Greensand

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Greensand is an olive-green coloured sandstone rock which is commonly found in narrow bands, particularly associated with bands of chalk and clay worldwide; it has been deposited in marine environments at various times during Earth history, such as during the Jurassic and Cretaceous periods.

Formation and Locations

Greensand forms in anoxic marine environments that are rich in organic detritus and low in sedimentary input. Greensands are occasionally fossil-rich, such as in the late Cretaceous deposits of New Jersey.^[1] Important exposures are known from both Great Britain and North America.

In Britain, "greensand" usually refers to a specific geologic formation. In some areas, a distinction is made between Upper Greensand and Lower Greensand; these strata tend to occur just beneath the Chalk and can be separated by a layer of Gault. Greensand is quite common in northern and western Europe. In Great Britain, prominent seams are to be found in the Vale of White Horse, in Bedfordshire, in Kent, Surrey, the South Downs National Park^[2] and Hampshire and the Jurassic Coast in Dorset. In Kent, greensand comes in four layers: the Folkestone Beds 60-250 ft thick; the Sandgate Beds 5-120 ft thick; the Hythe beds 60-350 ft thick and Atherfield Clays 15-50 ft thick.^[3]Thus, a large band of greensand runs under the bowl of the Thames valley, appearing on scarp slopes of the surrounding ranges of hills. The soil of the greensand is quite varied, ranging from fertile to fairly sterile. On the fertile soils we see chestnut and stands of hazel and oak, while Scots Pine and Birch colonise the poorer soils.^[4] These Greensand Ridges are popular long distance walking routes, for instance the Greensand Way in Kent.

Properties and Uses

The green colour of greensand is due to variable amounts of the mineral glauconite, an iron potassium silicate with very low weathering resistance; as a result, greensand tends to be weak and friable. It is a common ingredient in garden fertilisers, such as in organic gardening and organic farming. Due to its chemical exchange properties, the glauconite of greensand is used as a water softener. Greensand coated with manganese oxide, known as manganese greensand, is used in well water treatment systems to remove insoluble ferric (oxidized) iron and manganese. It is also used as a type of rock for stone walls in areas where ever greensand is common.

References

1. ^ Dale A. Russell, An Odyssey in Time (Toronto: University of Toronto Press, 1989), pp. 137-9.
2. ^ http://www.southdowns.gov.uk/content/page/1249/LCA_N.pdf
3. ^ Britain's Structure and Scenery, L.Dudley Stamp., Pub Sept 1946, Collins New Naturalist Series.
4. ^ Greensand Way in Kent, 1992, Kent Count Council, ISBN 1-873010-23-0