Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/152

58 counties of Kent and Essex, and separate maps, on a large scale, of the Chalk-pits and Brickearth-Pits 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 mammaliferous 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 &c. 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 tendency 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 off 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. We are therefore justified in stating that the character of the denuded 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 extensive area, and reach to a height more than 150 feet above the level of the Thames. It is equally impossible for the present volume 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