by Matt Law
Used to describe sediments when discussing site formation processes, the adjective 'aeolian' means wind blown. In the UK, aeolian deposits are most commonly found dating from the Pleistocene, where they were laid down in periglacial tundra-like conditions, and particularly during the Devensian period; although the same processes of erosion and deposition by wind can occur today (English Heritage 2004, 6), especially in coastal dune environments.
Pleistocene aeolian deposits commonly fall into two categories, determined by the dominant particle size:
- Coversands. Coversands are the remnants of dune systems. They are usually formed of moderately well or well sorted sand sized material, comprising predominantly rounded to well rounded particles. If the material originated in a coastal environment, there may be fragments of marine shell present (English Heritage 2004, 7). The process by which coversands form is called saltation. During this process wind picks up exposed sediment - for example from the coast or from glacial outwash - and deposits it as the wind strength subsides. Subsequent gusts may repeat the process, allowing coversands to be transported in stages (Evans 1978, 72).
- Loess. Loess deposits are commonly formed during glacial periods when the sea-level lowered, leaving large areas of fine-grained sediment exposed. They are predominantly formed of silt-sized material, although there may also be a large proportion of clay-sized material. Loess can be carried hundreds or thousands of miles. When loess is re-sorted and deposited by fluvial action (e.g. being carried in floodwaters), it gives rise to brickearth (English Heritage 2004, 7).
Video explaining saltation, by Matt Law
Holocene wind-blown deposits tend to derive from local conditions and only travel comparatively short distances (English Heritage 2004, 8). Sand dunes, for example, are common in most coastal locations where there is low-lying gently sloping terrain. Many dunes in Britain formed during prehistory when there were abundant supplies of sand in shallow coastal seas. These were brought onto land by waves where some of the sand dried enough to be carried by the wind beyond the reach of the sea (Ritchie 2001, 3). As there are no longer abundant supplies of offshore sand, most British dune systems are now subject to erosion.
Aeolian deposits are usually well-sorted, meaning that they are dominated by one particle size (Goldberg & Macphail 2006, 16). Quartz grains in aeolian sands are typically rounded, as aeolian transport is more effective than water in rounding grains (Goldberg and Macphail 2006, 17).
English Heritage, 2004. Geoarchaeology: Using Earth Sciences to Understand the Archaeological Record, Eastney: English Heritage Centre for Archaeology.
Evans, John G., 1978. An Introduction to Environmental Archaeology, London: Elek.
Goldberg, Paul, and Macphail, Richard I., 2006. Practical and Theoretical Geoarchaeology. Oxford: Blackwell
Ritchie, William, 2001. Coastal dunes: resultant dynamic position as a conservational managerial objective. In J.A. Houston, S.E. Edmonson, and P.J. Rooney, eds., Coastal Dune Management: shared experience of European conservation practice. Liverpool: Liverpool University Press. pp. 1-16.
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