Structure Of Dunes Research Articles

The sedimentary structure of linear sand dunes

Linear sand dunes--dunes that extend parallel to each other rather than in star-like or crescentic forms--are the most abundant type of desert sand dune. But because their development and their internal structure are poorly understood, they are rarely recognized in the rock record. Models of linear dune development have not been able to take into account the sub-surface structure of existing dunes, but have relied instead either on the extrapolation of short-term measurements of winds and sediment transport or on observations of near-surface internal sedimentary structures. From such studies, it has not been clear if linear dunes can migrate laterally. Here we present images produced by ground penetrating radar showing the three-dimensional sedimentary structure of a linear dune in the Namib sand sea, where some of the world's largest linear dunes are situated. These profiles show clear evidence for lateral migration in a linear dune. Moreover, the migration of a sinuous crest-line along the dune produces divergent sets of cross-stratification, which can become stacked as the dune height increases, and large linear dunes can support superimposed dunes that produce stacked sets of trough cross-stratification. These clear structural signatures of linear dunes should facilitate their recognition in geological records.

The morphodynamics of fluvial sand dunes in the River Rhine, near Mainz, Germany. I. Sedimentology and morphology

The dynamics of large isolated sand dunes moving across a gravel lag layer were studied in a supply‐limited reach of the River Rhine, Germany. Bed sediments, dune geometry, bedform migration rates and the internal structure of dunes are considered in this paper. Hydrodynamic and sediment transport data are considered in a companion paper. The pebbles and cobbles (D50 of 10 mm) of the flat lag layer are rarely entrained. Dunes consist of well‐sorted medium to coarse sand (D50 of 0·9 mm). Small pebbles move over the dunes by ‘overpassing’, but there is a degree of size and shape selectivity. Populations of ripples in sand (D50 < 0·6 mm), and small and large dunes are separated by distinct breaks in the bedform length data in the regions of 0·7–1 m and 5–10 m. Ripples and small dunes may have sinuous crestlines but primarily exhibit two‐dimensional planforms. In contrast, large dunes are primarily three‐dimensional barchanoid forms. Ripples on the backs of small dunes rarely develop to maximum steepness. Small dunes may achieve an equilibrium geometry, either on the gravel bed or as secondary dunes within the boundary layer on the stoss side of large dunes. Secondary dunes frequently develop a humpback profile as they migrate across the upper stoss slope of large dunes, diminishing in height but increasing in length as they traverse the crestal region. However, secondary dunes more than 5 m in length are rare. The dearth of equilibrium ripples and long secondary dunes is probably related to the limited excursion length available for bedform development on the parent bedforms. Large dunes with lengths between 20 m and 100 m do not approach an equilibrium geometry. A depth limitation rather than a sediment supply limitation is the primary control on dune height; dunes rarely exceed 1 m high in water depths of ≈4 m. Dune celerity increases as a function of the mean flow velocity squared, but this general relationship obscures more subtle morphodynamics. During rising river stage, dunes tend to grow in height owing to crestal accumulation, which slows downstream progression and steepens the dune form. During steady or falling stage, an extended crestal platform develops in association with a rapid downstream migration of the lee side and a reduction in dune height. These diminishing dunes actually increase in unit volume by a process of increased leeside accumulation fed by secondary dunes moving past a stalled stoss toe. A six‐stage model of dune growth and diminution is proposed to explain variations in observed morphology. The model demonstrates how the development of an internal boundary layer and the interaction of the water surface with the crests of these bedload‐dominated dunes can result in dunes characterized by gentle lee sides with weak flow separation. This finding is significant, as other studies of dunes in large rivers have attributed this morphological response to a predominance of suspended load transport.

Development and age structure of Holocene coastal sand dunes at Horn Head, near Dunfanaghy, Co Donegal, Ireland

