Page:VCH Surrey 1.djvu/51

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GEOLOGY rolled black flint pebbles, thin layers and nodules of iron-sandstone, and concretionary masses of hard siliceous sandstone. The last-mentioned material is also developed abundantly in the Thanet Sands and Woolwich and Reading Beds ; and from its superior durability large blocks, popularly termed ' grey-wethers,' are frequently strewn over the surface when the remaining portion of the bed of which they formed part has been wasted away. The thickness of the Lower Bagshot division varies from 100 to 150 feet. Its outcrop corresponds in shape to that of the London Clay, being similarly contracted to a narrow belt between the western border and the valley of the Wey and then broadly expanded as it sweeps round northward on both sides of that river up to its junction with the Thames, whence it returns westward to the county boundary in the neighbourhood of Virginia Water and Egham. Many detached ' outliers ' also occur on the London Clay beyond the limits of the main outcrop. The middle division of the Bagshot series, or ' Bracklesham Beds ' as they are usually called, from a place on the coast of Sussex where beds supposed to be of the same age are typically exposed, consists in Surrey of laminated clays and sands (sometimes containing a little lignite) from 40 to 60 feet thick. The sands are often of a dark-green colour, and occasionally include layers of flint pebbles. The clayey character of this division serves to distinguish it from the other portions of the series ; the water percolating through the porous overlying beds is thrown out along its outcrop in a line of small springs and boggy patches. The soil also is less sterile than on other parts of the Bagshot Beds, on which as a rule the proportion of uncultivated common is great. The outcrop of the Bracklesham Beds in Surrey occupies a broad much-indented tract between the valleys of the Blackwater and the Wey, lying within the crescent-shaped rim of Lower Bagshots. Outliers are also found between the Wey and the Mole, and again to the north of the main mass. The Upper Bagshot Beds of Surrey consist of light-yellow or ochreous sands, from 120 to 300 feet in thickness, which form for the most part dry barren land covered with heath and in places with fir, occurring only in a limited tract near the western border within the bounds of the Bracklesham Beds. Their most continuous development in the county is in the Chobham Ridges, extending thence southward into Fox Hills and Ash Common ; a large outlier occurs on Pirbright Common to the east of Pirbright, and a northerly chain of smaller outliers to the north of Bagshot and Chobham. They contain few pebbles, but many ferruginous concretions which sometimes exhibit the casts of marine shells. The ' grey-wether ' sandstone of these sands has already been mentioned. With these strata our survey of the materials which have gone to the making of the county is nearly concluded. Up to this time we have concentrated our attention upon the piling up of masses of strata by the accumulation of matter removed from other areas and deposited in our i 17 c