The resent period of lower than average rainfali in the United Kingdom, together with changes in commercial factors and land use policy have been contributory aspects to an increased rate of the construction of water reservoirs for agricultural use in Eastern England. In this area there are large areas with chalk bedrock and glacially or fluvioglacially derived superficial deposits. The chalk is often modified or peri-glaciaHy weathered and the superficial deposits vary between glacial tili

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... a permeability of l0 cm s 1 and fluvial sands with a permeability of 102cm 51 Reservoirs constructed with an artificial liner are expensive compared with a structure with a natural mineral lineri Consequently sites with thick, laterally consistent, impernieable superficial deposits or outcropping day strata are preferred.