1870.] WOOD WEALD- VALLEY DEKXJDATION". 15 



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 Green- 

 sand 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 permit- 

 ted 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 moiith, traversed by the Erighton 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 



