A scanning electron microscope study of surface ...

Sedinientology ( I 974) 21, 87-1 05

A scanning electron microscope study of surface textures of quartz grains from glacial environments

W . B R I A N W H A L L E Y * and D A V I D H . K R I N S L E Y

Department of Geography, Downing Place, Cambridge, England and Department of Earth and Environnzental Sciences, Queens College, City University of New York, Flushing, New York 11367, U . S . A .

ABSTRACT Surface textures of quartz sand grains from several glacial environments at the Feegletscher, Switzerland examined by means of scanning electron microscopy are described. The difference between supraglacial and subglacial material is very slight. At the moment the interpretation of these textures must be done with care until the full statistical relation to environments can be ascertained. This does not necessarily invalidate earlier investigations although it does mean that glacial and extraglacial environments in the vicinity of glaciers are apparently not distinguishable. Examination of surface precipitation features suggests a sequence of events which can be used to help discriminate between different ages of deposits in an area. A similarity between some of the surface debris from moraine samples and those seen in loess and quickclay deposits is also suggested.

INTRODUCTION Scanning electron microscopy (SEM) of quartz grains has enabled several different sedimentary environments to be discriminated on the basis of their surface textures (Krinsley & Donahue, 1968; Krinsley & Margolis, 1969; Krinsley & Doornkamp, 1973). Among these environments glacial deposits are included (Krinsley & Takahashi, 1962; Krinsley & Funnell, 1965; Coch & Krinsley, 1971 ; Hillefors, 1970). N o attempt has been made to further differentiate such sediments or to examine glacial processes by SEM. This paper presents an attempt to distinguish glacial sub-environments via SEM in a series of samples from known environments. The samples were collected from around the Feegletscher, Kanton Wallis, Switzerland, an area with schists and gneisses as the source material.

* Present address: Department of Geography, University of Reading, Whiteknights, Reading, England. 87

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RATIONALE In glacial geology and geomorphology there is frequently a need to be able to distinguish between sedimentary sequences associated with glacial deposits in order to investigate operative processes (Boulton, 1968, 1970; Whalley, unpublished) or, more frequently, to help determine an associated chronology. Usually this entails granulometric analysis coupled with other lengthy methods of examination (e.g. Shepps, 1953; Gross & Moran, 1972). It was hoped that different processes would provide sufficiently different energy and environmental conditions to affect the surface texture of the grains in recognizably different ways and thus help solve the problem of discrimination with much greater ease. In particular, it was presumed that, as an initial hypothesis, different processes operated on the surfaces of subglacially ground grains than on grains which had fallen off mountain sides upon glacier surfaces. knglacial grains were expected to have substantially similar textures to grains from supraglacial deposits. As englacial grains are carried in the ice surrounded by a thin film of water (Feegletscher is a ‘temperate’ or ‘wet base’ glacier) they should show considerable surface alteration. Deposits washed down from the glacier into a stream and in an esker were sampled and these would therefore be expected to show both sub- and supra-glacial textures with superimposed fluvial characteristics (Coch & Krinsley, 1971). Moraines dating from post c.1600 AD (Neuzeitlich) and at about 4000 BP were also sampled for comparison.

TECHNIQUE Samples were prepared by rinsing well in distilled water to remove adhering surface mica flakes. Individual grains were picked up with either a wetted single paint brush hair or with micro-forceps. Double-stick tape was used on the standard mounting stubs for the Cambridge Instruments Stereoscan S2 and S4 instruments. Grains in the size range 0.25 mm-1 mm long were examined and about fifteen grains of each sample were selected as randomly as possible. More than one sample was taken from any given topographic position. Coating was either gold or gold-palladium, both giving good results except for occasional charging of angular edges. A tilt of 30” was used for most of the photographs in the emissive mode; a dispersive X-ray analyser (EDAX) was used to check that representative grains were indeed quartz and to examine the surface debris on the grains. It was often difficult to distinguish unaltered feldspars from quartz as the feldspars also give fracture patterns as well as cleavage traces. OBSERVATIONS In this paper a number of new surface textural elements are recognized and described. These terms are listed with brief comments in Table 1. The textures are described according to the environment in which they were found and brief mention is made of their possible origin. Only surface textures are reported in this study; we hope to present investigations of angularity, roundness and other form measures at a later stage.

