition on an uneven surface there is a general increase in the rate of inclination of thebase of the formation toward the coast. Thesefacts are well shown by the section across southwestern Mississippi and eastern Louisiana,vhere the contact of the Citronelle with theolder formations descends at an average rateof a little less than 6 feet to the mile from apoint 6 miles north of Wesson to Osyka, Miss.,and at a much higher 'rllite from Osyka, Miss.,to Hammond, La. (See Pl. XXXII. Othersections show steeper inclinations, but theyare based on less accurate and less detailedinformation. The base of the Citronelle formation near the coast is drawn at the transition

1 Unpublished notes.


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176 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1916.from sands, sandy clays, and gravel to finergrained sediments, chiefly clays, containingcharacteristic fossils of the Pascagoula ~ l a yand the correlations areprobablyapproximatelycorrect, though there may be errors of a fewfeet in the position of the contact because ofslight inaccuracies in the well logs used for thepurpose of obtaining measurements of depthsto the base of the Citronelle formation. Wherethe computa twns of the rate of dip of the baseof the formation are made on a line severalmiles in length, the amount of error in the rateper mile is minimized. '

The actual dip of any portion of the formation may be less than the inclination of thebase, because deposition probably began nearthe coast and. was gradually extended inland.This is suggested by the fact that the coarsegravels that form outliers along the landwardmargin of the formation rest on finer depositsbelonging to the Citronelle farther south. Forexample, the coarse gravel rests on the Pas-cagoula clay near Wesson, but at Brookhavenit overlies about 130 feet of red sand with interbedded clay that is included in the Citronelle.The gravels are covered near Summit by stillyounger sediments that are more argillaceousthan those below.Near the coast the upper portion of theformation was laid down on the seaward slopeof the earlier deposits, and in this portionthe actual dip is probably greater than the dipof the base of the formation. In the absenceof reliable data to use in computing the truedip, however, it seems best to give the inclination of the base. The exact dip of the baseof the formation is not easy to determine,because near the coast there are no exposuresthat reach the contact with the Pascagoulaformation and it is difficult to interpret theinformation supplied by some of the logs ofwells. Such computations as have been madegive discordant results that are probably notentirely due to the incomplete informationbut are partly explained by local variationsn the rate of dip.In the northern part of Mobile County, Ala.,the base of the Citronelle formation is about240 feet above sea level, and at Citronelle, 4miles south of the county line, the contactwith the underlying Pascagoula clay is at220 feet above sea level, giving an average inclination of about 5 feet to the mile. From

Citronelle southward to Semmes the contactdescends from 220 feet above sea level to 92feet below sea level, or at the rate of about16 feet to the mile.At Laine, Miss., where the surface is about10 feet above sea level, characteristic fossils

of the Pascagoula formation were obtainedfrom the De Lamorton well at a depth of 615feet (?), and the base of the gravel beds wasreached at 605 feet. his well is 53 milesS. 28 W. from the place in Mobile County,Ala., where the elevation of the base of theformation was determined, indicating a dipof nearly 16 feet to the mile, provided the gravelreached in the well is at the base of the Citronelle formation; but as the gravel may be somewhat above the contact the dip thus determinedshould be regarded as the minimum inclination of the base of the formation rather thant,he exact amount.Near Maxie the base of the Citronelle formation is about 215 feet above sea level, and atBond, 6 miles S. 20 E., a well reached thecontact of this formation with the clays of thePascagoula at an elevation of 115 feet. Thesteep dip between these two points, nearly 17feet to the mile, is explained by the fact that theformation was deposi.ted on an uneven surface.Computations of the rate of dip between Maxieand Laine give about 14 feet to the mile, andthe direction is about S. 45 E. At Fontaine- -bleau,1 about 20 feet above sea level, fossils belonging to the Pascagoula clay were obtained ina well at a depth of 500 feet. These fossilsprobably came from beds near the top of thePascagoula, and they show that the dip of theCitronelle formation between Maxie and Fontainebleau is between 14 and 15 feet to the milein a directionS. 35 E.In western Mississippi measurements of thedip of the Citronelle formation have been madefarther inland than in eastern Mis sissippi and inAlabama. See Pl. XXXII. ) Six miles northof Wesson the base of the formation is 435 feetabove sea level, and just eas t of Brookhaven itis 330 feet above sea level. Here the contact ofthe Citronelle with the older formations descends about 115 feet in 15 miles, or at the rateof nearly 8 feet to the mile. From Brookhavento Osyka the contact descends from 330 to 100

1 Crider, A. F. nnd Johnson, L. C., Summary of the undergroundwater resou rces of Mississippi: U. B. Gaol. Survey WaterBupply Paper15\ r p. 32, 1906.


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U. S GEOLOGICAL SURVEY PROFESSIONAL PAPER 98 PLATE XXXV

A SANDS AND SANDY CLAYS WITH PLATES OF LIMONITE-CEMENTEDSANDSTONE NORTHERN ESCAMBIA COUNTY FLA.

