Page:Quarterly Journal of the Geological Society of London, vol. 28.djvu/560

464 Near the mouth of the Brent, at Kew, piles may be seen beneath the water, marking, no doubt, the site of a river habitation, similar to those of the Swiss lakes; and at Barnes, in the opposite bend of the river, similar piles have been found, associated with a number of flint celts and dolichocephalic skulls of the form believed to have belonged to the earliest inhabitants of these islands. As these relics, to the best of my belief, are not found in the Thames valley elsewhere than in the existing bed of the Thames, it follows that the river must have flowed in the same meandering course for two thousand years, and in all probability for a much longer period, the bed of the river having actually risen during this period, as appears by the depth at which these ancient relics are found beneath the sedimentary deposits. We are thus led to form an idea of the enormous time that would have been required to erode the whole valley by means of a river flowing under the same conditions as the present one, to effect which it would have been necessary for the river not only to have shifted its bed over every portion of the present surface of the valley, but to have done the same thing repeatedly at different levels, over an extent of country which, as shown by the section, must, on the 100-feet line, have exceeded four miles and a half. This theory of the gradual erosion of the valley by means of a river of the same size as the present one, however, appears to have been abandoned by some of the best authorities.

The presence of large tracts of brick-earth overlying the gravel, argues, as I venture to think, the existence of large volumes of water at the time they were deposited. Then to what cause are we to attribute the strips of the London Clay laid bare on the sides of the valley, and of the tributary streams, at the average level of 50 feet? Obviously to denudation of some kind. Why, then, is this denudation not continued along the sides of the same streams into the mid terrace and down to the present river? The mid terrace, instead of being broken into patches by denudation, like the high terrace, is continuous, following the sinuosities of the valley up to the limit of the 50-feet line, or thereabouts. I venture to suggest, though not without diffidence, that a body of water, occupying the whole valley up to the 50-feet line, would account for the phenomena presented. The denudation of the patches of the high terrace would be caused by the drainage into this body of water. The mid-terrace gravel would be the result, in a great measure, of accumulations beneath the surface of the water. If this hypothesis were admitted, the implements of the high terrace must belong to a period anterior to that at which the river or lake stood at this level; and there is no reason, therefore, why they should not be found in the gravels still higher above the river.

The presence of these implements in the high terrace, their absence, so far as our researches go, in the mid terrace, and their