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Table 1. Term

Comments

Inherited characteristics

The surface texture resulting from crystallization or lithification found a t the outset of a sedimentary cycle.

Subsequent characteristics

a sedimentary cycle.

Discussion or First use

The textures imparted on the inherited characters during

Facet

Distinct flat area but X 5000) on cleavage surfaces, sometimes in association

with cleavage flakes. Arc-steps

A series of concentric stepped (usually) arcs on conchoidal ‘Arc-shaped steps’ of breakage faces. The spacing is usually regular and size Krinsley & Donahue range may be considerable. (1968)

Parallel steps The upper size of arc-steps. Their form depends on surface ‘Semi-parallel’ and and sub-parallel configuration. ‘Hogback imbricate steps breakage blocks’ of Krinsley & Donahue (1 968) Arc-stepped grooves

Arc steps which occur in distinct grooves. The more pronounced the groove the greater the steps. Can occur as very small features and grade into ‘micro-steps’.

Micro-steps

A series of fine parallel to sub-parallel lines with spacing about 0.5 pm. They are usually short and can be seen as traces in a variety of locations up to 50 pm long in small zones.

Precipitation rounding

The late stage in precipitation where previously sharp edges become blunted with deposited silica and the outline becomes less angular.

Carapace

Adhering mixture of fragments (usually comminution debris of various kinds) which form a very uneven cover to a grain.

Related to the ‘prismatic pattern’ of Krinsley & Donahue (1968)

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Supraglacial grains This material, which originated from cliffs above the glaciers, would be expected to show only the ‘inherited’ textural characteristics as weathering has only released the grains onto the passively transporting glacier surface. Some crushing by rockfall could also play some part in the formation of these textures. Usually, however, the inherited features are due to stresses imposed on quartz grains in its late crystallization or lithification in the parent rock (Smalley, 1966). It is on top of the basic inherited texture that modifications are made. Most of the quartz grains are of high sphericity and angularity. Figure 1 shows a grain which is typical of freshly released material with a flat ‘cleavage face’ (C) and ‘cleavage flakes’ adhering to it; these flakes are not removed by rinsing and even boiling fails to remove all of them. The area between the facet C and the cleavage face of Fig. 1 is shown in detail in Fig. 2 . Though the grain looks very fresh, at high magnification ‘precipitation platelets’ can be seen ; these represent the earliest phase of deposition. The platelets may form in some cases by the ‘welding’ of cleavage flakes onto the fresh surface by amorphous silica in solution. Platelets and cleavage flakes

Fig. 1. Supraglacial grain showing ‘typical’ glacial features, including ‘s~ib-parallelsteps’ (D). Some comminution debris is present. The area round (C) is shown in Fig. 2. Scale bar represents 20 pm. (Krinsley, D.H. & Smalley, 1.J. 1972. Am. Sci. 60, 28C~291,Fig. 7.)

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Fig. 2. Detail of grain in Fig. 1 showing the small amount of silica precipitation and the sharp edges. (A) is a ‘cleavage flake’ and two ‘precipitation platelets’ are a t (B), Scale bar represents 1 pm.

appear to provide centres for resolution of silica, especially after the deposition of a grain, e.g. in a moraine. The ‘sub-parallel steps’ (Fig. 1, D) are probably an inherited characteristic which are thought t o be related to a cleavage direction ; probably representing one of the main quartz cleavages (Frondel, 1962). This grain is similar in both magnitude of surface relief and angularity of features to others recognized from glacial sediments (Krinsley & Donahue, 1968, Plate 6; Krinsley & Margolis, 1969, Figs 11 and 13). Sometimes heavily weathered quartz surfaces can be found supraglacially, as even on cliffs or glacier surfaces there may be sufficient water and high surface temperatures to cause considerable weathering (Reynolds & Johnson, 1972). There is a good deal of uncertainty about the time any particular grain has been waiting for transport down the cliff or along the glacier. Figure 3 shows a further stage in surface alteration; the edges are blunted by precipitation of silica. Precipitation on upturned plates is apparently one of the first stages in deposition of surface silica. A wide range of surface textures was observed in the supraglacial deposits. The basic form is of angular grains with the fracture textures-arc step, sub-parallel and arc-stepped grooves-being predominant. However, the frequency of these textures is

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Fig. 3. A supraglacial grain showing comminution debris (niainly mica and crushed quartz fragments) and some precipitation rounding the edges. Scale bar represents 20 pm.