B SAND CONTAINING A LENS OF CLAY WITH A PLATE OF SANDSTONECEMENTED WITH LIMONITE AT THE CONTACT OF THE SAND ANDCLAY NORTHERN ESCAMBIA COUNTY FLA.


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U S GEOLOGICAL SURVEY PROFESSIONAL PAPER 98 PLATE XXXVI

A SAND AND CL AY FLOMATON ALA

B SAND AND GRAVEL FLOMATON ALA.


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PLIOCENE CITRONELLE FORMATION OF GULF COASTAL PLAIN. 77feet above sea level, or 230 feet in about 39miles, which is at the rate of 6 feet to the mile.From Osyka southward the dip is difficult todetermine because of meager information, butit apparently becomes steeper, amounting toapproximately 810 feet between Osyka andHammond, a distance of 35 miles, which wouldbe at an average rate of about 23 feet to themile. These determinations are all made alonga line extending nearly north and south.In Louisiana many wells have been. drilledsouth o the margin of the Citronelle formation,but the materials penetrated in most of themare not described in the records with sufficientexactness to permit correlations. BetweenCenterville, Miss., and Bass, La., the rate of dipof the base of the formation is about 19 feet tothemile, and it must increase rapidly towardthe south, as is shown by the great thickness ofthe overlying Pleistocene deposits at New Orleans. Between Osyka, Miss., and Bass, La.,the rate of descent is about 10 feet to the mileand the direction about southwest. This is lessthan the slope from Centerville to Bass andprobably indicates that there was a delta in theMississippi Valley during this epoch rather thanan embayment. West of the Mississippi fewdata that can be used in computing the dips ofthe formation have been obtained. About amile north of the Jennings oil field, where thestrata are undisturbed and the surface is onlyabout 25 feet above sea level, Miocene fossilswere found at a depth of 1,960 feet. The baseof the Citronelle formation near Woodworth,about 70 miles north of the Jennings oil field, isapproximately 140 feet above sea level. Thiswould give a dip of nearly 30 feet to the mile,provided the fauna found near Jennings camefrom the top of the Pascagoula clay. This diprepresents a maximum, and the actual rate ofdescent between the two points is probably notso great.The figures given in the foregoing discussionshould be regarded as only approximate measures of the rate of inclination of the base of theformation. Tl;le general dip of the depositsbeneath the Pleistocene along the southernmargin of the formation is shown in PlateXXXII and amounts to less than 20 feet to themile. The general dip is toward the south,though there are doubtless many local variations in direction. Dips determined from the

Harris, G. D. 011 a.nd gas In Louisiana: U. 8. Geo l. Survey Bull. 429,p. 58,1910. .

elevations of the bases of outliers and from logsof wells drilled where the Citronelle formationis at the surface are much lower than 20 feet tothe mile, and the rate is variable because theformation was deposited on a surface that wasuneven and had a steep slope near the Pliocenestrand line. This steep slope accounts for thefact that near the coast the dip becomes steeper,probably exceeding 20 feet to the mile. In thevicinity of New Orleans the original slope nearthe Pliocene strand line may have been increased by subsidence during Pleistocene time,as shown by the great thickness of Pleistocenebeds 2,443 feet) overlying the Tertiary formations.1The thickness of the Pleistocene beds atFort Morgan, Ala., suggests a steep descent ofthe Pliocene Citronelle formation. The log ofthe well at that place, shown in Plate XLIIIin pocket, was made from a series of samplesof drillings, and the subjoined list of fossils wasfurnished by Mr. Dall.Fragments of echinoderms and two Recentshells, Dona:r variabilis Say and Paramya Depth feet).subovata Conrad 30-32Dentalium acus Dall, NG 88a acuta Say, Ervi-

lia. sp. iunior, Ostrea., and echinoid frag-ments, all RecentRecent shells, Mulinia lateralis Say, Dona:rvariabilis Say, Area transversa Say, etcRecent species, Abra requalis Say, Areatransversa Say, Strigilla carnaria Linne,Mulinia lateralis Say, Leda, Pecten, and

Dolium fragmentsStrigilla carnaria Linne, Recent Ostrea fragments Recent species, Ervilia cf. E nitens Montgomery, Phacoides crenella Dall, Mulinialateralis Say, Diplodonta sp., Modiolusdemis8U8 fnigments, Cadulus and Cyma-

tium fragments .Fragments of Phacoides, Mulinia, Ostrea,and Marginella sp.; probably Recent .Divaricella cf. D dentata Wood, fragments;Mulinia cf. Jlf. lateralis Say; Cadulus frag-ments; and Phacoides ,sp., young; prob-

3 )-4 )

32-87

100-112169-175175-256

217-321322-337

ably all R ecen t 523-570Nassa acuta Say, upper :Miocene to Recent. 1, 076-1,089Ostrea fragments. . . . . . . . . . . . . . . . . . . . 1, 089-1, 121Conus pealei Green, Recent ..... 1, 244-1,290Mulinia lateralis Say, Recent to Miocene . 1, 290-1,330Chione sp., fragments._.. 1 350-1,370Ostrea fragments, Anachis cf. A avara Say,

and a fragment of Turritella sp... . . . . . . 1, 378-1, 573The material above 1,290 feet is nearly all Quaternary;

that belo:w may possibly be Pliocene.1 Bnrris, G. D., op. cit., p. 170.


