Eolian deposits of the ice-free Victoria Valley, Southern Victoria Land, Antarctica

Transcription

1 New Zealand Journal of Geology and Geophysics ISSN: (Print) (Online) Journal homepage: Eolian deposits of the ice-free Victoria Valley, Southern Victoria Land, Antarctica M. J. Selby, R. B. Rains & R. W. P. Palmer To cite this article: M. J. Selby, R. B. Rains & R. W. P. Palmer (1974) Eolian deposits of the ice-free Victoria Valley, Southern Victoria Land, Antarctica, New Zealand Journal of Geology and Geophysics, 17:3, , DOI: / To link to this article: Published online: 12 Sep Submit your article to this journal Article views: 119 View related articles Citing articles: 51 View citing articles Full Terms & Conditions of access and use can be found at

2 No EOLIAN DEPOSITS OF THE ICE-FREE VICTORIA VALLEY, SOUTHERN VICTORIA LAND, ANTARCTICA M.]. SELBY*, R. B. RAINst, and R. W. P. PALMER* *Department of Earth Sciences, University of Waikato, Hamilton, New Zealand tdepartment of Geography, University of Auckland, New Zealand (Received 30 May 1973) ABSTRACT The most widespread eolian deposits of the Victoria Land dry valleys, Antarctica, are pebble ripples which appear to b" of larger size than those r,eported from other continents. The crests of the ripples are composed of grains with a greater diameter than the grains of the flanks. The mean ripple index of 23 is higher than that of large ripples reported from outside Antarctica, and there does not seem to be a simple relationship between index and grain size. The high winds and high density cold air of Antarctica appear capable of moving the largest pebbles which have a diameter of 19 mm. Sand forming transverse and whaleback dunes in the Victoria Valley is derived from the outwash from the Victoria Lower Glacier. The sand in the dunes is interbedded with snow, and moisture from this allows the dunes to be permafrosted except for a thin surface layer of dry mobile sand. The mobile sands of the transverse dunes are seasonally reversing. The sands of the dunes are well sorted and there is no selectiv5 deposition of a particular grain size in any area of the dunes. A characteristically granule-size lag deposit occurs on some interdune areas, and on the wha1ehack dunes, where it controls the size of the ripples. The granules may be in the critical size range for stabilising ripples which move only during infrequent very high velocity winds. INTRODUCTION The ice-free valleys of southern Victoria Land occupy an area of approximately 4000 km 2 and form the largest ice-free area of Antarctica. In the Koettlitz, Taylor, Wright, and Victoria Valleys, which with their tributaries are collectively known as the McMurdo "oasis" or "dry valleys", are several areas of eolian deposits. Elongated pebble ridges have been briefly mentioned by Nichols (1966, p. 34) and McCraw (1967, p. 413.) Sand dunes and sand sheets in the Victoria Valley have been mentioned by Webb 8< McKelvey (1959, p. 127), Alle,1 & Gibson (1962, p. 240), Cailleux (1968), and Lindsay (1973). The purpose of this paper is to describe the form, sediments, and structure of the eolian deposits more completely. Up-valley and down-valley winds blow through the dry valleys with easterly winds from the Ross Sea dominating the eastern ends of the valleys, and westerly winds from the Polar Plateau dominating the western ends. The central parts of the valleys experience the alternating winds most fully. In the Victoria Valley, from which little climate data are available, steady N.z. Journal of Geology and Geophysics 17 (3):