very variable. Though the fracture textures are thought to be inherited characteristics it is possible that they could be formed by impact breakages during rock and debris falls. Though surface precipitation is not common it can modify the grain texture to some extent. This supraglacial material can be deposited directly in moraines or washed away in streams to become part of another deposit. Englacial grains Englacial material is unlikely to be encountered on its own in a geological situation without a glacier but will contaminate ablation till or ground moraine. Englacial, as used here, is taken to mean material within the body of the glacier but excluding that which is probably derived from the rock bed beneath the glacier (Boulton, 1970), which is here called subglacial. The grains have become englacial due to having fallen in the accumulation area of the glacier and become buried by snow. Grains from this environment again show considerable variability; despite being transported for long distances many show remarkably little silica deposition. Figure 4


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shows a medium amount of precipitation on some areas with upturned plates, certain facets are still apparently devoid of precipitated silica. Figure 5 shows precipitation overgrowth and cemented debris on top of a cleavage facet. This debris is composed of quartz, feldspar and mica fragments. The lack of alteration on many faces is probably due to the fact that englacial grains, though surrounded by a pellicle of water, suffer little inter-particle contact and hence little disrupted lattice quartz is formed. This djsrupted lattice layer (Rieck & Koopmans, 1964; Lidstrom, 1968) is thought to be fundamental in providing a source of so-called ‘amorphous silica’ which has a high solubility (Iler, 1955, p. 6). The low solubility of ‘healed’ quartz lattice surfaces permits only small amounts of silica to go into solution. Furthermore, the disrupted lattice layer area also provides sites for precipitation. Hence, once in the glacier, surface texture undergoes relatively little modification. There is, therefore, little distinguishable difference between quartz grain textures in an englacial position and those which are found supraglacially.

Fig. 4. Englacial grain from the snout of Feegletscher. ‘Upturned plates’ (A) are tending to become obscured by precipitation, though some facets are still relatively clear. Scale bar represents 50 pm.

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Fig. 5. Englacial grain showing slight precipitation on a cleavage facet but with precipitation on the left and probably precipitation silica cementing comminution debris (mica, feldspar and quartz particles with only very small amounts of clay minerals) to the top right. Scale bar represents 20 pm.

Subglacial grains I n a temperate glacier, subglacial material is that which has come from the glacier rock bed by weathering or glacier erosive processes. Temperate glaciers do not have thick layers of debris-rich ice at their bases though the lee sides of rock outcrops provide a mechanism for temporarily freezing subglacially ground material onto the glacier ice. Samples were taken from debris thus frozen as well as from the small quantities found between ‘clean’ ice and rock bed. No distinguishable difference was found between grains from the two situations. The basic mechanism of subsequent texture formation in temperate glaciers would be expected to consist of grinding in a wet environment. Figure 6 shows the ‘typical’ glacial features together with comminution debris which many of the subglacial grains possessed. Wet grinding sometimes appears to give a ‘micro-block‘ texture as in Fig. 7. This texture is not common and is, like the upturned plate texture, apparently related to the imperfect quartz cleavage. It has occasionally been found in artificially prepared samples. Figure 8 shows a quartz grain

and traces of ‘micro-block’texture on the top facet. Scale bar represents 20 pm.

Fig.6. Subglacial grain showing ‘arc steps’ and ‘sub-parallel steps’ with comminution debris

Fig.7. Angular subglacial grain with some micro-blocks at A as well as larger blocks and only feeble conchoidal structures and sub-parallel steps. Scale bar represents 50 pm.

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Fig. 8. Grain produced by crushing Feegletscher country rock in a closed container. Shows a facet and extensive areas of micro-block texture which tend to become upturned plates near the facet edges. The micro-block texture is here thought to be an inherited rather than a subsequent texture. Scale bar represents 20 pm.

