of being lost. There are considerable differences in this tendency both betweenethnic groups and within those groups. Traditional culture is sometimes rejectedin its entirety by young people as a reacti.on against superstitition etc, ratherthan selectively extracting that which is worth keeping and worth examiningfurther. The local climates of Kenya are imperfectly understood and the pre­servation of local weather lore would therefore have more than a historicalvalue; it could be an important means of identifying significant local variationsin climate.

DIRT WAVES ON SNOW SURFACES

By R. CLARK

Portstewart, Northern Ireland

INTRODUCTION

THE appearance of ordered, repetitive patterns of dirt distribution onablating snow surfaces has frequently been described (Ashwell and Hannell

1966; Ball 1954; Chuter 1959; Richardson and Harper 1957). Dirt polygons andcones are among these features. The purpose of the present note is to describe awave-form dirt-on-snow pattern from Deception Island, South Shetlands(62°59'S, 60034'W).

During 17-18 July 1955 a deep depression passed eastwards across theScotia Sea. Temperatures above o°C are not frequent during mid-winter in theSouth Shetlands - Antarctic Peninsula region, but in the warm sector of thisdepression such temperatures accompanied by drizzle and sleet were experiencedclose to sea level. The saturated surfaces of snowfields near the coast then frozeto a crust. The passage of the cold front was associated with gale force north­west winds gusting at sea level to about 40 m s -1 and generally above 20 m s -1 ,

with pressure rises of 16 mb in one hour (30 mb in nine hours).At this time a small area free of snow, dry and kept unfrozen by volcanic

heat existed at the head of Whalers Bay. Its surface was mainly of loose blackvolcanic ash in the fine sand-small gravel size range. Morphologically this surfacewas a combination of beach, raised beach, and outwash flats. It extended about50 m across the run of the wind and was about 100 m long, lying between theshore and a steep, snow-clad, faulted slope of an extinct volcanic hill; it was theonly area of bare unfrozen ground in the vicinity.

FIG. 1. Opposite Ablating snow surface showing pitting and irregular small dirt cones. Thecut by the ice-axe shows the debris to be entirely superficial. Areas of new snow are apparent,and the recrystallized nature of the old snow may be seen in fragments near the ice-axePhotograph by R._Clark

458

l'holograph by U. W ..·/tilinson

Pileus. ;\lid-morning, 4 September 1969 over Nebraska

459

FIG. 2. General view of two groups of dirt waves. Waves are partially concealed by snowfrom two separate falls. Note in the foreground the alignment of dirt and recent snow normalto the trend of wave crests. Note alsu the tone distinction between drier and damper ash

Photographs by R. Clark

Sand and grit were carried up to I km from this source and noticeabledeposits were traced to a height of 250 m, though most was deposited below 30 m.The area of deposition formed a narrow fan extending south-east from its apexat the bare ground by the shore. The heaviest deposition was within 500 m ofthe apex. It was estimated that ash was spread over an area of about 23 000 m2

with a mean accumulation of 1300 gm m -2. Something of the order of 30 metrictons of dirt was derived from about 3500 m2 of bare ground during one night.

WAVE FORMS

The strong winds also eroded the ice crust of the snowfield in a regularpattern of troughs transverse to the wind direction. Types of troughs weredifferentiated by size and by the position in the troughs where the mixture ofdirt and coarse ice fragments lodged. Minor transverse troughs were 20-30 mmdeep and about 80 mm wide. The finest material accumulated on the downwindsides of these troughs. Trough centres were about 150-160 mm apart. Imposedon this quite regular pattern were larger, irregularly-spaced, transverse troughsabout 80 mm deep and 600 mm wide. In these a mixture of coarse sand and gritand ice fragments was banked against the upwind slopes of the troughs in a bandabout 350-300 mm wide. The intervening uneroded strips of ice-crust surfaceremained quite clean.

Thus the dirt was distributed under the influence of the trough pattern,heaviest deposition being in the larger troughs, light deposition in the minortroughs, and intervening areas being almost clean. Away from the fan of dirtdeposition the snow surface was diversified by clean newly-eroded troughs butthe area over which these occurred was not recorded. It is clear that the troughswere cut during the 17-18 July depression and as the area from which they wererecorded was open to the wind and relatively smooth no important topographiceffects were likely. Measurements were not made at the time of spacings betweenmajor troughs.

