Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/109

 mulated shrank into river- channels, through which the drainage of Essex and Middlesex flowed southwards to this disturbed sea. In order to make this intelligible I have placed beside the larger map, which shows the detrital beds and contour surface, two smaller ones illustrating the succession of events thus supposed. The first of these shows the distribution of land and water when the Thames and coeval gravels were accumulating, and the other this distribution when the sea had deserted the chalk country and retired within the chalk escarpments of the Weald.

Now, in addition to the three openings in the North Downs, through which the rivers Stour, Medway, and Darent flow, and which expand trumpet-mouthed towards the Weald, and are regarded by me as the remains, first of old channels, and afterwards of old river-mouths, there is another precisely similar mouth further to the west, through which the Brighton Railway passes. This mouth forms now a dry valley extending from Croydon to Merstham, but so elevated and shallow in comparison with the three others that the railway has to pass out of it into the Weald at Merstham by means of a tunnel. Elevated and destitute of water as is this trumpet mouth, it is identical in form with those through which the rivers Darent and Medway flow, showing undeniably, as it seems to me, that this trumpet-shaped feature is not due to the erosive action of a river flowing outwards from the Weald, for no river at all is there.

The explanation of this dry, shallow, and elevated trumpet mouth seems to me to be this, viz. that it represents another of the channel-, and eventually river-mouths opening into the Weald which became established when the sea was retreating to the chalk escarpments.

As we go eastwards from this point, the chalk and Lower Greensand have an easy dip ; but as we go westwards from it the dip becomes much sharper, until between Guildford and Farnham the chalk is all on edge, and at angles varying from 35° to 45°. It seems to me therefore that while this more easy upcast eastwards permitted the fluviatile wearing down combined with tidal erosion to keep pace with the upcast, and so maintain these mouths as points of river discharge, the more abrupt character of the western upcast did not allow of this being done ; so that the drainage into the Weald through this trumpet mouth, traversed by the Brighton Railway, was put an end to at an early stage in the retreat of the sea Weald- wards. The gorges through which the Mole and Wey now flow were probably similar mouths, which (although the sharpness of the chalk upcast in their neighbourhood, has somewhat destroyed their trumpet-mouthed character, as well as the coast-contour that was synchronous with them) nevertheless were cut through during the disturbances so as to allow the drainage to flow into the Weald.

That part of this easterly drainage which flowed through the Darent gorge seems to have terminated before the Lower-Greensand escarpment became the sea-margin ; but that flowing along the lines of the Medway and Stour remained unarrested, the mouth of the