3 544 N.z. JOURNAL OF GEOLOGY AND GEOPHYSICS VOL. 17 winds of up to 10 mls are common in summer and gusts of up to 20 mls have been noted (Calkin 1971, p. 377). At Vanda Station in the Wright Valley winds exceeded 17 mls on 55 days in 1970, and a maximum gust of 41 mls was recorded on 6 August in that year (Thompson et ai. 1971, p. 248). Strong winds are thus common in the dry valleys. Within the dry valleys a considerable supply of sediments of pebble size and smaller is present, both in the extensive glacial moraines and fluvioglacial outwash materials which cover the valley floors, and also as the products of physical' and chemical weathering of bedrock and moraine.. The most commonly occurring eolian depositional features are ripples in pebbles, granules, and sand. Ripples occur in areas varying from a few square metres to several hundred square metres, wherever there are extensive deposits of material sufficiently fine-grained to be moved by the wind. Sand deposits occur in all of the valleys, but the only areas where major eolian sand depositional forms occur are in the Victoria Valley between Lake Vida and the Victoria Lower Glacier (Fig. 1). In the Lower Victoria Valley easterly winds dominate the summer air flow and move sand from the Victoria Lower Glacier outwash. Sediment is also removed from the same area by meltwater streams which flow intermittently in December and January each year. In the remaining ten months of the year wind can remove sediment from the broad, braided, stream channels. Material picked up by the wind has accumulated into a group of transverse dunes on the north side of the valley near the Packard Glacier or in a set of whaleback dunes closer to Lake Vida. To study the transporting and possible sorting processes of the wind, sediment samples were collected from the Victoria Lower Glacier outwash, the transverse dunes, and whaleback dunes. In the following descriptions grain size is given by the phi (</» scale (Krumbein 1934) and Wentworth's (1922) size classes. The methods of analysis and statistical measures of grain size are those of Folk (1968). PEBBLE RIPPLES Elongated ripples of pebbles occur on moraines and fluvio-glacial outwash in the Koettlitz, Taylor, Wright, and Victoria Valleys. The greatest number and best developed ripples were found on an area of glacial outwash in the Upper Victoria Valley west of Lake Vida (Fig. 2). The outwash here has been deposited between moraine ridges and spread out as flat sheets in several zones which have areas of up to 1 km 2 The sampled ripples are subparallel in plan and slightly asymmetrical in profile, resembling transverse dunes; they have a mean wave length of 3'5 m with a range from 2'4 to 4 6 m, and a mean wave height of 0'15 m with a range from 0'10 to 0'25 m. Sharp (1963), working in the Kelso Dunes of the Mojave Desert, found granule ripples to have. a ripple index* *Ripple index = the ratio of wave length to wave height (Sharp 1963).

4 No.3 SELBY et al. - EOLIAN DEPOSITS, ANTARCTICA 545 FIG. l-the Lower Victoria Valley seen from the east. U.S. Nat'Y photo range of 12 to 20 with a mean of 15. The Victoria Valley ripples have a mean index of 23, which is possibly the result of higher wind velocities. Sharp believed that the index varies inversely with the grain size, but as the Antarctic ripples have a modal grain size in the pebble grade this does not appear to be true. A section through one ripple (Fig. 3) shows that the pebbles form a lag deposit overlying fluvio-glacial sediments. The pebble ridge is a wedge about 80 mm thick which thins laterally, but is thicker on the advancing edge than on the windward slope. Beneath this is a horizontal, 60 mm thick, layer of mixed sand and gravel overlying laminae, with lenticular bedding, whose particles range in size from fine sand to gravel. In contrast

5 546 N.2. JOURNAL OF GEOLOGY AND GEOPHYSICS VOL. 17 FIG. 2-Pebble ripples west of Lake Vida. Scale is given by the 4-m-long surveying staff which straddles a polygon contraction crack. FIG. 3-Trench through a pebble ripple showing the accumulation of pebbles forming the ripple, the lag deposit of the inter-ripple area, and the underlying fluvio-glacial deposits.

