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Rh above the level of the highest flood waters in the river. The gravels hold a considerable amount of water, which rises according to the rain- fall, or in other words, according to the amount of water in the river, and thus basements of houses may be rendered damp.

Looking generally to the relation between the geological structure and the early settlements, we find in Buckinghamshire as in other counties that the question of water supply from river, spring, or well was the natural guide in the fixing of sites. In the deeper Chalk valleys where springs break out and streams flow, along the base of the Chalk escarpment and of the Upper Greensand, on outliers of Lower Greensand and Portland Beds or along their margin, and on the Great Oolite Series, there we find the principal villages and towns. On the clay areas the settlements were fewer and less important, for only where patches of gravelly Drift occur could local supplies of water be readily obtained.