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178 SHORTER CONTRIBUTIONS 'IO GENERAL GEOLOGY, 1916.his well is 35 miles S. 35 E. from St. Elmo,where the Pliocene reaches an elevation of 130feet above sea level. The surface of the Citronelle formation descends about 1,220 feetbetween these two points, or at the rate ofmore than 35 feet to the mile, but the original

inclination may have been much less, becausealthough the Pliocene has been eroded at bothlocalities the amount of post-Pliocene erosionwas greater at Fort Morgan, which is near theaxis of Mobile Bay.Near Pensacola the available informationsuggests a somewhat lower dip for the top ofthe Citronelle formation. At Pine Orchard,nearly 7 miles north of Pensacola, the top ofthe formation is about 160 feet above sea level,and about 4 miles south of Pensacola the shellsobtained at a depth of 256 to 276 feet belowsea level were tentatively referred to the Pliocene by Mr. Dall. This indicates a maximumdip of 416 feet in a distance of about 12 miles,or at the rate of about 35 feet to the mile,approximately toward the south.

From the foregoing statements it seems evident that there is a rapid increase in the rateof inclination of the top of the Pliocene nearthe coast at Pensacola and Mobile and north ofNew Orleans, and similar conditions doubtlessprevail along the entire shore line. This apparent steeper dip at Pensacola and Mobile is,however, partly explained by the fact that inthe wells cited the seaward equivalent of theCitronelle formation may have been reachedsome distance above the level of the fossilbeU;ring beds. Still, even after liberal allowances have been made for inaccuracies due tothis cause, it is obvious that the top of theCitronelle formation must originally have hada steep seaward slope near the coast.

THICKNESS.The thickness of the Citronelle formation isvariable, ranging from a few feet near the landward margin of the formation to several hundred feet near the coast. Exact determination

of the thickness near the coast is difficultbecause it is necessary to rely in part on logsof wells for data, and the information obtainedfrom this source is not entirely satisfactory.The uncertamty is increased by the fact thatthe overlying Pleistocene beds consist of sandsand clays resembling those belonging to theCitronelle formation.

In southern Alabama the Citronelle formation forms a high upland reaching nearly 350feet above sea level, and in the adjacent valleysthe older formations are not exposed in thevalleys where the elevation is less than 100feet. Thus the formation may have a thickness of more than 250 feet. The log of a welldrilled about 12 miles west of Mobile showssand tLD d gravel to a depth of 245 feet and anadditional thickness of pink sand and clayamounting to 95 feet. This gives a minimumthickness of 245 feet and a maximum of 340feet.In Mississippi the thickness of the outliers ofthe Citronelle formation as a rule does notexceed 20 to 30 feet. At Bond a well penetrated 90 feet of sands and gravel belonging tothis formation before encountering the underlying Pascagoula. The De Lamorton well atLaine passed through sand containing sometree trunks at a depth of 141 to 153 feet. Themateria ls beneath this sand are described assand, shale, and gumbo to a depth of 615 feet,where Pascagoula fossils were obtained. Thesematerials included a gravel bed with its baseat 605 feet, and i this is at the base < ~ f theCitronelle the thickness of the formation atthat place may be as great as 452 feet. AtFontainebleau the Pascagoula clay was reachedat a depth of 500 feet, and as the Pleistocenebeds are probably as thick there as at Laine,the thickness of the Citronelle formation atFontainebleau is estimated to be about 350feet.A well at Osyka passed through 150 feet ofsand and gravel belonging to the Citronelleformation, and the formation has an additional thickness of about 50 feet beneath theupland. Farther south, at Hammond, thethickness may amount to several hundredfeet, though the formation is not readily separable from the Pleistocene, and accurate determination of thickness is impossible. NearBrookhaven the Citronelle formation has athickness of about 130 feet, but it thins to afew feet within a distance of 20 to 30 milesfax:ther nerth.Information concerning the thickness of theCitronelle formation in Louisiana is meager.The formation has been observed through avex:tical range of over 150 feet, and may safely

Crider, A. F ., nnd Johnson, L. C., Underground water resourcesof Mlsslsslppl: U. 8. Geol. Survey Wntei'-Supply Pnper 159 p. 1906.


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U. S GEOLOGICAL SURVEY PROFESSIONAL PAPER 98 PL TE XXXVII

A

BSILT UNDERLAIN BY SAND AND GRAVEL OF THE CITRONELLE FORMATION ROCKYSPRINGS MISS.


