58 PEOCEEDIJSTGS OP THE GEOLOGICAL SOCIETY. 



counties of Kent and Essex, and separate maps, on a large scale, of the 

 Chalk-pits and Brickearth-]3its of Erith, Crayford, and Grays define 

 the exact position of the sections across the valleys of the Thames and 

 in the pits ; and the heights explain their relation to the present river 

 Thames. Sections of the raised beach and mammaliferons gravel at 

 Brighton, and of the fossiliferous gravel and raised beach at Sangatte, 

 showing the contour and the modelling of the upper surface of the 

 gravel-series, have been also drawn. A comparative Table of fossil 

 shells &G. from the above localities is also added. 



The evidence of numerous sections teaches us that, prior to the 

 deposition of the gravel, there was a land- surface smoothly denuded 

 by rain and streams so as to form a perfect system of principal and 

 minor valleys, the ground sloping from higher to lower points, so as 

 to admit the rainfall to flow with the minimum of obstruction into 

 the side- valleys and thence into the ancient Thames. The subsequent 

 deposition of the gravel-series did not in any way alter old lines of 

 drainage ; but where concavities existed the new deposit had a ten- 

 dency to fill them up with a thicker stratum of material than was 

 spread over the general surface of the chalk or clay. Thus the 

 Quaternary beds reach a thickness of 80 feet at the maximum, while 

 the average is perhaps only 25 feet in the whole district. Some 

 boulders on the upper part of the gravel- deposits reach to many tons 

 in weight ; and they'are as large or larger than those in any part of 

 the series, so that there is evidence of as great intensity of pluvial 

 action at the end as at the commencement of the Quaternary series. 



Except where the old river-channels and concavities are filled up, 

 the contour and the modelling of the upper surface of the gravel-series 

 resembles that of the clay or chalk on which the lower gravel rests, 

 and is perfectly adjusted to carry ofi" the rainfall occurring in the 

 later part of the gravel-period into the ancient Thames without any 

 impediments. The contour of the land is such that it could only 

 have reached its present form by pluvial and fluvial action, and not 

 by marine denudation. Many of the minor valleys in which gravel 

 and brick-earth were deposited with each flood in the Quaternary 

 period are now dry. After the heaviest rainfall in recent times there 

 is not sufficient force of water to remove vegetation, so as to make 

 any change in the present surface. 



AVe are therefore justified in stating that the character of the de- 

 nuded surface of the London Clay and Chalk above the level of the 

 Thames is evidence of the occurrence of an enormous rainfall in the 

 commencement of the gravel-period, and that the character of the 

 surface-deposits of gravel is evidence of nearly as much rainfall at 

 the close of that period. 



With a rainfall such as we now have, it would be impossible 

 that such widely extended gravel-beds could be spread over an ex- 

 tensive area, and reach to a height more than 150 feet above 

 the level of the Thames. It is equally impossible for the present vol- 

 ume of the Thames to have produced fluviatile beds at all equivalent 

 in size to those of the ancient Thames. 



The condition of the beds which rise above the 50-feet level points 