Two areas of Holocene coastal sand dunes on the western side of the Horn Head peninsula extend from low ground immediately inland of their respective source beaches onto flanking higher ground (> 100 m OD). Stratigraphic investigations supported by 14C dating of buried organic-rich horizons and shells demonstrate several phases of sand accumulation in the Pollaguill-Croaghadara system in contrast to the larger Tramore-Anloge Hill system, where the available evidence points to relatively recent dune emplacement. A broadly synchronous phase of dune development occurred during the thirteenth to fourteenth centuries AD and is viewed as part of a regional (northwestern Europe) dune-building episode resulting from climatic deterio ration. Earlier phases (pre-3300-2800 14C years BP) of aeolian sand deposition at Pollaguill-Croaghadara have no apparent equivalents at Tramore-Anloge Hill and repeated fluvial incision and sand reworking during the last 2000 14C years is also evident at the former site. Dated organic horizons, in combination with site location, sand source and transport direction, suggest the earliest phase of aeolian sand accumulation and subsequent stabilization was time-transgressive. The respective roles of relative sea-level change, climatic change and anthropogenic influence in dune evolution are difficult to evaluate in the absence of detailed information, although documentary evidence indicates human-induced destabilization of the Tramore-Anloge Hill dunes during the 1920s and 1930s. Thus, despite similar topographic settings and close proximity, the dunes do not share entirely similar histories and caution against the development of regional dune chronologies from investi gations of single systems. The observations and data suggest that a mix of local and regional controls have combined in the construction of these dunes.

Internal structure of aeolian dunes in Abu Dhabi determined using ground‐penetrating radar

ABSTRACTA ground‐penetrating radar survey of aeolian dunes in the Al Liwa area of Abu Dhabi reveals a variety of dipping reflectors which are interpreted as primary sedimentary structures. The interpretation of the radar profiles has been confirmed by bulldozing trenches through the study area and comparing logged sections in the trenches with the radar profiles. NNW— SSE‐orientated radar profiles, approximately parallel to the prevailing wind direction, show two sets of dipping reflectors which are interpreted as sets of cross‐stratification and second‐ and third‐order bounding surfaces. Radar profiles orientated WSW—ENE across the prevailing wind direction are dominated by concave‐up reflectors which are interpreted as trough‐shaped scours and sets of trough cross‐stratification produced by oblique progradation of barchanoid dunes. Nested troughs, with small sets of trough cross‐stratification within larger troughs, may be due to reactivation following wind reversal, or the superposition of small dunes on larger dunes and the fill of large dune troughs by smaller dunes. Convex‐upwards reflectors are interpreted as linear spurs on the convex portions of sinuous dunes or erosional remnants between troughs. Overall there is a tendency for the larger second‐order bounding surfaces to dip downwind, which confirms Brookfield's ideas of the relative migration paths of dunes and draa.

Features of landscape structure of the coastal dunes of the East Caspian Sea

A band of coastal dunes on the eastern shore of the Caspian Sea is found in the Middle Caspian, where plateaux of the Turan platform (Mangyshlak and Kendirli-Koyasan) come up to the shore, and in the South Caspian, bordering with the West-Turkmenian lowland, situated in the inter-mountain depression of alpine folding. Coastal dunes of the Middle and South Caspian were created like barrier islands, which attached to the shore during sea-level fall and separated lagoons from the sea. Later, these lagoons were divided into small salt lakes which dried out and changed into solonchaks. In spite of the general method of development of coastal dunes, there are some differences in the landscape structure of the dunes of the plateau and those of the lowland. For example, on the shore of the plateau the main shore developing fluctuations are waves, whereas on the shore of the lowland, wind-induced surges and, to a lesser extent waves, are the driving force. The elevation of the coastal dunes is higher and their surface is wider on the lowland shore. The surface of dry solonchaks is also higher here. On the shore of the plateau rapid sea-level rise causes destruction of old patches and the creation of new ones. On the shore of the lowland this leads to transformation of old patches, located in the zone of sea/land interaction.

Population Structure of a Sand Dune—Obligate Beetle, Eusattus muricatus, and Its Implications for Dune Management