6 No.3 SELBY et at. - EOLIAN DEPOSITS, ANTARCTICA 547 with the ripples which have a pebble accumulation up to 15 particles thick, the inter-ripple areas have a lag deposit which is usually only one pebble thick. Samples from the upper 50 mm of the deposits of six ripples were taken for particle size and shape analysis. Four samples were col1ected from each of the crest, lee, windward, and inter-ripple areas. The ranges of particle sizes are indicated in Fig. 4. The particles of the crests are 950/0 pebbles (by weight) and only 2% are in the sand fraction. The lee sides of the ripples have about 550/0 of their particles in the pebble grade, 15% are granules, and 300/0 are in the sand grade. The grain-size distributions of the lee slopes of the ripples are, like the crests, coarsely skewed, but the mean grain size of the crests is greater than that of the lee slopes. The windward sides of the ripples are not significantly different in particle sizes from either the inter-ripple areas or the leeward sides. Samples from the inter-ripple zones and windward slopes have a bimodal distribution because they contain a lag deposit composed predominantly of pebbles, and an underlying sand fraction with a mode in the medium sand grade. A study of the roundness of the coarse particles using the Powers (1953) scale shows that the differences between the particles of the crests and interripple areas are small. The accumulation of pebbles in a ripple crest does not, therefore, appear to be attributable to a variation in the roundness of the particles. The Victoria Valley ripples appear to have a greater wave length, height, and modal grain size than those described from elsewhere (Sharp 1963; Allen 1970). Wind can support in suspension grains of a maximum diameter of 3 mm (-1'5</» (Kukal 1971, p. 116); larger grains are moved by traction. Although a wind of given strength cannot, by itself, move grains larger than a definite size, Bagnold (1941, p. 154) has shown that a saltating grain can, by impact, move grains exceeding six times the size of those composing the saltation load. Large granules of approximately 3 mm diameter were observed to strike a tent 2 m above ground level during a blizzard with wind gusts in excess of 41 m/s (80 knots) in the Koettlitz Valley during the field season. Such granules could then, theoretically, displace pebbles of 18 mm diameter. The largest grains in samples from the crests of the Victoria Valley ripples have a diameter of 19 mm; hence it seems that extreme winds can take into suspension sufficiently large granules to produce traction by impact on the largest pebbles of the ridges. It has been su~gested by Smith (1966, p. 160) that, because drag is directly proportional to the density of the air, the low temperatures of the Antarctic winter would permit a substantial increase in the transporting power of winds of a given velocity. To test this hypothesis, the carrying capacity of the wind for various temperatures has been calculated. Experimental work by Bagnold (1941) has established the general relationships for the conditions governing the movement of sand. The threshold velocity,

7 548 N.Z. JOURNAL OF GEOLOGY AND GEOPHYSICS VOL r;: Q) u... Q) a. Q).~ -C :J E :J U mrnn CRESTS ~ LEE SIDE ~-=-=-=-=3 WINDWARD SIDE c:::=:.j INTER RIPPLE Pe b b I e fine sand sand sand sand I 0.11 Ii' i I I I Granule I c~~~re I Coarse I Medium I o Size ( ) FIG. 4-ParticJe size distribution for the pebble ripples west of Lake Vida.

8 No.3 SELBY et at. - EOLIAN DEPOSITS, ANTARCTICA 549 at any height above the ground surface, needed to take a grain into saltation is given by Bagnold (1941, p. 101) as: ----, j u- p z V t = 5'75 A ~~Kdlog~ \f p k where: V t is the threshold velocity, A is a coefficient, a is the density of the grain material, p is the density of the fluid, g is the acceleration due to gravity, d is the diameter of the grain, z is the maximum height of the trajectory, k is a measure of the roughness of the bed; all measures are in e.g.s. units. By taking the observation, given above, that in a 41 m/s wind at -looc a 3 mm diameter grain reached a height of 2 m, the equation has been solved to give a value for A and a value for k has been used which recognises the very irregular surface of a moraine. The density for dry air at various temperatures is taken from Weast (1969, p. F-ll). Using the following values the equation was then solved for a range of temperatures likely to be experienced in the dry valleys and in trop~cal deserts, and the results are displayed in Fig. 5: a = density of quartz = 2'65 g/cm 3, d = 0.3 cm, z = 200 cm, k = 30cm, g = 982'86 cm/s 2 at 75 latitude, g = 978'64 cm/s 2 at 20 latitude, p = value derived from Weast (1969), A = 1'14. It can be seen that a granule of 3 mm diameter can be carried to a height of 2 m by a wind of velocity 36'05 m/s (69'93 knots) when the air temperature is -70 c, but at ooc the velocity required is 4L'70 m/s (81'0 knots), and at c it is 45'42 m/s (88'25 knots). Thus the velocity required to lift the particle at the extreme differences of temperature found in the Antarctic and tropical deserts differs by approximately 10 m/s (19'5 knots). It appears probable, therefore, that sufficiently large granules can be thrown into saltation, by infrequent wind storms, to move by traction the largest pebbles found in the pebble ripples in the dry valleys, and that the differences in air density account for the differences in maximum particle size of pebble ripples in polar and tropical deserts. The differences in the acceleration due to gravity in these areas have only a small effect, represented by the difference in the two threshold velocity values at ooc shown in Fig. 5. The grains composing the ripples and inter-ripple areas in the Victoria Valley are mainly of granite and dolerite, unlike the surrounding moraine ridges which are almost entirely of dolerite. This indicates that the pebble