formed by crushing a sample of the Feegletscher country rock in a closed container with a little water; the micro-block texture is very widespread on this example. There were few of the features associated with brittle fracture in the specimens examined. Precipitation was found to be relatively rare on grain faces although it can occur in hollows and corners where a water film can remain undisturbed. The general absence of precipitation is thought to be due to the fact that in wet grinding the disrupted lattice layer is thin, as relatively little free energy compared with dry grinding (Lidstrom, 1968) and hence contributes little to precipitation-solution effects. Edges and corners are most susceptible to grinding while faces may be much less so. Most lattice disruption is therefore at ground edges. It is possible that this factor might account for the fact that ‘typical’ glacier textural features on quartz grains as described by Krinsley & Donahue (1968) do not appear to be predominant in the particular subglacial environment examined. Laboratory experiments are planned to investigate the effects of wet and dry grinding with different ice/water/particle mixtures. Grains from boulders over-ridden by the glacier In the course of the recent advance of the Feegletscher, some large blocks have been over-ridden (c. 1970 AD); one of these was found in a tunnel excavated into the snout of the glacier. The block was frozen into the glacier by the winter cold wave passing

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into the ice. Fragments, apparently freshly weathered from the surface, were found on the top of the block. The grains revealed many more parallel and arc steps than the subglacial samples (Fig. 9) though the micro-block texture is also occasionally seen. Very little precipitation was observed, probably due to the short period of time the grains had been released from the boulder. This observation suggests that in fact the ‘typical’ glacial conchoidal fractures and associated step features are mainly produced by the release from the parent rock or by percussion impacts in falling off cliffs.

Fig. 9. Freshly weathered grain from under the glacier snout but with no grinding. There is only slight precipitation in the minor hollows with ‘arc-stepped grooves’. This may be due to the wet conditions under the snout in the summer. Scale bar represents 20 pm.

Grains from a stream, lake and esker A stream and lake below the glacier were also sampled as it was assumed that further alteration of textures would be observed. The meltwater stream was sampled 150 m below the snout of the glacier; textures were again variable reflecting the mixed debris input. Figure 10 shows a blocky texture with some modification to the facet edges and some precipitation. Lake core samples exhibited a variety of textures as did an esker formed about 1945. Here, fresh conchoidal fractures (Fig. 11) as welt as heavy

Fig. 10. Grain from the glacier outflow stream; little rounding but some precipitation in hollows and on micro-blocks (A). Scale bar represents 20 pm.

Fig. 11. Grain from an esker showing micro- and sub-parallel steps but little modification to these inherited characters. Scale bar represents 20 pm.

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Fig. 12. Grain from an esker showing pitting (presumably by fluvial action) on the lower face. Precipitation over upturned plates on the upper facets. Scale bar represents 20 pm.

precipitation and pitting can be seen on the surface (Fig. 12). The amount of precipitation and general surface alteration was variable in all these environments. All the grains had a certain amount of cleavage flakes and some other comminution debris adhering to their surfaces but those in the esker had the least. Stream, lake and esker grains did show an apparent increase in perimeter angularity/roundness over the samples on and in the glacier but this has not yet been examined statistically.

Grains from moraines Samples were taken from a lateral moraine dating from &heNeuzeitlich (c. 16001900 AD) period of advances. The moraines are composed of material which has been dumped from the glacier surface plus a small amount which has melted out from within the ice. The older, basal deposits within the moraine showed considerable silica precipitation on grain surfaces. Some grains had a ‘carapace’ of what is thought to be quartz cleavage flakes and comminution debris together with muscovite and feldspar fragments cemented by silica between the very small ( < 20 pm) flakes and the quartz grain surface (Fig. 13). The mineralogy of these fragments was confirmed with the EDAX. The precipitation on these grain surfiices is thought to be due to the high stress of

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interparticle contacts creating an active disrupted lattice layer in the quartz (Schneider, 1968; Lidstrom, 1968). Mafic minerals, when ground with water give a high abrasion pH (Keller & Reesman, 1963; Keller, Balgord & Reesman, 1963); above a p H value of 9 ‘amorphous’ silica readily goes into solution (Iler, 1955). Small amounts of capillary water in a compacting sediment might well give suitable conditions for the dissolution of silica from quartz grains which have a disrupted lattice. These conditions are especially likely to occur where a feldspar grain is in a high stress contact with quartz grains. The site of some disrupted lattice quartz also likely to induce precipitation at the same spot.

Fig. 13. Grain which has been deeply buried (post 1600 AD) in a lateral moraine. The distinctive ‘carapace’ is difficult to remove by ordinary washing. Scale bar represents 50 ym.