The approximately 45 unconsolidated sand dunes of the Great Basin and Mojave Deserts are unique insular habitats in that they are generally spatially dynamic, small in size, and of recent origin (Smith 1982). The dunes support diverse sand-obligate biotas that include mammals (Brown 1973), plants (Pavlick 1985, 1989), and insects (Hardy & Andrews 1976, 1979; Andrews et al. 1979; Bechtel et al. 1981,1983; Rust 1985, 1986). Several duneobligate species are candidates for protection under the U.S. Endangered Species Act (Federal Register 1994). Most sand dunes in the Basin and Range Province are associated with pluvial lake basins (such as Sand Mountain, Little Sahara Dunes, and Kelso, Ibex, Dumont, Big, Eureka Dunes) or major delta areas (such as Algodones and Gran Desierto Dunes) (Morrison 1964, 199 1a, 199 1b; Smith 1982; Lancaster 1988a, 1988b, 1989; Dohrenwend et al. 1991). The dunes resulted from sand deposition during the pluvial-glacial periods of the PleistoceneHolocene, followed by eolian transport of these sands to their present locations (Bagnold 1941; Morrison 1964, 1991a; Smith 1982; Benson et al. 1990; Eismann 1990; Dohrenwend et al. 1991). Sand deposited in this manner can be stabilized through plant growth or remain unconsolidated, forming dunes. Our study organism, Eusattus muricatus, is one of the most common dune-obligate tenebrionid beetles in western North America (Hardy & Andrews 1976, 1979; Andrews et al. 1979; Bechtel et al. 1983; Rust 1985, 1986). It is flightless, fossorial, and restricted to very sandy, friable substrates, especially eolian dunes (Doyen 1984). We used starch-gel electrophoresis to elucidate the

International borders and range ecology: The case of Bedouin transborder grazing

Recent research suggests that, under unconstrained human circumstances, pastoral nomads within arid environments have at their disposal means of evading ecological stress that could impel them to cause damage to their grazing and land resources. The Israeli-Egyptian border has imposed a severe constraint upon the range management strategy of the Bedouin whose traditional territory it bisects. The border forced them to exert an increased pressure upon local resources. Considerable damage was thus caused to the perennial vegetation cover (both macrophytes and microphytes) and to the structure of sand dunes on the Egyptian side of the border, with opposite effects on the Israeli side to which the Bedouin had no access. This case study adds a further dimension to the discussion of range management by pastoral nomads in arid and semi-arid areas.

Particle packing from an earth science viewpoint

Particle packings are relevant to many aspects of the Earth sciences, and there is a long history of the study of packings from an Earth science viewpoint. Packings have also been studied in connection with other subjects and disciplines. Allen (1982) produced a major review which provides a solid base for Earth science related studies. This review complements Allen's work and in particular focuses on advances in the study of random packings over the last ten years. Transitions from packing to packing may be as important as the packings themselves, and possibly easier to model. This paper places emphasis on certain neglected works, in particular Morrow and Graves (1969) and the packing transition envelope, Kahn (1956) and the measurement of packing parameters, Griffiths (1962) on packings in one-dimension, and Getis and Boots (1978) on packings in two dimensions. Certain packing problems are relevant to current areas of study including structure collapse in loess (hydroconsolidation), flowslides in very sensitive soils, wind erosion, jewel quality in opals and the structure and functions of sand dunes. The region where interparticle forces become active (particles < 200 μm) is considered and the implications for packing are examined.

Assessment of sand dune change detection in Rajasthan (Thar) Desert, India

Abstract Landsat MSS digital data provide an important means for the study of sand dune morphology and distribution, land cover change and desertification monitoring. Two images, one each of Landsat-l and Landsat-5 MSS were used to determine sand dunes, their change and desertification by human activities from 1973 to 1986. Five dune classes were observed: parabolic, barchan, longitudinal, transverse and sand sheet. Parabolic dunes and sand sheet are widely distributed and are dominant classes in this area. Other classes of land cover observed were canal, saline area, salt lake and protected plantation. Various changes in dune structure are observed due to human activities followed by wind erosion which indicates that desertification is enhancing severely. But there was no major shift in previous dune boundaries from 1973 to 1986 in the study area. Excavation of parabolic dunes for a canal also affected the dune patterns in the Bikaner district. Recent salinization in the vicinity of the Indira Ghandhi Ca...

Morphology, Internal Structure, and Reversal of Asymmetry of Large Subtidal Dunes in the Entrance to Gironde Estuary (France)