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'MM


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PLIOCENE ClTRONELLE FORMATION OF GULF COASTAL PLAIN. 179be credited with a minimum thickness of thatamount, and this agrees with the thickness assigned to the formation in the well at Ludington. (See Pl. XLIII, in pocket.) East ofMississippi River the well at Bass penetrated90 feet of yellow clay and 10 feet of sand, likethe materials exposed at Port Hickey. Thisportion of the section is referred to the Pleistocene. It rests on 220 feet of clay that is regarded as the southward extension of the claysfound in the upper part of the Citronelle formation in southern Mississippi, and the chertysands and chert gravels immediately below thisclay may safely be correlate; with the sand andgravel that form the basal part of the Citronelle formation. This would give a thicknessof 400 feet for the formation at Bass, and i f thesubjacent clay is included the thickness is stillgreater. Near the coast the deposits of Pleistocene sand, gravel, and clay can not be separated from the Citronelle formation, and itsthickness has not been determined, though itprobably thins rapidly in that direction.

TOPOGRAPHYGENERAL FEATURES.

The Citronelle formation occupies an areawhere the chamctel"istic topography of the seawal. d margin of the Coastal Plain changes tothe more diversified topography of the Tertiarybeds. The contrast between the varied physiographic expression o this formation and therelatively uneroded terraces of Pleistocene agefarther south is striking and would s ~ r v e as abasis fol mapping the. seaward margin of theformation except where remnants of the Pliocene plains have been preserved. The land-ward margin of the formation could not be soreadily distinguished by this method, becausethe topogmphy of the older formations is similar to that of the Citronelle.

The original surface of the formation was aseries of plains with gentle slopes toward theGulf (see Pl. XXXII , but its present topography is the result of dissection so thoroughthat only a smal l percentage of the area remainslevel, the surlace being mainly reduced toslopes interrupted by terraces, t hough near thecoast a coating of m aterials of more recent ageobscures the original surface. The generalhomogeneity of the deposits that constitute theformation permits the drainage channel& to formunder the influence of the original slope and

41307 1 6 2

independent of the stratigraphy. Each of themajor streams has numerous tributaries, providing a dendritic drainage that is adequate fornearly all the area covered by the formation.r a few places may be seen remnants of theoriginal surface of'the formation, with drainageso deficient that they are partly submergedduring periods of prolonged rainfall. On theseremnants the variations in level are so slightthat they are scarcely noticeable except wherethere are ponds either in the form of smallrounded or elongated depressions ~ v r g r o w nwith cypress and other swamp vegetation,Some of the higher hills, especially those withflat tops, are remnants of the surface of theoldest plain, and by noting their elevations itis in places possible to reconstruct an uplandhaving a maximum elevation of a little morethan 500 feet above sea level. However, someof the bills are mere erosion remnants, theoriginal s u r f c ~ having been wholly removed,though the rate of degradation bas been sufficiently uniform locally to preserve a certainamount of uniformity in elevation over smallareas. This condition is found where a bed ofresistant material, such as gravel, is encountered, and a general seaward inclination resultsbecause such beds have a slight dip in thatdirection. The slope of the original surface ofthe formation remained nearly undisturbedafter the time o deposition, because the formation was mised bodily to its present heightwith only slight change in attitude resultingfrom a gentle warping o its surface. Thepresence of remnants of terraces in the valle.ysthat cross the Pliocene upland shows that theCitronelle formation has been terraced sincethe formation o the highest plain. The lowerplains resemble the uppet one in topography,but they are much na:r.rower and the continuityo their deposits with those beneath the upperplain suggests that the materials of the lowerplains wel. e laid down in the closing stages ofthe Pliocene.

The range in elevation of the exposures belonging to the Citronelle formation amounts toover 450 feet, because the formation crops outin a few places near the level of the Gulf, andits outliers cap some of the high hills in theareas underlain by Oligocene formations. Theaverage difference is about 250 feet, and if theupper surface of the formation is considered,the range is from about 170 feet on the sea-


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180 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY1 1916.ward margin to more than 500 feet on some ofthe higher outliers farther inland. In general,the n ~ r e s e in altitude is comparatively rapid,the surface rising from about 170 feet to approximately 300 feet within a distance of a fewmiles, and upon this portion of the formationnarrow seaward-facing plains are developed.

PLAINS

SUBDIVISIONS AND GENERAL CHARACTER.The surface of the Citronelle formation isdivided into four plains, designated, in descending order, the Brookhaven plain, theSardis plain, the Canton plain, and the Loxleyplain. See Pl. XXXII. his arrangement isalso in the order of age, the Brookhaven plainbeing the oldest and the others being successively younger. Each plain may be divided

into two portions that merge with each otherat their points of contact and may be calledstream terraces and interstream plains. t hasbeen customary to speak of these features asfluviatile and marine, though the propriety ofsuch a designation may well be questioned,because some portions of the interstream plainsare of as distinctly fluviatile origin as anyportion of the stream terraces. The exactposition of the contact between the deposits ofthe two types no doubt varied during theprogress of deposition, the fluviatile sedimentsbeing pushed beyond the limits of the adjacentvalley walls where the material was beingcarried seaward in the form of deltas andretreating into the valleys where estuarieswere developed. For these reasons the termsstream and interstream, although arbitrary,are used instead of the more expressive butless accurate terms fluviatile and marine.