Figure 14 shows some light precipitation on a grain from a moraine deposited about 1880 AD and which has not had time to undergo a great deal of diagenesis, nor has the overburden been great enough to induce particle breakage or to form a carapace. However, free drainage and low permeability may have increased silica concentration and precipitation. Generally, there is little difference between the grains deposited in the moraine about this time and those which are on the surface of the glacier at the present time.

Surfuce textirre~u j qiturtz gruins

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Fig. 14. Grain deposited in a lateral moraine within the last 100 years. There is slight precipitation on some facets while others are clear. Compare with Fig. 1 and Fig. 3. Scale bar represents 20 pm.

A grain from a late glacial moraine deposited from the glacier surface about 4000 is shown in Fig. 15. This particular example has a thinner silica cover than many of those observed from this moraine; the semi-parallel steps can still be clearly seen. Grains from near the surface of this deposit apparently have only poorly-developed carapaces if they are present at all. This suggests that the process which cements comminution debris to the surface of a grain is primarily a pressure rather than a time effect. A series of moraines is being investigated in this area; it is hoped that they will give a dated sequence for moraines under similar geological and topographical conditions as well as of climate. Preliminary results suggest that the surface alteration of grains from a similar depth is time dependent in some way though as yet there is no absolute dating of the sequence. With suitable control however, the use of SEM on quartz grain weathering may provide a dating technique for comparative studies. Andrews & Miller (1972) have confirmed the chronology of some glacial deposits in Baffin Island by comparison of SEM photographs of quartz grains from different weathering zones. BP

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Fig. 15. Grain from a moraine estimated as being 4000 BP. Precipitation on top of step features and parts of a carapace of detrital breakage fragments. Scale bar represents 20 pm.

GENERAL C O N C L U S I O N S Certain of the textures discussed here show differences from those in other publications (Krinsley & Donahue, 1968 ; Krinsley & Margolis, 1968; Hillefors, 1971); there may be several reasons for this. First, the nature of the quartz grainsas weathered from the parent rock (the inherited characteristics) is of prime importance. Smalley (1966) has discussed the factors which could influence the shape of quartz grains; similar mechanisms could also have some effect on the surface textures. Textures of a Precambrian granite quartz grains found on the surface of the Arapaho Glacier, Colorado, exhibited relatively few conchoidal fractures and steps on some grains, considerable amounts on others. The full range of possible inherited textures is not yet known, but subsequent modification is relatively unlikely to add more of the breakage type of texture, though Warnke (1971) and Setlow &. Karpovich (1972) have described ‘glacial’ type fractures from littoral environments. These textures require high energy environments to induce brittle fracture though it is possible that grooves


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may be formed if aided by cleavage. An exception is breakage along hairline cracks (Blatt, 1967); this may occur some time after release and during transport. Brown (1 973) has recently shown the non-glacial nature of some quartz grains, and a paper examining a variety of inherited characteristics from a variety of lithologies is being prepared by one of the present authors. Secondly, the nature of wet grinding may obscure some of the breakage textures and replace them by microblock and upturned plate surfaces. Both these features are thought to be related to the weak quartz cleavages and. as Fig. 8 shows, may also come from crushing as well as grinding. In some of the glacial deposits examined by other authors it may be that the grinding took place under ‘dry’ conditions under a cold (i.e. dry-base) glacier; in the present case of a wet-base glacier there would be a cushioning effect from water surrounding the grains. Hence, it is not known if wet grinding has removed some of the brittle fracture surface texture (and perhaps replaced it with micro-block texture in some cases) or the texture is in fact all inherited. N o surface texture was observed which could characterize any particular glacial sub-environment. The general variability of the grain surfaces from all the glacier positions makes it impossible to tell by examining grains in a deposit from what glacial position it was deposited. The frequency of the fracture textures (arc steps, parallel and sub-parallel steps and arc-stepped grooves) on grains from deposits which have not been subglacially ground is important. Freshly weathered-out grains from cliffs above the glacier also show a predominance of these textures. It is possible therefore, that the glacial (taken to mean subglacial) origin of some deposits identified by SEM techniques may be in question. Further work on some of these questions js at present under way. It appears that the larger the grain the less likely it is to show precipitation of silica at an early stage. The smaller the grain, the greater the opportunity for surface contacts and disruption of the crystal lattice. There is a general sequence of increasing precipitation which can be recognized under ideal conditions. Not all types are necessarily represented in a given sampled deposit. Initially, precipitation of silica is only a light cover which forms on some cleavage surfaces of freshly-released grains, but this later extends to stressed cracks and hollows. Sometimes the hollows may retain precipitation in increased amounts even though the rest of the texture remains uncovered. Conchoidal breakage surfaces, on the other hand, remain relatively free of precipitation until a late stage. Depressions tend to contain more and smaller irregularities than conchoidal surfaces and there is a greater opportunity for silica to go into solution. Corners between micro-blocks generally attract precipitation at the same stage as the outer rims of upturned plates are being modified. The plates and the surface eventually become covered though breakage may again occur during subsequent abrasion. Areas where there are no upturned plates or micro-blocks and which are at an angle to welldeveloped cleavages then become affected. Precipitation on adjacent faces or facets then follows and the last stages involve increasing precipitation, rounding and encrustation, with or without a complete carapace. The nature of the carapace is undergoing further investigation but a similarity with some loess grain surface textures is suggestive; see e.g. Smalley & Cabrera (1970) and Warnke (1971). It is also of interest to note the great similarity in appearance between some of the comminution and carapace debris (e.g. Fig. 5 and Fig. 15) and some of the silt-clay sensitive clays (e.g. Leda and Drammen clays; Tovey, 1971). Smalley, Cabrera & Hammond (1973) have discussed the general properties of soils of this type and

