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GEOLOGY TABLE OF THE GEOLOGICAL FORMATIONS OF BERKSHIRE Period Formation Character of the strata Approximate thickness in feet Recent to Alluvium Silt clay marl, peat 2O Neolithic Brickearth Loam IO Pleistocene, Palaeolithic, Valley Gravel .... Gravel, mainly flint, both sub-angu- lar and pebbles and sand . to 30 and of doubtful Plateau Gravel .... Gravel and sand, with ferruginous conglomerate .... to 2O age Clay with Flints . . . Pebble Gravel .... Clay, flints and pebbles .... Gravel, mainly flint pebbles . . . to 20 IO Barton or Upper Bagshot Bracklesham or Middle Bagshot Light coloured sand Sand, clay, pebbles ; much green coloured sand 2OO 45 Eocene Lower Bagshot Beds . London Clay .... Reading Beds .... Yellow sand, some clay .... Blue clay, with Septaria Sand and pebbles at the base Mottled clay, grey clay, pebbles and sand IOO 52 to 3491 70 to oo Upper Chalk .... Chalk with flints, the Chalk Rock at the bottom 720 Upper Cretaceous Middle Chalk .... Lower Chalk .... Chalk with very few flints, the Melbourn Rock at the bottom . Chalk and Chalk Marl with no flints 170 215 Upper Greensand . Gault .... Malmstone, green sandy marl . 90 22O Lower Cretaceous Lower Greensand . Sand with ironstone, chert, pebbly gravel and calcareous sponge gravel to 60 Upper Oolites Portland Beds .... Kimeridge Clay Pebbly limestone, sand .... Clay and shale, with septaria and nodules of earthy limestone . to 20 140 Middle Corallian Limestone coral rag sand and clay 50 to 80 Oolites Oxford Clay Clay with septaria 4. CO In course of time a subsidence of the land took place and the sea again spread over Berkshire, and in so doing no doubt effected consider- able destruction of the older oolitic beds and apparently removed all land or freshwater deposits which may have accumulated during the time of emergence. The result is that our next formation, the Lower Green- sand, rests upon the denuded folds of the Oolite in an unconformable manner. Our Lower Greensand is a marine formation, but there was probably land in the western counties, Wales, etc., and also in Kent and Belgium. 3