9 550 N.Z. r- JOURNAL OF GEOLOGY AND GEOPHYSICS VOL. 17 A I - ~ 40 :> y/,,/?>e'>y,,,, , I Temperature eel FIG. 5-Threshold wind velocity required to lift a 2 mm granule to a height of 2 m at various temperatures. lag is derived by selective deflation from the underlying outwash deposits. If the present lag gravel of the interdune areas were removed, a further mm of outwash material would have to be winnowed in order to produce another pebble lag. Upwind from the main Victoria outwash area dolerite pebbles have been blown from the moraine on to the outwash deposits. The supply of such pebbles seems to be less than the rate of reduction and removal by wind, for a shallow depression 5-20 m wide and up to 1 m deep has formed there, while downwind of the outwash the wind blown material is overlapping the next moraine ridge and burying the base of the morainic boulders. TRANSVERSE DUNES The largest group of dunes in the ice-free valleys of the McMurdo region have an area of about 2 km 2 They occupy the north side of the Lower Victoria Valley in a belt which is about 4 km long and 0'5 km wide. There are about 30 distinct dunes in the field and all have a crescentic form although the degree of development of the dunes is variable. Three barchan dunes were observed but the remainder of the big dunes are better described as transverse dunes, and the small ones as sand mounds (Fig. 6). The dunes are oriented with slip faces towards the west and with crest lines running north to south in response to the dominant summer wind which is from the east. Some dunes have a southern horn which is elongated towards the west. A group of three transverse dunes was surveyed and the profiles across them are shown in Fig. 7. The maximum height of the dunes is 13 m. The easterly, or back slopes, of the dunes have gentle slopes inclined at 3 to 18

10 No.3 SELBY et at. - EOLIAN DEPOSITS, ANTARCTICA 551 FIG. 6-Transverse dunes of the Victoria Valley; looking eastwards towards the Victoria Lower Glacier. which are usually concave upwards and steeper towards the crest of the dune. The crest is usually convex and the top brink of the slip face is sometimes 1 m or more below the highest point on the dune crest. Nine slip-face profiles were measured. The wings have slip-face angles of '5 and the centres, and hence longest, part of the faces have slopes of At the base of the slip face there is an abrupt change of angle to a straight ap.ron with slopes of 17'5 ± 3'5. The lower part of the apron becomes gradually concave upwards and then grades into an interdune area which is nearly horizontal. Dune Surfaces The backs of the dunes are everywhere covered with regular, nearly parallel ripples, with a wave length of mm and a wave height of about 10 mm. These ripples are transverse to the wind direction. The slip faces are smooth sand slopes when they are undisturbed, but dry sand flows are very common. Most of the flows observed began within a metre of the top of the slip face. Disturbed sand moved downwards, overwhelming the lower sand in a flow that was never observed to be more than 10 mm thick and seldom more than 300 mm wide. Simultaneously, the flow scar worked head wards from the initial site of the disturbance as crescent-shaped wedges of weakly adhering sand were undercut and collapsed into the flow, where they lost their adhesion. The flow scar progressed up-

11 A West,.. ~"""J"'",,''''''''''''''''''-'''': ~~ ~~ I -. -.,.. d.. ~ ~4k*Wtmlt"... -, Cross section A-AI AI l~ 0q 5 10m Scale, B.." ~ "2W.4i!.i ~.. t~.4f!!is:~h' F, ,"" "," -.',",,.1",""" ~~~ C lp-chj;:un-hiilwif.w",ii\i0ffi 4i i,);:.- C ross section C _(I (' ACj\ '''' \\ I,1\1 III \ Is' ~Apron ~Backslope mmrrmmrn Slip face Interdune zone III 1'/ r) c' Location sketch w::tntm Convex crest FIG. 7-Profiles through three of the Victoria Valley transverse dunlls.

