Forensic geology - Sand and gravel

(a work in progress)

The term sand can be used loosely to mean a quartz-rich fine unconsolidated sediment - like we find on a sandy beach. It can also mean a medium to coarse grained un-consolidated sediment (sand and gravel). But is used scientifically to mean a sediment with a grain size of between 2mm and 63um, containing mineral clasts, rock fragments and sometimes fossils.

Sand can form or be deposited in several different ancient and modern environments - deep water marine, marine beaches, coastal sand dunes, estuaries, rivers, streams, fresh water lakes, and deserts. It can also be subsequently transported and use by man - builders sand. And even re-used after that in recycled aggregates.

By studying a sample of sand we can accurately quantify its properties which will enable us to compare it with other samples, obtains clues about the environment it was deposited in and perhaps help us to locate its origin.

Factors that affect the composition of the sand are:-

• The composition of the parent rock or rocks - the minerals originally present.
• The weathering of the parent rocks - resistance of some minerals to weathering
• Distance from the source - how far the sediment had travelled from the parent rocks.
• Mode of transport - wind, water, ice.
• Speed of transport agent - affects sorting.
• Depositional environment - sorting, flora and fauna.

Clast type

Mineral fragments, rock fragments, microfossils, broken fossils, minerals formed at that site. Identify the clasts and record the percentage of each type. Have the rock fragments been derived from a nearby source?

Clast size and Sorting of clasts

Size is measured against a standard scale. This can be done by putting the sample through a series of standard sieves and weighing the fractions. A "well sorted" sand will have little variation in grain size; a "poorly sorted" sand will show a greater variety of grain sizes.

Clast shape

Are the grains angular, sub-angular, rounded or well-rounded? Are they oval or spherical?

Clast surface

Is the surface rough or polished?

Sedimentary structures

Were there any sedimentary structures to be seen in situ when the sample was collected? Was the bed of sediment homogeneous, or was it coarser at the base or top? Was the bedding horizontal or was there evidence of cross-bedding (caused by ripples)?

Fossils and microfossils

Are there any shells or microfossils present in the sample? Can you identify them? What do they tell you about the environment? What can they tell you about the geological age of the sample?

Trace fossils