BROOKHAVEN PLAIN.DISTRmUTION.

The Brookhaven plain, named from Brookhaven, Miss., where it is well known, is theoldest and highest of the Pliocene plains andformerly occupied an area larger than thec o m ~ e d area of all the other Pliocene plains.Its interstream portion forms a belt from 10 tomore than 60 miles in width, extending fromwestern Florida across Alabama, Mississippi,and Louisiana into southeastern Texas. Theoriginal width of this plain can not now bedetermined accurately because some portions

of the deposit have been removed by erosion,but it must have been more than 30 mileswide north of the Alabama-Florida boundaryand 60 miles wide in western Mississippi, fromwhich it narrowed rapidly to somewhat morethan 10 miles in southeastern Texas.

The stream terraces of the Brookhaven plainare not well known outside of the MississippiValley, where they have been studied byE. W. Shaw.

ALTITUDE.Because of subsequent erosion and deformation the altitude of the Brookhaven plain asj tnow appears is variable, being from about 350to 420 feet above sea level in western .Alabama,from 420 to more than 520 feet in westernMississippi, about 320 to 380 feet in southernLouisiana, and about 340 feet in southeasternTexas.The landward boundary of the plain is notmarked by any distinct scarp but by outliersof sand and gravel, locally overlain by sandyclay. These outliers are widely distributed, but

their detailed distribution has not been mapped.SLOPE.

The interstream portion of the Brookhavenplain slopes seaward at the rate of about 6inches to the mile, though local variationsare to be found in places where the surface hasbeen eroded. From the highest point on thisplain in western Mississippi, where its surface isas much as 520 feet above sea level, the plainslopes eastward to about 420 feet above sealevel in western Alabama, or at an average ofabout 6 inches to the mile. The slope fromwestern Mississippi to southeastern Texas isfrom 520 to 340 feet, or at an average rate ofabout 5 inches to the mile, in a direction southof west. As the greater portion of this descentappears to be in the eastern portion of Louisiana, the actual rate of slope near MississippiRiver may have been nearly a foot to the mile,though the subsequent removal by erosion ofthis porti on of the plain has made exact determinations impossible. The width of this plainin Alabama is variable but is much less than in:Mississippi, and the slope toward the south isapparently about 2 feet to the mile.

DISSECTION.The interstream portion of the Brookhavenplain has been deeply dissected by streams,

and the original surface is preserved only on the1 Shaw, E. W., unpublished notes.


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U. S GEOLOGICAL SURVEY PROFESSION L PAPER 98 PL TE XXXIX

A IRREGULAR STR TIFIC TION OF SAND AND GRAVEL GRAVELPIT MILES SOUTH OF BROOKHAVEN MISS.

B COARSE FRIABLE CONGLOMERATE MILES SOUTH OF BROOKHAVEN MISS.


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PLIOCENE CITRONELLE FORMATION OF GULF COASTAL PLAIN. 181divides and in isolated hills. These divides aregenerally in the form of even-topped ridges orin some localities flat areas several miles inwidth. A large proportion of this dissectionwas accomplished during the closing stages ofthe Pliocene, and the valleys developed at thattime have not been broadened except at afew localities. This is shown by the fact thatthe Pliocene plains younger than the Brookhaven are represented by stream terracesin the valleys that cross the Brookhaven plain.

LITHOLOGY Al ID SOn.S.The material tmderlying the Brookhavenplain is in general coarser than that underlying

the younger plains. Fine sediments, however, are the rule on the surface of the plain,and the soil derived from them is either loamor sandy loam, with subordinate amounts ofclay. Where the plain has been sufficientlyeroded to uncover the coarse gravels, as, forexample, near Crystal Springs and Hazlehm-st,

~ f i s s . stony or gravelly loams are common.STRATIGRAPmC RELATIONS OF THE DEPOSITS.The strata forming the interstream areas of

the Brookhaven plain rest unconformably onthe underlying formations, of upper Tertiaryage, and merge with the materials beneath thenext younger Sardis) plain. The history ofthe formation and dissection of the Brookhaven plain forms a part of the history of theCitronelle formation, as a whole, which is discussed elsewhere.

SARDIS PLAIN.Below the level of the Brookh.aven is another plain that is well. developed in thevicinity of Sardis, Miss., and is called theSardis plain. The stream terrace of this plain

in the Mississippi Valley north of the latitudeof Vicksburg has been studied by E. W. Shaw.1DISTRmUTION.