12 No.3 SELBY et at. - EOLIAN DEPOSITS, ANTARCTICA 553 FIG. 8-A transverse dune: a sand Row is occurring in the foreground; ripples are visible on the back slope and the apron is visible at the foot of the slip face. wards until it reached the top of the slip face (Fig. 8). The top of the apron was initially overwhelmed by the sand flow deposit but cross winds rapidly smoothed both the apron and the slip face, eliminating evidence of the flow. Some of the aprons have smddth surfaces but most have rippled surfaces. Where the sand has the same texture as in the dunes the ripples are like thdse Df the backs, but in several places a lag gravel has been carried into the dune field from the stream bed immediately south of it. In such areas the ripples are much larger with a wave height of up to' 0 5 m and a wave length Df up to' 4 m. The DrientatiDn Df these ripples is irregular and frequently shows two' preferred alignments. A dominant crest line is Driented parallel to the transverse dunes, but a subsidiary ripple forms at right angles to the main Dne, suggesting that a local cross wind is responsible for the secondary crests (Fig. 9). The interdune areas are varied in their surface features, having areas of exposed glacial moraine, simple small sand ripples, or granule-surfaced ripples. Dune bedding A pit in the back of a large dune south of the Packard Glacier has revealed something Df the internal structure of the dunes (Fig. 10). The upper 50 mm Df sand has a uniform bedding of fine laminae parallel to the surface Df the dune. The near-surface 10 mm Df sand is dry and easily mdved by the

13 554 N.z. JOURNAL OF GEOLOGY AND GEOPHYSICS VOL. 17 FIG. 9-Granule-surfaced ripples of the interdune areas with major and minor crest lines. The scale is 300 mm long. FIG. 10-A trench in the back of a transverse dune shows a section with interbedded snow and sand dipping at 29, and an overlying bed of near! y horizontally bedded sand. The bedding is discernible as partl y erased ridges in the surface of the dune (top).

14 No.3 SELBY et al. - EOLIAN DEPOSITS, ANTARCTICA 555 wind and the lower 40 mm of sediments are damp. Below this layer the sand is interbedded with snow in beds which dip at 29 in a direction opposite to that of the slip face. Permafrost was encountered in this pit at a depth of 390 mm. Over 20 pits were dug in the dunes and it was found that similar bedding occurs on the backs of many of the dunes and in small ridges which rest on the backs of the main dunes. Depth to permafrost varies from 250 to 460 mm (mid-summer). Permafrost was not discovered in the loose sand of the apron, nor was bedding discerned in the apron, but this may have been because the sides of the pits collapsed easily. Most of the slip faces are in loose sand, without interbedded snow and with only faintly visible laminae. In some of the interdune areas laminae, preserved by permafrost and interbedded snow outcrop and form distinct, firm ridges of arcuate shape with the concave side facing east. The bedding forms suggest that in summer the dry near-surface sand of the transverse dunes advances towards Lake Vida by the blowing of sand up the backs of the dunes onto the upper part of the slip face which periodically collapses in a dry sand flow. Field observations were made in mid-summer when about 1 m of loose sand was found on the slip face, suggesting that the face may advance about 2 m a year. In winter it is probable that the upper surface of loose sand is interbedded with snow, and the direction and dip of the bedding indicate that, during the period in which snow can fall and not sublime, the wind direction is dominantly from the west. At this time the free sand (i.e.) not permafrosted) may be formed into a reverse dune. The bedding of this reversed dune may be preserved when, during the following summer, moisture from the interbedded snow freezes in the sand. It seems unlikely that the whole dune is seasonally reversing (McKee 1966) as much of the dune is permafrosted, but the dry sand of the surface can certainly be formed into superimposed dunes of reversed direction on the plinths of fixed dunes. Similar observations and conclusions have been made by Lindsay (1973). Further evidence of the importance of a westerly wind off the Polar Plateau in winter is provided by the thin sheet of sand which can be observed, early in summer, covering parts of the Victoria Lower Glacier. This is at a time when winds from the Ross Sea, with no sand source, dominate the Lower Victoria Valley. Sand Characteristics Six 50-mm-depth core samples of sand were collected from each of five zones on the fransverse dunes: aprons, wings, faces, backs, and interdunes. The sands have a modal size of 2'251> and nearly all of the sand is within the fine to medium grade. There appears to be no significant difference in the mean sizes of grains from the five zones (Fig. 11). The Inclusive Graphic Standard Deviation (Folk 1968) reveals that the sands of all five zones are well sorted (ITr = 0'431>, 0'491>, 0'361>, 0'411>, and 0'421». The lag deposits on the ripples of the interdune areas consist of granules. Application of the Inclusive Graphic Skewness test (Folk 1968) indicates that the distributions of sand from the interdunes, faces, and backs are near-symmetrical (Skr = -0'06, +0'04, 0'00) with the