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emphasize the importance of the inactive, non-clay mineral, constituent. Moraine material from Feegletscher, and indeed many other alpine areas, seems to be of similar composition with low percentages of material less than 2 pm.X-ray diffraction analysis of the < 20 pm and < 2 pm fractions from lateral moraines round Feegletscher shows considerable amounts of inactive mica, quartz and feldspars compared with only small (approx. 0.1 % of the whole) amounts of active clay minerals (vermiculite). The nature of the quartz particles and the adhering debris from the moraines may apparently have close connections with both the problems of loess formation and with the behaviour of quickclays. However, as the moraines are formed of surface debris from the glacier and the formation of both loess and quickclay has been attributed to subglacial grinding, the relationship between erosive processes in the material source area is in need of extensive examination by SEM techniques.

ACKNOWLEDGMENTS This work was carried out at the University of Cambridge, England while one of us (DHK) was on sabbatical leave at the Sedgwick Museum; and he would like to thank Professor H. B. Whittington for the use of facilities there. WBW carried out this work while holding a Natural Environment Research Council Studentship at the Department of Geography and completed it while on a Leverhulme Trust European Studentship at the Swiss Federal Institute of Technology, Zurich. We would like to thank Drs P. Wroth and K . Tovey of the Engineering Department, Cambridge; Mr D. Gray of the Department of Metallurgy and Materials Science, Cambridge; Mr R. McGrew, Department of Molecular, Cellular and Developmental Biology, University of Colorado; Dr B. M. Funnel1 and Mrs J. Irving, School of Environmental Sciences, University of East Anglia for assistance with their various instruments. The Graduate School of the University of Colorado assisted with finances for microscope time at Boulder and WBW would like to thank Dr J. D. Ives of the Institute of Arctic and Alpine Research at Boulder for his help and also to Mr R. Coe for printing the photographs.

REFERENCES ANDREWS, J.T. & MILLER,G.H. (1972) Chemical weathering of tills and surficial deposits in east Baffin Island, N.W.T. Canada. In: International Geography 1972 Vol. 1 (Ed. by W. P. A d a m & F. H. Helleiner), paper 0102 Geomorphology pp. 5-7. University of Toronto Press, Toronto. BLATT,H. (1967) Original characteristics of clastic quartz grains. J. sedim. Petrol. 37, 401-424. BOULTON, G.S. (1968) Flow tills and related deposits on some Vestspitsbergen glaciers. J . Glaciol. 7, 391-412. BOULTON, G.S. (1970) On the origin and transport of englacial debris in Svalbard glaciers. J . Glaciol 9, 21 3-229. BROWN,J.E. (1973) Depositional histories of sand grain textures. Nature, Lond. 242, 396-398. COCH,N.K. & KRINSLEY, D.H. (1971) Comparison of stratigraphic and electron microscope studies in Virginia Pleistocene coastal sediments. J . Geol. 79, 426-437. C . (1962) The System of Mineralogy of J. D. Dana and E. S. Dam, Vol. 3, Silica Minerals. FRONDEL, Wiley and Sons, New York. GROSS,D.L. & MORAN,S . R . (1972) Grain-size and mineralogical gradations within tills of the Allegheny Plateau. In: Ti///ASymposium (Ed. by R . P. Goldthwait), pp. 251-274. Ohio State University Press, Columbus, Ohio.

