A HISTORY OF BERKSHIRE as we travel from the south-east towards the north-west, and we find the oldest, the Oxford Clay, forming a strip of low land along the banks of the Isis from the Cole to the Thames near Oxford. The Oxford Clay is not however a very ancient formation if looked at from a geological point of view ; it belongs only to the middle part of the Jurassic Series, and it and all the other strata which form the surface of Berkshire and extend to a depth of many hundreds of feet rest upon a platform of very much older rock. What these older rocks are is not certainly known. A boring at Burford Signet, 13 miles west of Berkshire, reached the Rhastic Beds at a depth of 717 feet and the Coal Measures at a depth of 1,184 feet, 1 and a boring at Richmond in Surrey, 1 2 miles east of Berkshire, reached rocks which were pro- bably New Red Sandstone at a depth of 1,239 feet below the surface ; * so it may be assumed that the platform of old rock, New Red Sandstone with possibly Coal Measures, etc., lies somewhat over 1,000 feet beneath the surface of Berkshire. Resting on these old rocks are probably representatives of the Lias and Lower Oolites, for the former is believed to have been reached in a boring at Wytham near Oxford, and rock of Lower Oolitic age was found in the Richmond boring. 3 There is however very little evidence on these points, so we pass on to consider the formations which are found at the surface of the ground ; but perhaps it may be as well to point out that these forma- tions, which are coloured on the geological map in this volume, are in fact very often hidden from view by beds of gravel, sand, clay, etc., which in places attain a thickness of many feet and are included under the general term ' Drift.' They will be dealt with after the formations indicated on the map have been described. One of the recent deposits, the Alluvium which accumulates along our rivers, is, it will be seen, marked upon the map. The geological history of Berkshire may be said to open in the period .of the Oxford Clay. At that time the sea extended over the whole county and also over nearly all England, though there was probably land to the west in Cornwall, Wales, etc., and perhaps also to the east from east Norfolk to east Kent. This submergence continued through succeeding periods, though in Portlandian times the land seems to have closed in on the north, and eventually the Purbeck continent arose and separated the northern or Aquilonian from the southern or Tithonic sea. Probably the whole of Berkshire then became land, and so it continued through the Purbeck and Wealden periods. This change in the distribution of land and sea was due to great earth movements which eventually resulted in a considerable folding of the Oolitic strata. 1 H. B. Woodward, 'Jurassic Rocks of Britain,' Geol. Survey, iv. 303. 2 J. W. Judd and C. Homersham, Quart. Journ. Geol. Soc. (1884) xl. 724, (1885) xli. 523. Compare fig. 22 (p. 44) and fig. 145 (p. 299) in 'The Jurassic Rocks of Britain,' Geol. Survey, vol. v. (1895).
