Page:VCH Surrey 1.djvu/47

 GEOLOGY of surface waters ; and the Clay-with-flints represents the insoluble residuum of the rock thus dissolved, often with the addition of a little detritus from the Tertiary strata which have once overlain the Chalk. 1 From this cause many parts of the Chalk Downs possess a deep loamy soil, and are either enclosed and cultivated, or support a thick wood- land growth, the beech, yew and ash all thriving well in these tracts. The Chalk is of economic consequence as an inexhaustible source of lime, and perhaps still more on account of its valuable properties as a water-bearing formation. The rain which falls on its surface is rapidly absorbed and stored in the pores of the rock and in the innumerable cracks or joints by which it is traversed, so that at low levels the Chalk is in a state of saturation, and yields a copious supply of water when wells are sunk, besides feeding the springs which are thrown out along the base of its escarpment, and in other places where the surface falls below the plane of saturation. Between the deposition of the uppermost part of the Chalk in Surrey and that of the lowermost Tertiary beds now overlying it there must have been a long lapse of time. Not only do we find that the physical conditions of the area were completely changed during this interval, but also that a new group of life-forms were developed, so that the species of fossils in the Tertiary deposits are quite different from those in the Chalk. In both respects the Chalk bears witness to a re- mote past in which there was no approximation towards the present conditions, while the Eocene strata dimly foreshadow the existing state notwithstanding the great cycles of change which had still to pass over our country before the present distribution of land and sea was attained. As to the course of events during the transition from Cretaceous to Tertiary times we can glean very little information in England, since there are no deposits in this country which bridge the gap. The bottom of the Chalk sea seems to have been gently and evenly uplifted, until brought within the influence of erosive agencies ; and these agencies affected wide tracts so equally that when the newer deposits were formed, their stratification was almost parallel to that of the Chalk on which they rested. Hence in actual sections no discordance can be traced between the Chalk and the Tertiary Beds, although other data indicate that con- siderable erosion of the older rock had occurred before the newer strata were laid down. The great change in physical circumstances which had taken place in the meanwhile is shown in the character of the sediments. The homogenous mass of chalk denoting widespread and long-continued marine deep-water conditions is succeeded by a changeful group of sands, clays and pebble-beds, constituting the ' Lower London Tertiaries,' the earliest division of the Eocene Period, which was accumulated in part in a shallow sea and in part in the estuary of a large river. In the aggregate this group rarely attains a thickness of more than 100 feet in 1 See W. Whitaker, Mem. Geol. Survey, 'The Geology of London' (1889), chap, xviii., where this subject is fully discussed, with references to previous literature. 13