Page:VCH Berkshire 1.djvu/39

GEOLOGY BERKSHIRE is in parts decidedly hilly, but nevertheless much of the high ground takes the form of flat-topped plateaux or rises with a regular and gentle slope ; indeed it might fairly be described as a county of tableland in which long and deep valleys have been carved out. The highest levels are on the ridge of Chalk which crosses the county in a nearly east and west direction from Streatley to Ashbury, it being in places over 800 feet above the sea, but elsewhere the hills and plateaux rise little above the 400 feet contour. The county is naturally divided into three very well marked dis- tricts. They are indicated by colours or groups of colours on the geological map, but are almost as clear on any map where the hills are shaded or the contours marked. The first of these districts forms the northern end of the county, and there a succession of Oolitic and Cretaceous formations cross the county in bands, with an east and west trend approximately parallel to the ridge of high ground already mentioned. These formations consist largely of clay, though there are also sands and prominent limestones. The second comprises the central part of western Berkshire, extending from the Wiltshire border to the Thames, and forming the sides of that river's valley from Wallingford to Reading. The tract included in the bend of the Thames between Twyford and Maidenhead belongs mainly to this district, as also does the ground upon which Windsor Castle stands. The geological formation is Chalk, and it is wholly calcareous. The third district includes the south-eastern end of the county, most of the area south of the Kennet and some tracts west of Reading and north of Newbury. The geological formations belong to the Eocene System, and are composed of clay and sand. The Chalk extends under the whole of these formations, forming a hollow or basin in which they rest, and this is the western end of the London Basin. In this third district therefore formations newer than the Chalk form the surface of the ground, in the second district the Chalk is itself the surface rock, and in the first formations older than the Chalk lie at the surface. Speaking generally, we pass from newer to older geological strata