ABSTRACT The Gironde is a macrotidal estuary characterized by important changes in tidal-current velocity and river discharge. Two surveys were made in the lower estuary in June 1987 and October 1989 in an area with large dunes. The symmetry of these dunes (1.5-6.7 m high) changes from flood-dominated (in upstream areas) to ebb-dominated (in downstream areas). A transition zone consists of symmetrical dunes with a very low vertical form index. The internal structure of the dunes, revealed by seismic records, shows a hierarchy of bounding surfaces related to fluctuations in movement speed and asymmetry of the dunes. The lowest part of symmetrical dunes consists of ebb-oriented reflectors, while the upper part has flood-oriented reflectors. The same profile lines run in 1989 show that each large dune can be identified, despite an upstream shift of the transition zone of about 1000 m. Comparison of seismic records shows that some lower reflectors are preserved between the two surveys, and that the dunes moved downstream a mean distance of 30 m. In 1989 the internal structure of symmetrical dunes was reversed, with flood-oriented reflectors in the lower part overlain by ebb-oriented reflectors. Both seasonal changes in river discharge and fortnightly oscillations of the tidal range control the magnitude and orientation of the net bed-load transport in the study area. Because the response time of the large dunes is longer than that of the superimposed small dunes, the reversal of asymmetry of the large dunes requires a long-period process, while fortnightly oscillation or even semi iurnal reversal of the tidal current (during spring tides) could cause reversal of small dunes.

Flow separation and the internal structure of shadow dunes

In the sand-undersaturated aeolian terrains of southern Kuwait the only dune bedforms are nebkhas and shadow dunes, both of which are confined to the surfaces of evaporitic pans. Shadow dunes are aerodynamic bedforms with a quasi-pyramid profile and caused by horizontal flow separation to the lee of impermeable barriers (the nebkhas). Their internal structure consists of: (a) bipolar azimuths and bimodal dip distributions of tabular-planar grainfall cross-strata with a chevron-like pattern in the dune crest, and is formed by variable secondary winds, and (b) alternating reactivation surfaces probably representing third-order bounding planes on each slip-face. This structure is developed in an essentially prevailing, unidirectional wind field with minor fluctuations about a vector mean. With a rising water table, causing sand sheets to migrate over and cover the evaporitic pans, these cross-strata could be preserved. In ancient sequences, the internal geometry of shadow dunes could be of potential value in estimating regional palaeowind directions and in palaeogeographic reconstructions of sand-starved desert terrains.

Development of hybrid aeolian dunes: the William River dune field, northwest Saskatchewan, Canada

Observations have been made on the structure, morphology, and pattern of sand movement on large-scale, roughly elongate, northwest–southeast-aligned aeolian sand dunes in a desert area of northern Saskatchewan, Canada. Transverse profiles show steeper northeast flanks, the lower parts of which are covered with psammophilous grasses. Dune structure is dominated by northeast-dipping accretion laminae, and 14C dates of organic material trapped between such exposed laminae on the southwest flank indicate migration to the northeast at about 0.5 m/year in the last few centuries. On the other hand, there is a progressive increase in height, bulk, symmetry, and peakedness of the dunes from northwest to southeast, suggestive of substantial along-dune sand movement. The present-day wind regime shows a potential resultant sand-transport vector to the southeast, virtually parallel to the dune axis; winds from the north-northeast and northeast dominate the first 6 months of the year, followed by winds from the west-southwest in the latter half. Field evidence of airflow and sand-movement patterns upon the surfaces of two dunes also indicates a strong along-dune component. The dunes are interpreted as hybrid landforms reflecting both transverse and longitudinal processes acting at the present time. Of particular sedimentological significance is the discordance between dune stratigraphy and the wind regime. Dune structure is controlled by a southwest–northeast imbalance in sand movement assumed to result from an asymmetric distribution of sand-trapping vegetation and from a seasonal contrast in sand mobility that partly correlates with seasonality in the wind regime. Both factors promote northeast migration normal to the potential resultant of effective winds.

Hybrid Eolian Dunes of William River Dune Field, Northern Saskatchewan, Canada: ABSTRACT

