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AGI DATA SHEET 10.1Criteria 'or Determining Top and Bottom O, Beds.by Siemon W. Muller, Stanlord UnlversityPHYSICALTraclng 01 beds or recognltion 01 a known normal sequence. The top and bottom 01 vertical or sleeply inclined beds may be delermined by Iracing to or correlating with theknown normal (upright) sequence (A) in the area where these strata are either only gentIy lolded (at B) or not at all delormed (at A).

Scouring or channeling. Scouring or channeling 01 strata with subsequent filling 01the channels will truncate the underlyingstrata. The concave sides 01 channels willgenerally point upward. The edges 01 bedstruncated by erosion (unconlormity) aretoward the original topoConglomerates. Basal and intralormationalconglomerates may contain pebbles andboulders which can be recognized as having been derived Irom the beds below.

Solution surfaces. Irregular solution surlaces may lorm along the top 01 limestonebeds or other relatively soluble rocks.

Graded bedding. In sediments with gradedbedding, the texture will grade lrom coarsebelow to line above. This olten does nothold true in currentbedded deposits.Graded bedding may be present undervarious current, including turbidity current,conditions. However, it should be borne inmind that under these conditions a grada

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ion in texture may be Irom coarse to line as well as Irom line to coarse. As origlnally de-r ~ n t ~ ~ ~ o ~ ~ ~ ~ ~ , ? r ; e ~ ~ I ~ n ~ e ~ ~ ~ u g ~ ~ s ~ g ~ ~ a ~ : t r ; ~ ~ i ; t W , d ~ t ~ : : ' ~ : ? : r ~ ! ~ ~ ~ ~ ~ ~ ~ ~ ~ ; ~ o ~ ~ ~ ~ rent bedding" which is produced by "resorting and redistribution 01 material."Crossbeddlng.ln crossbedding one set 01layers or laminae are truncated by overlying layers, but away Irom this contact thelayers sweep along a concave curve to aconlormable contact with the underlyinglayers. The concave si de 01 crossbeddinggenerally points toward the original upperside. Individual crossbedded laminae mayshowa downward gradatlon in textureIrom coarse to line.

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AGI DATA SHEET 10.2Mud cracks. Mud cracks generallydecrease in width downward and may beIlIled with material which composes theoverlying beds.

Rlpple marks. In symmetrical ripple marksthe crests (tops) are sharper than thetroughs. Occasionally minor crests may bepresent in troughs.

Sole marks. Small, wave or tongue-likepenetrations 01 a coarse clastlc materialIrom aboye into a liner clastic materialbelow along minor surlace irregularities ona bedding plane. Some 01 these marks areexplained by delormation or Ilow 01 unconsolidated (and diluted or watersaturated)sediments by gravity' sliding along aprimary incline 01 a bedding plane andpossibly triggered by earthquakes. Theseshale), but are rarely (il ever?) lormed at the contact 01 clay overlying sand.leatures tend to develop along a contact 01 sand (now sandstone) overlying a clay (nowFracture cleavage. In the upright section 01lolded rocks, the Iracture cleavage isgenerally steeper than the beddlng (a), butwhen the beds are overturned the reverseis true (b) .

Curved fracture cleavage. Curved Iracturecleavage may be observed in delormedbeds where individual beds have a perceptibie gradation 01 texture Irom coarsebelow to line aboye (graded bedding). Onthe side with coarser texture (bottom) theangle between the Iracture and the bedding will be larger or more obtuse than onthe side with the liner texture (top or lace01 the bed). The convex side 01 the curovature 01 the Iracture cleavage will bow outtoward the original top 01 the bed.

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Pebble dents. When the matrix is bulgedaround an imbedded pebble on one sideonly, this side is the original bottom (a). Apparent denting 01 laminated sedimentsaboye and below an embedded pebble mayresult lrom the subsequent compaction orcompression 01 sediments (b) . bReferenceShroek, R.R. , 1948 Sequence in Layered Rocks. MeGraw-Hill Baak Ca, Ine ,New Ya rk-Taranta-Landan, 507p ., 397 figs.