Stream terraces. A stream terrace of theSardis plain probably originally occupied anarea in the Mississippi Valley extending fromeastern Adams County, Miss., nearly to centra lLouisiana. This is the area where i t shouldhave been widest, because here it was formed bythe combined action of Mississippi River and

1 Shaw, E. w., unpublished notes.

its largest tributaries in the region. Subsequent erosion has reduced this terrace to aband less than 15 miles wide bordering the .east side of Mississippi River,. The width ofthis terrace on Pearl River, farther east in thesame latitude, was from 15 to 20 miles, and thewidth on both Leaf and Chickasawhay rivers,still fp.rther east, was nearly as great. OnAlabama and Tombigbee rivers in Alabamathis terrace was apparently twice as wide ason the rivers in eastern Mississippi. In Louisiana the stream terraces belonging to theSardis plain have been so much dissected byerosion that it is difficult to estimate theirwidth.

lnterstream areas. The interstream portionof the Sardis plain borders the southern marginof the Brookhaven plain and extends fromcentnd Alabama westward to southeasternTexas. In western Mississippi it has a breadthof about 30 to 35 miles, and it narrows gradually eastward to the Alabama-Floridaboundary, where it is only about 15 miles wide.In western Louisiana and eastern Texas theinterstream areas are still narrower, having anaverage width of about 10 miles.

ALTITUDE .The altitude of the stream terrace ; of theSardis plain is in general greatest near Mississippi River and declines gradually eastward

to Florida and westward to Texas. They havea general slope in the direction of stream flowexcept in western Mississippi, wheFe deformation has altered the OJ;iginal slope. On theeast side of Mississippi River the s t r e ~ terrace of the Sardis plain has an altitude of ~ t b o u t380 feet above sea level east of Vicksburg andnear Woodville and Centerville. Near Natchezit is about 75 feet higher. In the Pearl Rivervalley the plain slopes from about 410 feet afew miles north of Jackson to about 400 feeteast of Brookhaven and to 360 feet near theMississippi-Louisiana boundary. On ChickasawhayRiver and along the streams in Alabamaand west of Mississippi River in Louisiana theslope appears to be steeper, though exactdeterminations of altitude have not been procured.

The altitude of the inner margin of the interstream plain in western Mississippi is about.420 feet, in e r n Alabama about 350 feet,


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182 SHORTER CONTRIDUTIONS TO GENERAL GEOLOGY 1916.and in western Louisiana about 320 feet.The outer margin of the plain has an altitudeof about 350 feet in southwestern Mississippi310 feet in western Alabama and 270 feet inwestern Florida. In western Louisiana andeastern Texas the outer margin has an altitude of about 270 feet.The width of the plain is variable beingfrom 30 to 35 miles in western Mississippi 10to 15 miles near the Alabama-Florida boundary and 8 to 10 miles in eastern Texas. Thonarrowing of the plain both to the east andwest of its place of maximum width is gradual though marked local variations are found.where the terrace has been partly removed byerosion.

SLOPEThe slope of the stream terraces of theSardis plain was probably originally st.eep but

the exact rate can be determined on very fewstreams because there are few reliable determinations of altitude. In western Mississippideformation has given the stream terrace aslope away from the coast from N atchoz toVicksburg at the rate of about 6 inches to themile. From Natchez southward it slopes seaward at the rate of about 9 inches to tho mile.Tho slope of the interstream plains has beenless affected by changes resulting from deformation and if the steep descent from theBrookhaven plain is omitted amounts to about1 foot to the mile in western Mississippibetween 2 and 3 feet to the mile near thoAlabama-Florida boundary and about 3 feetto the mile in western Louisiana and easternTexas.

DISSECTIONThe Sardis plain has undergone somewhatless erosion than the Brookhaven plainthough locally the stream terraces have beenentirely removed. In generol the tmTaceshave been reduced in area by the meanderingof the major streams and have been separatedinto small isolated plains by the erosion of

tributary streams.The interstream plains are extensivelyeroded near the principal streams but onsome of the divides there are still remnants ofthe original surface from 5 to more than 10miles wide. Most of these remnants areClossed by shallow channels of wet-weatherstreams that provide adequate drainage but

in a few places as for example near Atmoreand Georgetown Ala. the surface of the plainis so flat that shallow ponds form during wetweather. Even near the Mississippi Valley atCenterville and Woodville some portions of thisplain are well preserved being trenched by veryshallow valleys but the conditions here arefavorable for the preservation of the originalsurface because during the development of thedrainage of southwes tern Mississippi the streamencountered lodges of hard rock : ;imilar to thatat Fort Adams at depths of 40 to 120 feet belowthe surface of the Sardis plain and this hardrock retarded the erosion and resulted in thepartial preservation of the original Pliocenesurface where it would otherwise have beendestroyed.

LITHOLOGY AND BOILS.The materials beneath the Sardis plain are

essentially the same as those beneath theBrookhaven plain consisting of coarse crossbedded sands and gravels near the base of thedeposits overlain by :finer sands and sandyclays. In general the gravels are finer than inthe deposits of the Brookhaven plain thoughnear Mississippi River coarse gravels are foundbeneath the landward margin of the Sardisplain. Gravels are not abundant except in thearea where sediments were supplied by Mississippi River. The upper member of theplain is a sandy clay of red or yellow colorand where the surface of the underlyingTertiary formations was high as at Clintonand Woodville Miss . this material restsdirectly upon them the basal sands .andgravels being absent.The soils derived from the weathering of thesurface materials of the Sardis plain are eitherloam or sandy loam though in a few placeswhere the upper member has been removed byerosion gravelly loam is found.