A series of northwest-southeast aligned, large-scale (up to 30 m high) eolian dunes, occurring in a confined (600 km2) desert area in northern Saskatchewan, Canada, was examined in the field. Observations were made of dune morphology and internal structure, and patterns of sand movement on the dunes were analyzed in relation to wind events during the summer of 1981. Present cross-sectional profiles exhibit steeper northeast slopes, the lower segment of which are intermittently covered by psammophilous grasses. Dune structure is dominated by northeast-dipping accretion laminae. Three 14C dates from organic material cropping out on the lower southwest slopes reveal that the dunes have migrated as transverse bed forms at rates of roughly 0.5 m/yr during the last few hundred years. However, a progressive increase in height, bulk, and symmetry along the dune axis from northwest to southeast, suggests an along-dune component of sand transport. This view is supported by (1) field measurements of airflow and along-dune sand transport patterns on 2 dunes, and (2) the present-day wind regime (1963-78). Dominated by north-northeast to northeast w nds from January to June and by west-southwest winds from July to December, the resultant potential sand transport vector is toward the southeast, virtually identical to the dune axis. The dunes are viewed as a hybrid type, forming in response to a combination of transverse and longitudinal processes and are probably not uncommon in many deserts. The discordance between the dune structure End_Page 242------------------------------ (and migration direction) and the resultant effective wind vector may have significant implications for paleowind analysis in the geologic record. In this instance, the structure reflects only west-southwest (July-December) winds, owing to a southwest-northeast imbalance in sand movement. This imbalance is due mainly to the effect of sand-trapping by vegetation on northeast slopes, as well as to seasonal differences in sand mobility, the latter correlating partly with the semi-annual shift in the wind regime. End_of_Article - Last_Page 243------------

Internal Structure and Surface Geometry of Longitudinal (Seif) Dunes

ABSTRACT A mutual relationship exists between the morphology of longitudinal (seif) dunes and their dynamics and internal structure. The basic mechanism of longitudinal dune movement results from the deflection of oblique wind flow into a longitudinal flow on the lee flank. The magnitude of the longitudinal flow depends on the angle of the wind incidence with the crest line. Deposition occurs when this angle of incidence increases through meandering of the longitudinal dune crest line. After heavy rain exposed the internal structure of the dune, two kinds of depositional unit were found: the main unit forms in the meandering area on both sides of the dune; the other, secondary unit is a narrow belt on both sides and parallel to the crest line. The two units have different strikes. Because the laminae deposited on the dune are of the three primary stratification types--climbing translatent strata, sand flow cross-strata, and grainfall laminae--the range of dips is wide (10° to 30°). These dips accord with topographic profiles across the dune. A model of the internal structure of a longitudinal dune has been developed from the field data. The model shows the erosional and depositional areas along this linear dune.

Factors Affecting the Lithification of Eolianite and Interpretation of its Environmental Significance in the Coastal Plain of Israel

ABSTRACT Analyses of carbonate content in a partially consolidated eolianite showed a highly significant correlation between the amount of carbonates in the rock and the degree of consolidation. At least 8% CaCO3 is required for the cementation to proceed. The cementation results from wetting and drying cycles and proceeds from the upper surface of the dune downwards, through the percolation of ram water and the recrystallization of the dissolved CaCO3. Lamination in the dune is a result of particle size differentiation during ripple formation; the calcareous skeletal fragments being coarser grained than quartz sand accumulate on the crest of the ripples. The preservation of the laminated dune structure with alternating laminae of smaller and larger amounts of carbonates supports Friedman's (1964) observation that the reprecipitation during cementation takes place adjacent to the place of dissolution. The environmental analysis of the factors controlling the formation of eolianite suggests that the correlation of the eolianite ridges with definite climatic phases of the Quaternary is not warranted unless supported by independent evidence. All the eolianites and associated sediments and soils of the coastal plain of Israel were formed under seasonally wet and dry climatic conditions that were not much different from those prevailing today. Because of local factors and disturbances, the sedimentary cycles of the coastal plain were not necessarily time equivalent to the major cyclic events of the Quaternary.

STRUCTURES OF DUNES AT WHITE SANDS NATIONAL MONUMENT, NEW MEXICO (AND A COMPARISON WITH STRUCTURES OF DUNES FROM OTHER SELECTED AREAS)1