15 556 NZ. JOURNAL OF GEOLOGY AND GEOPHYSICS VOL. 17 -'" Ol "w 40~ ~ -" >-, fj r ~ 30,f J: l, ~ c ~ -U ~ ~ > c o L 1: Back --'- 60-! I 50- i! 4 f ~ 3 1 ~ I ~ v 20.E! ::r-t ~ Ol ~ ~ of! r.r-i ~ i Wing c-=---d _-'_. I Size( 1 I~~-~'--' Face r -j-~2~--i Size( , ~, ~ I ~-- ~--r ~ 150, I r n 1.0 '---~-'------' l60 Interdune -':30 j, A:'"JJ': I J I -30 ~20 j " Size ( I 1- --" Size( 1 i lo '3 >-..c *' r 9~' 1 Size( 1 --'--1 FIG. ll-grain size histograms of deposits in the Victoria Valley.

16 No.3 SELBY et al. - EOLIAN DEPOSITS, ANTARCTICA 557 ~ ~-~-/ / / / lor / / ( 5- "... '--_, ~~, ~~! Scole:l11) -..., 6 L -20-4'0 U-y, '., :..-..-r<: /' /K'.' <~... --~-----'--~ w-x U-y " \ Y \ \ x w-x Y-I z S/ _~~~ ~~. ~ FIG. 12-Profiles of a whalehack dune east of Lake Vida. wings fine skewed (SkI = ) and the aprons near-symmetrical to strongly fine skewed (SkI = to ). The size distribution of the sand thus appears to be little affected by its movement over and around the dune. The presence of the lag deposits only in the interdune areas and on the base of the apron does, however, indicate that the wind is unable to move this load up the backs of the dunes. VICTORIA LOWER GLACIER OUTWASH Six 50-mm-depth core samples were taken from the outwash material at the base of the terminus of the Victoria Lower Glacier at points where the sample material might be expected to become part of the load of the streams draining the glacier. The sediment has a bimodal distribution, is strongly coarse skewed (SkI = to -0-52), and is moderately to poorly sorted (!TI = 1-67 to 0-91 cp) _ The modes occur in the fine sand and coarse sand grades, and the material ranges in size from very fine sand to pebbles (Fig. 11.) - WHALEBACK DUNES There are three whaleback dunes in the Victoria Valley, and they extend for a little over 2 km from the eastern end of Lake Vida. The dunes are aligned obliquely across the valley and exposures of ground moraine within the dune area suggest that the alignment and shape may be partly controlled by the underlying moraine_ Profiles of the dune which was carefully surveyed are shown in Fig. 12_ The measured dune has a longest axis of 350 m, a maximum height of about 9 m, and a width across the widest part of 180 m. The dune is asymmetrical; the oblique back slope is less steep than the lee slope (see slopes above points X and Z in Fig. 12)_ Gcology-4

