Page:VCH Kent 1.djvu/46

 A HISTORY OF KENT b. Strata Proved in Deep Borings only Period. Formation Character of Material Approximate thickness in feet Upper Jurassic PurbeckBeds. . . Portland Beds. . . Kimeridge Clay. . . Corallian Beds. . . Oxford Clay. . . . Clay, shale and cement-stone with gypsum Sandstone and limestone. . Dark clay and shale, with nodules and bands of cement- stone Coralline and oolitic limestone, etc. Firm grey marly clay. . . Very variable ; frequently ab- sent ; maxi- mum not yet known 14 at Brabourne up to 356 ft. proved, but maximum not yet known up to 305 ft. proved up to 243 ft. proved Middle Jurassic Great Oolite Series. . Chiefly pale oolitic limestone. up to 189 ft. proved Lower Jurassic Upper, Middle and Lower Lias Dark shale, marlstone, etc. up to 173 ft. proved Triassic Trias Pebble-conglomerate ; also 52 ft. red marl and sandstone at Chatham, either Trias or Devonian 48 ft. at Bra- bourne Carboniferous Coal Measures. . . Sandstone, shale and coal seams up to 1 157 ft. proved Devonian ? Doubtful ? Red marl and sandstone of Chatham boring ; see above. Dark slaty shale of Brabourne boring (Devonian or older) up to 52 ft. proved up to 88 ft. proved THE GEOLOGY OF THE SURFACE ROCKS » General Structure. — The predominant factor in the arrangement of the strata forming the surface of Kent is the general northward slope or ' dip ' of the beds already referred to, by reason of which the older rocks are seen only in the more southerly part of the county and the newer only along its northern margin, every formation in turn sinking north- ' The Geology of Kent has a voluminous literature, to which only passing reference can be made in this outline-sketch. For detailed information regarding the stratigraphy of the county the following works should be consulted : — Memoirs of the Geological Survey : ' The Geology of the Neighbourhood of Folke- stone and Rye' (sheet 4 of I -inch map) by F. Drew (1864) ; 'The Geology of the Weald' by W. Topley (1875), for the beds below the base of the Chalk and for matters connected with the valley systems of the We.ild and its denudation ; 'The Geology of the London Basin' by W. Whitaker (1872), for the Chalk and Eocene ; ' The Geology of London and of part of the Thames Valley,' vol. i., by W. Whitaker (1889), for later information respecting the Eocene, for account of the River Drifts and other superficial deposits, and for discussion of the deep-seated geology as then known ; and vol. ii.