A HISTORY OF SURREY Putting aside for the present the consideration of the deep-seated rocks, let us first examine the character and arrangement of the strata which occur at the surface, since it is in these that we shall discover the cause of its present configuration. The geological structure of the county is so simple and its existing features depend so closely upon this structure that it forms an ideal tract for the study of the elementary principles of the science. As indicated by the different colours on the accompanying map, the outcrops of the several geological divisions tend to form bands of varying width running nearly east and west across the county, with the older for- mations in the south and the newer in the north. This arrangement arises from the general northerly dip or inclination of the beds, due to an un- equal uplift of the land in past times by which the southern part of the county has been raised to higher levels than the northern portion. Hence we may walk east and west upon the same formation along the line of ' strike ' from one end of the county to the other, while if we go southward we soon cross to underlying, and if northward to over- lying beds. For our present purpose it is sufficient to note that, as shown in the section accompanying the map, this northerly dip prevails, with some minor irregularities, throughout the greater part of the county. We shall see later that it is dependent upon the presence of an elongated dome of elevation which included the whole country between the North and South Downs, having its axis a little to the southward of our county boundary and extending through Kent and Sussex into the eastern part of Hampshire. The central portion of this dome has been so greatly eroded that it is now for the most part lower than the sides, but at one time the successive belts of strata which now encircle it have extended across it in a flat arch rising many hundreds of feet above the highest ground now existing. The arch has been broken through by long-continued erosion, and as some of the inner or lower strata happen to be of less enduring composition than those by which they were originally covered, the wasting away after the removal of the higher portion has been more rapid towards the interior of the dome than at the sides. Deep borings in Sussex and Kent have proved that immediately be- neath the central portion of the dome there occurs a thick series of marine deposits (Portlandian and Kimeridgian) of Upper Jurassic age ; but these do not reach the surface. The oldest strata actually outcropping within the uplifted tract consist of an estuarine series of shaly clays with thin cal- careous stone bands, and with lenticular beds of gypsum in their deeper portion. These ' Purbeck Beds ' cover a small area in Sussex some miles to the south-eastward of the Surrey boundary. They pass upwards into the ' Hastings Beds,' a thick series of soft sandstones and semi-coherent sands, with intercalated bands of clay, apparently the sediments brought down by a large river into a lake or estuary.
