Field Analysis:Why Stones

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Why record stoniness?
The frequency, size and rounding of stones in natural deposits provides information about the nature and energy of the erosional and depositional environments. In general, the larger and more frequent the stones the higher the energy in the erosional and depositional environments.

Stone shape
Stone shape refers to the degree of rounding and also the sphericity or elongation of the stone. Rounding can result from abrasion processes in water and wind, and the degree of rounding is determined by the composition of the stone and its inherent hardness, by the mode and energy of transport, and by the length of time or distance the stone has been transported. Roundness may also be inherited from the parent material.

Type
The range of stone types in a deposit relative to its parent material can give an indication of the degree of modification of a deposit by human inputs or of potential source areas for sediments. Field identification of stone types is impractical without specialist help, but an idea of the number of different stone types can be recorded. This is particularly useful if you are trying to determine whether you have an in-situ deposit or a dump of material.

Distribution
Bands of stones may indicate surfaces, both natural such as desert pavements and laid floors or high energy depositional events. In top soils heavily mixed by earthworms, small to medium pebbles may also form a band at the base of the mixed ‘horizon’. These worm sorted layers are indicators of bioturbation and can help identify buried soils.

Orientation
If stones have a preferred orientation, i.e. most of the stones long axis are pointing in the same direction, this strongly suggests deposition from a moving body of water, mud or ice. The long axis will usually be aligned along the direction of flow of the glacier, river or mud flow, whilst pebbles in an anthropogenic dump, on a road surface for example, will usually be randomly oriented with no preferred orientation. In some situations pebbles and stones may also show imbrication (overlapping); this should also be recorded because it indicates deposition in moving water and current direction.

A record of stone frequency and stone characteristics can be useful when investigating the nature of the depositional environment and sediment sources, trying to identify buried soils and assessing loss of stratigraphy to bioturbation.

More information about how to record stoniness can be found here.

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