HILLEFORS, A. (1970) Deep weathered rock material and sand grains under the scanning electron microscope. Svensk Geogr. Arsbok, 46, 138-1 64. ILER,R.K. (1955) The Colloidal Chemistry of Silica and Silicates. Cornelf University Press, Ithaca, New York. A.L. (1963) Dissolved products of artificially pulverized KELLER, W.D., BALGORD, W.D. & REESMAN, silicate minerals and rocks, Part 1. J. sedim. Petrol. 33, 191-204. A.L. (1963) Glacial milks and their laboratory-simulated counterparts. KELLER,W.D. & REESMAN, Bull. geol. Soc. Am. 74, 61-76. J. ( I 968) Environmental interpretation of sand grain surface textures by KKINSLEY, D.H. & DONAHUE, scanning electron microscopy. Bull. geol. Soc. Am. 79, 743-748. KRINSLEY, D.H. & DOORNKAMP, J.C. ( I 973) Glossary of’ Quartz Sund Grain Textures. Cambridge University Press, Cambridge, England. B.M. (1965) Environmental history of sand grains from the Lower and KKINSLEY, D.H. & FUNNELL, Middle Pleistocene of Norfolk, England. Q. Jlgeol. Soc. Land. 121, 435-461. S.V. (1969) A study of quartz sand grain surface textures with the KRINSLEY, D.H. & MARGOLIS, scanning electron microscope. Trans. N . Y . Acad. Sci. Ser. 11, 31, 457-477. T. (1962) Applications of electron microscopy to geology. Trans. N . Y . KRINSLEY, D.H. & TAKAHASHI, Acad. Sci.Ser. IT, 25, 3-22. LIDSTROM, L. (1 968) Surface bond-forming properties of quartz and silicate minerals and their application in mineral processing techniques. Acra polytech. Chem. incl. Metall Ser. 75. S.V. & KRINSLEY, D.H. (1971) Submicroscopic frosting on eolian and subaqueous MARGOLIS, quartz sand grains. Bull. geol. Soc. Am. 82, 3395-3406. N.M. (1972) Chemical weathering in the temperate glacial environment REYNOLDS, R.C. & JOHNSON, of the North Cascade Mountains. Ceochim. cosmochim Acta, 36, 537-554. RIECK,G.D. & KOOPMANS, K. (1964) Investigations of the disturbed layer of ground quartz. Brit. J. appl. Phys. 15, 419-425. U. ( I 968) Makroskopische und mikroskopische Eigenschaftsanderungen von FestoffSCHNEIDER, pulvern infolge starkermechanischer Beanspruchung in Muhlen. Aufbereit. Technik, 9, 567-573. R.P. (1972) ‘Glacial’ micro textures on quartz and heavy mineral sand SETLOW, L.W. & KARPOVICH, grains from the littoral envronment. J. sedim. Petrol. 42, 864-875. SHEPPS,V.C. (1953) Correlation of the tills of north east Ohio by size analysis. J . sedim. Petrol. 23, 34-48. SMALLEY, I.J. (1966) Formation of quartz sand. Mature, Lotid. 211, 476-479. I.J. & CABRERA, J.G. (1970) The shape and surface texture of loess particles. Bull. geol. Suc. SMALLEY, Am. 81, 1591-1596. I.J., CABRERA, J.G. & HAMMOND, C. (1973) Particle nature in sensitive soils and its relation SMALLEY, to soil structure and geotechnical properties. Paper presented at: Int. Soil Structure Symp., Goteborg, Sweden, 1-2 August 1973. TOVEY,N.K. (1971) A selection of scanning electron micrographs of clays. Report CUED/C-SOILS/ TR5 (I 971), Department of Engineering, University of Cambridge, England. D. (1971) The shape and surface texture of loess particles. Discussion. Bull. geol. Soc. Am. WARNKE, 82, 2357-2360. WHALLEY, W.B. (Unpublished) Investigation of some moraines of valley glaciers in the Swiss Alps. PhD thesis to be submitted, University of Cambridge, England.

(Manuscript received 23 June 1973)