In the latter part of the winter further snowfalls concealed this unevendirt cover. The spring and early summer saw an alternation of snowfalls andlight scatterings of sand derived from newly-exposed bare ground. In the areaof the July ash deposition rapid summer ablation lowered an increasingly dirtysnow surface to the level of the July ash spread. By this time the snow wascoarseand open-textured. Pocking and miniature dirt cones already characterisedthe snow surface and the original appearance of the July ash-drop was notrevealed when its level was reached. Stripes of thicker, more continuous ashdeposits, most of them trending north-east, may have marked the lines of majortrough deposits or resulted from the amalgamation of the more closely-spaced

FIG. 3. Opposite. Closer view of an area on right of Fig. 2. Collapse of flanks is seen tohave taken place. Re-exposed snow is visible on the nearest ridge flank. The cut showsthe dirt to be superficial. Far end of foreground ridge is completely lowered

major troughs but this \Vas not une<[uivocably demonstrated. These stripes(Fig. r) became the loci for wave crests on a corrugated snow surface. Usually thecrests \Vere slightly sinuous and almost parallel though there were also link"between crests. There were also small areas with waves trending obliquely to themain series. The dewlopment of the ridges was complicated by further falls ofsnow being concentrated between the ridges and reducing the relief (Fig. 2).

However, the general development was for the continuous debris cover to delayablation and for light, discontinuous cover to hasten ahlation. Klay and Orheim(r969) noted of this \'olcanic ash on Deception Island that its' low thermalconducti\'ity has an insulating effect on underlying ice', and elsewhere on theisland thicknesses of about 200 mm effectivel~' inhibited ablation through thesummer. Differential ablation between thickest and lightest covers increasedsurface relief and hence the steepness of ridge flanks. Instability of steepenedflanks led to collapses of debris cover, exposure of snow, accelerated ablation,and ridge destruction. Ridge crests with dry surface debris were persistent untillower flanks with damper dirt covers collapsed (Fig. 3). After sufficient lateralattack had effected the l'Ollapse of the narrowed crest, the ridge tended to reformat a lower level. Ewntually the dirt was lowered to the underlying debris surfaceor, where it lay on permanent snow, was removed bv sheet wash over refrozensnow crusts or over congealed hard neve. Only in one or two places did the rlirtsurvive on a snO\\' surface to the end of the summer.

Though it was not established that dirt-ridges developed on the sitesof earlier major trough deposition, there are grounds for presuming this tube the case. The din·ction of ridges and the distances hetween them had 11()

apparent relationship to conditions during ablation. On the other hand theirlocation and alignment could he related to the winter gale. Distances betweencrests varied from zero at conjunctions to about 4 m, were mostly betweenr·8 m and J'() m, and mainl~' about 2'4 m. Hence it is possible that these figuresrepresent the spacings of major troughs cut in the ice-crust, and bear somerelationship to the windspeeds of the cold sector of the depression. Similarlvthe spacing of minor troughs and the orientation of both types across the flu,,'of wind indicates transverse turbulence apparentl~' operating at two scales.

Transverse erosional forms in snow have been discussed in general term~

by Seligman (1C)1>2), but the dual occurrence of erosion of snow and depositionof dirt followed by inn'rsiun of topography during ablation does not appear tube well-known.

HEFElU::;CE"

ASHWELL, 1. Y. andHAN:'EI.. F. C.

BALL. F. I":.CHUTEH, 1. H.KLAY, J. R. and

ORHEIM, O.}{ICHARDSO:-, \\'. E.RICHARDSO:-, \\". F. and

HARPEl<, H. n. \1.SELIG :>1.-\:-, (;.

195-\19591969

f()5-\

H)57

Experiments on a snow-patch in the mountains ofSweLlen. J. Gla.ciology, 43 (61, pp. 135-1H

Dirt polygons on snow. Wealhl'l', 9 (10), pp. 322-323Snow polygons. IbId., 14 (-\l, p. 139Glaciology and glacial geology on Deception Island.

.hllarclic]. of the Uuited Stales, 4 (-\), pp. I25-121>

Ilirt polygons. Weather, 9 (-\), pp. 117-121.\hlatioll polygons on SIlOW - further observations and

theories. J. Gla.ciolog.\', 3 (21), pp. 25-27S"ow struclure aud ski .fields. 1962 reprint, Brussels