STRATIGRAPmC RELATIONS OF THE DEPOSITSNear the north edge of the Sardis plain thestream-terrace deposits rest unconformably on

the deposits of the Brookhaven plain. Farthernorth remnants of the Brookhaven plain areabove the level of the base of the Sardis plainand the deposits of the la t ter plain rest unconformably on older Tertiary formations.The interstream deposits of the Sardis plainare continuous with those of the Brookhavenplain and were deposited when the sedimenta-


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U. S GEOLOGICAL SURVEY PROFESSIONAL PAPER 98 PLATE XLII

- . -- - 1 '

. p

' i { r -

_; ; -A CLAY FRAGMENTS DERIVED FROM UNDERCUTTING BY A SMALL STREAM, SULPHUR CREEK,LIVE OAK COUNTY TEX.

B A CLOSE VIEW OF THE FRAGMENTS SHOWN IN PLATE XLII 4Photographs by T W. Vaughan.


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PLIOCENE CITRONELLE FORMATION OF GULF COASTAL PLAIN. 183tion was pushed farther seaward because of aslight general change in the altitude of the land.The change was apparently gradual, and thezone of rapid sedimentation was carried progressively seaward with continuous depositionalong the margin of the older plain. This mayhave resulted in partial unconformity betweenthe deposits of the two plains, though probablythe unconformity was not greater than atmany other places within the deposits beneaththe plains.

ANTON PLAIN.The Canton plain borders the seaward andstreamward margins of the Sardis plain andoriginally formed a continuous belt at aslightly lower level than the Sardis plain.The name is applied by Shaw 1 to a stream ter-race of this plain at Canton, :Miss. and isextended to other stream terraces that can

be correlated with that one and to the inter-stream plains that merge with the stream ter-races at the seaward ends of the valley.

DISTRmUTION.Stream terraces. Although the stream ter-races of the Canton plain are better preserved

than those of the older plains, they are represented only by mere fragments of the originaldeposits. The terrace in the Mississippi Valley doubtless had a width of many miles, butremnants in western Mississippi are 10 to 12miles from the present position of the river.This terrace has probably been entirely removed by erosion in the area between Mississippi and OJ.Iachita rivers and it has not beenrecognized between Ouachita. and Red rivers.The stream terraces of the Canton plain havebeennoted at a number of places in the valley ofthe rivers of eastern Mississippi andwestern Alabama, but the observations have not been sufficiently systemat ic to warrant discussing the distribution of the terraces in detail. In westernLouisiana and southeastern Texas still lessdetailed information has been obtoined concerning their distribution, though a poorlypreserved terrace at an elevation of 385 feet onthe east side of Sabine River between Neameand Evans Ferry is tentatively correlated withthe Canton plain.Interstream areas. The interstream portionsof the Canton plain are well developed in

1 Shaw, E. W., unpublished notes.

Louisiana east of :Mississippi River and extendacross southern Mississippi and Alaboma intowestern Florida. In Louisiana the interstreainareas of this plain extend westward from apoint a few miles west of Woodworth, passingnorth of De Ridder and a.cross the State lineinto eastern Texas.The original width of the interstream areasof the Canton plain in southern Mississippi andthe adjacent portions of Louisiana was about12 to J5 miles, and they narrow eastward toBaldwin County, near the Alabama-Floridaboundary line, where the width is approximately 6 to 8 miles. In western Louisiana andeastern Texas the interstream areas are aboutas wide as those in Alabama.

ALTITUDE.The st ream terraces of the Canton plain arelower than those belonging to the older plains,though considerable local variation is found,the al titude depending on the size of the stream.As a rule, the stream terraces of all the plains

naturally rise to higher levels and away fromthe coast have steeper gradients on the smallstreams than on the large. An exception sfound in the Mississippi Valley, where the ter-race in the vicinity of Newmans Grove, east ofVicksburg, rises to only about 300 feet abovesea level and the altitude increases slightlytoward the coast, being nearly 380 feet east ofNatchez. Thence tho terrace slopes southward to about 350 feet near the southern boundary of Mississippi. Both inner and outer mar-gins of the stream terraces were originallymarked by scarps, but in many places tliesefeatures have been destroyl: d by erosion.The inters tream areas in southern Mississippi range in altitude from about 350 feet onthe landward margin to about 280 feet on theseaward margin. The altitude decreases gradun.lly from western :Mississippi to western Alabama, where the landward margin is about 310feet above sea level and the seaward margin 250feet and there is a still further decrease in.western Florida, where the altitude of the landward margin is about 270 feet and that of theseaward margin about 220 feet. A similar butslightly more rapid descent west of MississippiRiver brings the surface of the plain to aboutthe some altitude in western Louisiana andeastern Texas that it has near the westernboundary of Florida.