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AGI DATA SHEET 10.3PALEONTOLOGICALBryozoa. Shells 01 invertebrate organismsor other solid objects Iying with theirlonger axes in the plana 01 stratilicationmay be encrusted by bryozoans on the upper slde.Brachlopods. Crania-like brachiopods occur cemented on other shells or on sub-trate with their convexo conical val ves ' . pointing u p w a r d

Pelecypods. a)Shellsol Schizothaerus(fertiary) and Pholadomya (Mesozoic) are notuncommonly lound in their original buriedposition, "standing on end" with theirposterior (siphonal) end pointlng upward.

b) Rudists and rudist-like aberrant pelecypods are occasionally lound in theiroriginal upright position with the Iree valveat the topo

c) Disjointed or spread-out open val ves01 convex pelecypods are generallybrought to rest by wave action or by currents with their convex side up. Exceptionsto this rule are not uncommon. Observations based on a single shell or only a lewshells are not completely reliable.d) Inequivalved pelecypods il buried

alive will have their more convex valvepOint downward.e) Shells 01 marine organisms or othersolid objects on the substrate may be encrusted on their Iree, uppar surlace withcemented lorms such as oysters, barnacles, or other sessile organisms.

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Worm Iralls. Worm trails and trails 01 problematical organisms generally leavegrooves on the face of the bed. Wormsgrubbing near the surlace 01 the subslralewill leave raised, flaltened ridges wilh abarely perceplible groove in Ihe middle,presumably due lo the "caving" 01 Ihegrubblng "Iunnel" afler the organismpassed Ihrough Ihe lunnel.

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AGI DATA SHEET 10.4

PHYSICAL CRITERIA FOR IGNEOUS ROCKSLaya flows. Tops 01 interbedded lava Ilowsgenerally are more vesicular. In lavaswhich contaln branching tubules the direclhoen ~ ! r ~ ~ ~ r ; ~ i ~ ~ i j u ~ c ~ ~ ~ t i ~ e ! ' g ! ~ ~ ~ ~ ~ original top 01 the lava Ilow.A more or less perceptible contact metamorphlsm (brickrOO burned soil) may bepresent In the rocks below the bottom 01Ihe lava flow, but no metamorphism and adepositional contact will mark the top 01the lava Ilow.Crestsof wrinkles The crests or tops 01wrinkles on the surfaces 01 lava are generoally smoother and more broadly curvOOthan the spaces between the wrinkles.f ~ : f u ~ e ~ a ~ ~ ~ c ~ I : ~ I ~ ~ ~ ~ ; v g l \ ~ ~ e f l ~ ~ ~ w i n g al Upper surfaces 01 pllows are moderately or gently convex and relativelysmooth.b) Bottoms 01 pillows commonly havecusps pointing down Into Ihe inlerspacesbetween the underlying pillows.cl Pillows are generally more vesicularnear the top than near the bottom.

d) Smallscale columnar joinl ing may bemore or less well deyeloped around the up-per periphery. Columnar jolnting Is poorlydeveloped or is altogether absent on thebottom side 01 the pillow.e) Pillows exlruded upon unconsolidated sediments are likely lo ruffle andcrumple these underlying sediments andmay have enough heat to bake these conlorted sediments.1) The top 01 the plllow lava Ilow generalIy shows no heal ellect on the overlyingsOOlments. The "pillowy" surlace 01 theflow is gradually ellaced or leveled by the

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Gastropods. In vermetids and similarwormgaslropods Ihe linal leeding tube (aswell as scars 01 early stages) generallypoints upward duri ng Ihe lile 01 the animal.

Corals. Solitary corals cemented to thesubstrate are normally oriented with theirnarrow end down, widening (and branchi n ' B ~ ~ ~ i ~ { d o r a l s may be preserved in Iheiroriginal position wlth their calyxes point: ~ ~ g ~ P ~ ~ ; ~ d : ' l , o ~ ~ ~ ~ ~ i t ~ 9 . e ~ ? l c ~ ~ ~ f ~ r ; ; ! ~ cling to Ihe side or even the bottom 01 aprotruding edge and haye Iheir calyxespOint downward.Colonial corals (and calcareous algae) 01biscuit or bun shape normally grow andbecome buried wlth their convex side up.Echlnolds. Sea urchins when lound inlarge numbers are commonly oriented withIlat ventral (oral) side down and convex dorosal side up.

overlylng sediments, which tend to 1111 the depressed area more rapidly.