Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/99

 a late period as that when the conditions giving rise in these parts to gravels and brick-earths had ceased — the period in fact of the modern alluvium, which alone occurs in the valley, or more properly the wide gorge thus cut through the ridge.

I then pointed out that east of London, where it occupied the more seaward portion of the channel thus opening southwards towards, the Weald, the Thames gravel had been greatly broken up, denuded and elevated irregularly, by which action partial terraces had been formed — that under these terraces occurred the gravels and brick- earths of fluviatile origin with Cyrena fluminalis which had succeeded to the spreading out of the gravel occupying such terraces — and that these were the deposits of rivers into which the original gravel-inlets had by the elevation of their bottoms become reduced, such rivers, equally with the inlets that had preceded them, opening to a sea in the direction of the Weald. The mouths of these rivers, I considered, had followed the shore-line as this gradually receded southward from the rise of the Wealden area, until the sea, first becoming confined within an estuary of its own eroding, marked by the Wealden escarpments, was eventually expelled from the Wealden area — and that upon this event taking place, the drainage acquired its present reversed direction from the Weald into the Thames estuary, which then came into existence*.

At the time when by a study of the gravels without the Weald I was thus led to these views, I had not examined with any detail how far the constitution and position of the gravels lying within the chalk escarpments supported or conflicted with them. This I have now done, so far as concerns the north-eastern part of the area, which, from its contiguity to the mouths of the Thames and East Essex gravel inlets, is the part of principal importance in the question ; and I propose now to show its bearing upon it.

In doing so it will, I think, be advantageous to consider also a question that I had deferred for the occasion, viz. how far the theories of the denudation of the Weald by agencies which involve the escape of the material removed in the course of denudation outwards from the Weald and into the Thames area, be they atmospheric, fluviatile, or marine, receive support or meet with negation from the composition of the detrital beds lying without the north-eastern part of the Weald.

Taking up this latter inquiry first, we have two sets of detrital beds to consider, viz. the Glacial and the Postglacial. Of the first, we have in this part of England two formations, the Boulder-clay and the gravel underlying it, which I have termed Middle Glacial.

In neither of these deposits can it be said that the debris of the

the brick-earth of Erith and Crayford as distinct from that of G-rays, and as having preceded the Thames gravel. Finding afterwards, by a clearer section, that it did not pass under that gravel, I, in a letter published at page 534 of the fifth volume of the Geol. Mag., withdrew from that position, and admitted that the Grays and Erith and Crayford Brick-earths are identical, and belong alike to the lower terraces of the Thames-gravel formation.
 * In my paper in the Quart. Journ. Geol. Soc. vol. xxiii., at p. 408, I regarded