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184 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1916.The margins of the interstream plain are notbounded by distinct scarps but they are marked

by an increase in gradient. At many placesthe margins have been destroyed by erosion,because the changes in gradient have furnishedfavorable places for the development of lines ofdrainage. SLOPE

The slope of the stream terraces of the Cantonplain is in _general seaward, though the rate isnot easily determinable because of the lack ofdetailed information. The terrace on the eastside of Mississippi River from Newmans Groveto the latitude of Natchez forms an exceptionbecause of deformation subsequent to deposition. The slope of this terrace is reversed,being northeastward, and it amounts in theaggregate to 30 feet in 6 miles, or about 6inches to the mile. From the latitude ofNatchez to the northern boundary of Louisiana the slope of this terrace is southward atthe rate of about 9 inches to the mile.

The in erstream portions of the Can onplain slope more steeply than those of the Sardis and Brookhaven plains, the average slopebeing from 2 to 3 feet to the mile, i the steepened landward margin is disregarded. Localvariations are common, however, and in manyplaces the original slope has been destroyed byer9swn.DISSECTION

Both stream and interstream portions of theCanton plain have been greatly dissected, andthe remnants now appear as isolated areashaving nearly level surfaces or as even-toppeddivides between the major streams. n placesthese divides are several miles in width, butthe undrained areas are smaller than those onthe Sardis plain. Tllis is attributed to the factthat the Canton plain was narrow and theoriginal surface had a steeper slope than theSardis plain.

LITHOLOGY AND SOILSThe lithology of the Canton plain is, in general, like that of the Citronelle formation as awhole. Gravel is less abundant in the depositsbeneath this plain than in those beneath theSardis and Brookhaven plains, though it isreported in some wells and is exposed in a fewdeep valleys. The surface of the plain is composed of fine sandy clay and silt similar to thematerial forming the surface of the older plainsand the soils are chiefly loam and sandy loams.

STRATIGRAPmC RELATIONS OF THE DEPOSITSThe materials underlying the Canton plaindoubtless have the same stratigraphic relationsas those beneath the Sardis plain, but theexposures are not deep enough to permit sat-isfactory observations.

LOXLEY PLAINThe lowest of the Pliocene plains that areunderlain by deposits of the Citronelle formation is well preserved in the vicinity of Loxley,Baldwin County, Ala., and is therefore namedthe Loxley plain.

DISTRIBUTIONStream terraces. The Loxley plain has awidth of only a few miles where it is represented by remnants of stream terraces. n theMississippi Valley the original extent of the

stream terrace belonging to this plain can notbe easily determined, though in the vicinity ofSicily Island, where the terrace is the resultof erosion and deposition by 1\fississippi andOuachita rivers, it may have been 3 to 4miles wide. The average width in the Mississippi Valley between Sicily Island and Vicksburg was probably not much greater than 25miles. The original width was apparently lessthan 1 miles in the valleys of the rivers ineastern Mississippi and western Alabama andFlorida, except in the Alabama and Tombigheedrainage basins, where this terrace is morethan 25 miles wide at its southern margin.

Interstream areas. The interstream areas ofthe Loxley plain occupy a belt between theCanton plain and the oldest Pleistocene terrace.They vary much less in width than the olderterraces of the Citronelle formation, being aswide in western Florida and Alabama as insouthern Mississippi and eastern Louisiana.The average width of these areas is about 8miles and the maximum is about 15 miles. Onthe eroded outer margin of the plain the un-derlying deposits have been exposed in placesby the removal of the younger beds, and i fthe maximum width should be extended to include these exposures it would amount toslightly more than 30 miles. However, thesemore or less isolated exposures represent surfaces eroded during the Pleistocene epoch.The width of the interstream portions of theLoxley plain in southwestern Louisiana andsoutheastern Texas is about 8 miles. Thisplain is well developed in an area that extends


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PLIOCENE CITRONELLE FORMATION OF GULF COASTAL PLAIN 185from Forest ill to De Ridder La. and thencewestward passing a few miles south of NewtonTex.

ALTITUDEThe stream terraces of the Loxley plain vary

in altitude being higher on the small streamsthan on the large and they increase in altitudeaway from the coast. This generalizationhowever does not apply to the terraces of therivers in western Mississippi where the altitudehas been changed by deformation. East ofVicksburg the terrace in the :Mississippi Valleyis about 280 feet above sea level and east ofNatchez the altitude is nearly 40 feet higher.From Natchez southward to the Louisianaboundary it declines to about 270 feet abovesea level. The terrace on Pearl River has analtitude ranging from about 230 feet north-west of McNeil to about 300 feet near Monticello and 320 feet near Jackson. The altitudesobserved on Leaf River range from about 250feet near Hattiesburg to about 200 feet nearMerrill and those in western Alabama on thewest side of the Tombigbee from about 250feet east of Seaboard to about 200 feet north-west of Mobile. On Conecuh River they rangefrom 200 feet near the Alabama-Florida boundary to about 230 feet east of Kirkland. Terraces of this age were recognized as far east asYellow River in northern Florida where thealtitudes o

The Citronelle Formation (Pliocene) and Its Flora (Matson and Berry 1916)

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