17 558 NZ. JOURNAL OF GEOLOGY AND GEOPHYSICS VOL. 17 FIG. 13-Granule ripples on the whaleback dunes east of Lake Vida. The scale is 300 mm long. Bedding within the dune is only exposed at a few marginal localities where an ephemeral meltwater stream has eroded a flank. Here the bedding is seen to be nearly horizontal and the sand to be interbedded with lenses of snow. The snow releases sufficient moisture for the dune to be permafrosted to within a few tens of millimetres of the surface in mid-summer. A core sample of the upper 50 mm of the dune deposits shows a strong bimodality in particle sizes (Fig. 11). This is caused by a lag deposit of granules (-1 to -1'75cj» overlying the sand. The sand fraction has a mode at 2 25cj> and is predominantly in the fine to medium size range; the sorting is poor ((I"I = 1'28 to 1 15cj». Samples were collected at 50-m intervals along the longitudinal axis of the dune and although graphical analysis suggests that there is a slight coarsening of the sand and the lag granules towards Lake Vida, this is only a minor effect and unlikely to be the result of a significant sorting process. Ripples are particularly well developed on the surfaces of the whaleback dunes; they range in height from 20 to 200 mm and have wave lengths from 0'05 to 1'5 m (Fig. 13). The ripple index varies from 10 to 60. These extremes are respectively lower and much higher than the values discovered by Sharp (1963) for the granule ripples of the Mojave Desert. The ripples are extremely variable in shape, spacing and pattern although all are transverse to the dominant wind. These ripples all occur on the same dune and all have a lag gravel of granules with size ranging from -1 to -1'75cj> and a mode at -1 25cj>. It seems probable that this size range is the critical

18 NQI.3 SELBY et at. - EOLIAN DEPOSITS, ANTARCTICA 559 FIG. 14-Scanning electron micrograph of medium-size sand grains from outwash at the Victoria Lower Glacier. Most of the grains show irregular high relief and a few are rounded with subdued relief. Qne fqr stabilising ripples which mqve Qnly during infrequent very high velocity winds. Unless the wind is extremely variable in strength Olver very shqrt distances a simple direct.relatiqnship between ripple size and wind speed fqr particles Qf the same size doles nqt hqld in this area. DISCUSSION When the histqgrams Qf the sand fractions Qf the samples frqm near the VictQria Lower Glacier snout are cqmpared with thqse Qf the transverse dunes and the whaleback dunes (Fig. 11) it becomes apparent that the outwash from the glacier has a much higher proportion of sand in the fioe tq very fine grade (2 tq 4 ) than Qccurs in the dunes. AlthQugh more samples

19 560 N.z. JOURNAL OF GEOLOGY AND GEOPHYSICS VOL. 17 FIG. 15-Scanning electron micrograph of medium-size sand grains from the back slop:!s of the dunes. Most grains are rounded and have subdued relief. A few still show irregular and higher relief. would be needed to confirm this, a tentative conclusion is that some of the fine sand from the outwash is not carried down the valley towards Lake Vida because it is selectively blown back on to the glacier by wind from the Polar Plateau. The dominant easterly wind from the Ross Sea, however, can move sand of all grades from the outwash towards Lake Vida and into the dunes. Scanning electron micrographs (Fig. 14, 15) of sand grains from the back slopes of the sand dunes and from the outwash at the edge of the glacier confirm this conclusion. The grains from the outwash have irregular and high relief, with many fresh fractures, which would be expected of glacial sand (Krinsley & Margolis 1969), but some grains are well rounded and smoothed and thus have the characteristics which would be expected to develop in a dune environment. The grains from the dunes are generally

