Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/120

 contemporaneous, as I consider, with the similar rectilinear ridges of the Isles of Wight and of Purbeck, represent, I submit, the elevatory influences under which the chalk area of Hampshire and of Wilts became converted into land, and under which the Wealden upcast acquired that special configuration which gave to the up- channel tide its great scouring power. In No. III. the sea is shown as confined within the chalk escarpments of the Weald, with a barrier of land extending across to France and shutting off the British Channel from the North Sea ; while a part of the area now occupied by this sea between East Anglia and the north of France was in the condition of land supplying streams that found their way through the Stour and Medway gorges into the Weald, so much of the drainage as passed through the Thames valley* reaching the Weald through the gorges of the Wey and Mole to the west. The gravel-beds, with remains of an ancient beach, described by Mr. Prestwich and by others near Calais, appear to me to fall into their place between the two stages thus represented, while the Brighton bed seems to belong to the period represented by No. III. and to that following it. It will not be difficult to pursue the change from the stage thus represented in No. III. to that when the shore had become established at the Lower-Green-

by this preliminary postglacial denudation ; while to the north of the Essex heights, away to the northernmost extremity of Britain, the parts once covered by the same glacial sea, being remote from the theatre of these disturbances, felt their influence only in the form of a tranquil elevation, and were consequently only partially denuded of their covering of glacial beds. It was during a lull in these disturbances, and when this preliminary denudation and emergence had brought about the conditions represented in Map no. II., that the Thames gravel accumulated ; so that more properly it was a renewal, and not the setting-in of these disturbances, which, first completing the sharp inclination of the rectilinear ridges, then lifted them, together with those portions of the chalk and subcretaceous districts which had not yet emerged, above the sea, and, by renewing the causes of denudation, removed this accumulation everywhere except in the places where we now find it (which it seems to me were the parts of least disturbance at the particular epoch), eating also still deeper into the old strata as they underwent elevation from the waters. I have elsewhere (Quart. Journ. Geol. Soc. vol. xxiv. p. 174) endeavoured to connect this renewal of disturbance in the south with the setting-in of that depression in the north of England to which was due the postglacial clay of Hessle, which wraps like a cloth the deeply denuded glacial beds, and is underlain by a gravel containing the characteristic shell of the Thames beds and their allies, the Cyrena fluminalis.

this period could have been but small, because the entire bottom of this valley, east of London as far as Erith, which is now occupied by the marsh mud, and which, if the embankment were removed would be all flooded, is covered by an oak, yew, hazel, and fir forest, rooted into the gravel and overspread by the marsh mud. It is clear from this, that subsequent even to the latest part of that gravel, but prior to the general depression of England, which buried so many forests remaining round our coasts, as well as the forest in question, the bottom of this valley, far within limits that, but for the embankments, would now be water, was dry ground. It is this depression of so much of England, at a late period, that I associate with the opening of the mouths of the Thames and Crouch, and the occupation by the present North Sea of the large area to the north of Kent, shown in Map No. III. as land. This general depression seems to have been the recoil from the termination of the Wealden elevation.
 * The amount of drainage collected in the Thames valley east of London at