SUMMARYThe type, scale, and relative abundance of sedimentary structures in four kinds of dunes at White Sands National Monument, New Mexico, were determined by examination of vertical sections on walls of trenches cut through the dunes both in a windward direction and at right angles to this direction. Analysis of cross‐stratification in all dunes examined indicated certain common features: sets of cross‐strata mostly are medium‐ to large‐scale; nearly all laminae dip downwind at high angles (not uncommonly at 30°‐34°); most bounding surfaces between sets of cross‐strata are nearly horizontal on the upwind side, but have progressively steeper dips to lee, downwind; and individual sets of cross‐strata tend to be thinner and the laminae flatter near the top than at the bottom of a dune in vertical section.Sparse but distinctive structural features that are characteristic of the four types of dunes are varieties of contorted bedding, rare ripple laminae, and either local scour‐and‐fill bedding, or festoon bedding. Other structures, apparently limited to either one or two types of dunes, are the concave‐downward foresets in some parabolic dunes; the low‐angle reverse dips of upwind strata on high transverse dunes; and the almost horizontal laminae which represent apparent dip in sections normal to wind direction in dome‐shaped and transverse dunes.Describing cross‐stratification in terms of three dimensions, dune structure at White Sands consists dominantly of the tabular planar sets, with units thickest near the dune base, thinner above. To a lesser extent the sets are of simple (non‐erosion) tabular form and relatively uncommonly, of the trough type. Wedge planar forms are scarce. The planar forms characteristically are of two classes in nearly equal proportions: those in which bounding surfaces are virtually horizontal and those in which they dip at moderate to high degree.A brief comparison is made between the structures of dunes that are characteristic of one effective wind direction, as at White Sands, and certain others formed by winds of two or more directions. Seif dunes of Libya, reversing dunes of the San Luis Valley, Colorado, and star dunes in Saudi Arabia are discussed as examples of complex dunes formed by multi‐directional winds.

A RECONNAISSANCE CLASSIFICATION OF THE SOILS OF THE SOUTH TURKANA DESERT

Summary Development of the Soil Pattern As an outcome of an ecological reconnaissance this account provides a record of the occurrences of two major regional types, a system of consolidated sand dunes and derived sands, referred to here as the Chestnut Sands, and a wide assortment of stony truncated ‘desert pavement’ soils or soil remnants described as Stone Mantles. The latter involve areas both of basement complex and of lava origin. Briefer observations are also given on other soils of lesser extent, including flash‐flood, alluvial and lacustrine deposits.Although a present‐day desert or semi‐desert, the south Turkana region bears the stamp of successive erosion cycles, which have largely influenced its soil pattern. Modified remnants of the original end‐Tertiary surface are probably discernible in the higher‐lying stone mantles about the central hill ranges (Section 2, Introd.). Residual stone mantles of this class appear to be the last truncated remnants of an ancient soil cover. Ancient alluvial and lacustrine deposits, the former of conjectured and the latter of established Pleistocene age, have provided the material for other stone mantle areas (Section 2 (b)). The present surface of all these has been affected by more recent erosion and deflation.On top of these several types of stone mantle, or the stony soils from which they originated, seems to have been deposited the deep mantle of the Chestnut Sands, blanketing at one time the greater part of the Turkana plain. These dune sands are regarded as a mainly aeolian drift, deposited during a period of active arid erosion, though their materials may have been derived from older surfaces (Section I, Introd.). They must be subsequent to the deposits which they overlie, and may possibly be compared with the Goz or Kordofan Sands of the Sudan, which have been very tentatively referred to the late Pleistocene (Andrew, 1948).As with the Goz Sands, the dunes of the Chestnut Sands presumably became fixed under less arid conditions. Finally, in common with the rest of the Turkanalandscape, they have been subjected to the present régime of brief seasonal flash‐flood erosion and intervening wind action. Their formerly extensive dune structure has been greatly modified and from many areas they appear to have been removed wholesale. The redeposited Chestnut Sands, the flash‐flood deposits and sandy riverine alluvia are the outcome of this latest phase of erosion. At the same time tracts of comparatively recent lacustrine sands and possible deltaic silts (Section 4 (b) have been left in the east of the region as a result of the progressive retreat of Lake Rudolf.The influence of the resulting soil pattern on the vegetation of the region is very briefly indicated, with reference more particularly to the principal shrub components.

Grain Structure of Sand Dunes and its Relation to their Water Content

THE stratification of wind-accumulated sand beds is well-known, and the evidence of it can often be seen on the vertical faces of sand-pits. Beadnell1 has shown that the laminar structure of fresh non-cohesive dune sand can also be observed, if the dry dune is soaked with water to allow a vertical face to be exposed without collapse. The strata he thus exposed dipped at the angle of repose of sand, which experiments show to lie between 30½° and 35¾° to the horizontal2. In this case the succeeding layers have been formed by sand which, having been driven over the surface of the dune, has come to rest temporarily sheltered on the upper part of the slope, and has accumulated there, increasing the angle of the slope until shear occurs. An avalanche then takes place down the slope from top to bottom (Fig. 1). Fresh sand deposits formed in this way are characterized by their loose packing and by their extreme softness. I have called3 such a mode of deposition 'encroachment'.