Page:VCH Surrey 1.djvu/61

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GEOLOGY with a drop on the south-west side of about the same amount as the last. 1 Several smaller faults have also been recognized, chiefly in the Lower Greensand tract ; but these need not detain us, as the instances already given are sufficient to show the general character of this kind of disturb- ance. At the close, then, of this period of great earth-movement the strata in the south of the county, including the Chalk and the under- lying beds, formed part of the elevated dome and probably extended con- tinuously from the present line of the North Downs to the present line of the South Downs, while the north of the county was relatively depressed to form part of what we now know as the London Basin. It is, however, probable that before the elevation had attained its maximum the Chalk had already been partly worn away over the rising area, since in many parts of the Eocene deposits, as we have seen, there are water-worn flints derived from the Chalk ; but on the other hand there is evidence to show that some of the Eocene deposits were them- selves once continuous over at least a portion of the elevated tract ; so that we may estimate the original surface to have been not less than 2,000 feet higher than it now is in the middle of the tract. We have next to consider how this huge pile of material has been removed and the land brought to its present shape. At about the time of the relative upheaval of the Wealden dome there was, besides, a general elevation, by which the whole area became dry land. When the rain fell on this land and gathered into streams, these streams of course flowed down the slopes from the dome towards the lower ground. Hence in our particular district they flowed north- ward from the Wealden tract towards the London Basin ; and in this direction the main streams have never since ceased to flow, although what was once relatively the higher ground has now been worn away until it has become relatively low ground, and the streams have had to cut deep valleys across the hilly ridges of the present land to maintain their courses. Thus we find the Wey at Guildford and the Mole at Dorking crossing the Chalk in trenches which they have excavated transversely through the escarpment which threatened to bar their passage ; and the same conditions prevail with regard to the Darent and the Medway in Kent ; while on the opposite side of the dome the Arun and the Adur in Sussex break their way through the South Downs in an exactly similar manner, but in the reverse direction. If the configuration of the land when these streams began to flow had been even approximately that of to-day, such courses would have been impossible ; and we are compelled to recognize that these drainage-channels were established in a remote past before the escarpments were in existence. They are the sluices down which the greater part of the waste of the land has been conveyed to the sea, and their channels have been scoured and deepened at a more rapid rate than that at which the general level has been lowered. Meanwhile, as formation after formation crumbled away on the elevated tract, the 1 F. Drew, Mem, Geol. Survey, ' Geology of the Weald,' p. 233. 23