20 ND.3 SELBY et al. - EOLIAN DEPDSITS, ANTARCTICA 561 well rounded althdugh many grains still show the irregular relief and fractures characteristic of glacial grains which have ndt been subjected to an edlian transpdrt environment fdr very IDng (MargDlis & Krinsley 1971). CONCLUSIONS The most commdn features Df eolian depdsitidn in the McMurdo dry valleys are ripples which occur in sediments ranging in grain size frdm fine sand to pebbles. There ddes ndt appear td be a simple, direct relatidnship between ripple index and grain size. In VictDria Valley, sediment from the Victoria Lower Glacier is carried tdwards Lake Vida both by outwash streams, which fldw intermittently fdr about two mdnths a year, and by wind. This sediment is mostly incorpdrated into a series Df transverse sand dunes and, further down-valley, intd whaleback dunes. The transverse dunes and parts of the whaleback dunes have snow interbedded with the sand and this provides endugh mdisture fdr the dune td be partly permafrosted. ShallDw trenches dug in the backs Df the dunes revealed that reverse bedding is CDmmon and this suggests that seasonal reversal of the directidn of migration Df the dune DCCurS. In the absence Df deep trenches thrdugh the dunes it is ndt possible to ascertain if the entire dune cdmplex reverses directidn, or only the surface few tens-ofmillimetres Df sand, which allow superimposed dunes to develop. It is alsd unknown whether the reversing phenomendn occurs every year. The sands forming the transverse dunes are similar in grain size to those Df hdt deserts (see, fdr example, CODke & Warren 1973), and there is nd apparent slorting of the sand lover different parts of the dunes. A characteristically granule-size lag Dccurs in some interdune areas and on the whaleback dunes where it controls the size of the ripples. ACKNOWLEDGMENTS We are indebted to the University Grants Committee for financial aid and to the Antarctic Division, DSIR, and the U.S. Navy for logistics support. Dr C. S. Nelson commented on a draft of this paper. REFERENCES ALLEN, ]. R. L. 1970: "Physical Processes of Sedimentation". George Alien & Unwin, London, 239 p. ALLEN, A. D.; GIBSDN, G. W. 1962: Geological investigations in southern Victoria Land, Antarctica. Part 6-0utline of the geology of the Victoria Valley region. N.z Journal of Geology and Geophysics 5: BAGNDLD, R. A. 1941: "The Physics of Blown Sand and Desert Dunes". Methuen, London. 265 p. CAILLEUX, A. 1968: Periglacial of McMurdo Strait (Antarctica). Biuletyn Peryglacjalny 17: CALKIN, P. E. 1971: Glacial geology of the Victoria Valley System, southern Victoria Land, Antarctica. Pp in CRARY, A. P.; (Ed.): Antarctica snow and ice studies II American Geophysical Union Antarctic Research Series p.

21 562 N.Z. JOURNAL OF GEOLOGY AND GEOPHYSICS VOL. 17 COOKE, R. U,; WARREN, A. 1973: "Geomorphology in Deserts". Batsford, London. 374 p. FOLK, R. L. 1968: "Petrology of Sedimentary Rocks". Hemphill's, Austin, Texas. 170 p. KRINSLEY, D. H.; MARGOLIS, S. V. 1969: A study of quartz and grain surface textures with the scanning electron microscope. Transactions of tbe New York Academy of Sciences, Series II, 31: KRUMBEIN, W. C. 1934: Size frequency distribution of sediments. Journal of Sedi mentary Petrology 4: KUKAL, Z. 1971: Geology of Recent Sediments". Academic Press, London. 490 p. LINDSAY, J. F. 1973: Reversing barchan dunes in Lower Victoria Valley, Antarctica. Geological Society of America Bulletin 84: MARGOLIS, S. V.; KRINSLEY, D. H. 1971: Submicroscopic frosting on eolian and subaqueous quartz sand grains. Geological Society of America Bulletin 82: MCCRAW, ]. D. 1967: Some surface features of McMurdo Sound region, Victoria Land, Antarctica. N.z. Journal of Geology and Geophysics 10: McKEE, E. D. 1966: Structures of dunes at White Sands National Monument, New Melxico (and a comparison with structures of dunes from other selected areas). Sedimentology 7: NICHOLS, R. L. 1966: Geomorphology of Antarctica. Pp in TEDROW, ]. C. F.; UGOLINI, F. C. (Eds.): Antarctic soils and soil forming processes. American Geophysical Union Antarctic Research Series 8. POWERS, M. C. 1953: A new roundness scale for sedimentary particles. Journal of Sedimentary Petrology 23: SHARP, R. P Wind ripples. Journal of Geology 71: SMITII, H. T. U. 1966: Wind formed pebble ripples in Antarctica. Geological Society of America SPecial Paper 87: 160. THOMPSON, D. c.; CRAIG, R. M. F.; BROMLEY, A. M. 1971: Climate and surface heat balance in an Antarctic dry valley. N.Z. Journal of Science 14: 245-5l. WEAST, R. C. (Ed.) 1969: "Handbook of Chemistry and Physics". The Chemical Rubber Company, Clevedand, Ohio. WEBB, P. N.; McKELVEY, B. C. 1959: Geological investigations in south Victoria Land, Antarctica. Part I-Geology of Victoria Dry Valley. N.z. Journal of Geology and Geophysics 2: WENTWORTH, C. K. 1922: A scale of grade and class terms for clastic sediments. Journal of Geology 30: