Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/108

 subsequently to the accumulation of the Thames gravel, and that their effect was to put an end to that gravel and the coeval gravels of East Essex and of the Canterbury heights, and gradually to raise the sea-bed over Kent and Sussex, and away to the north of France, giving rise thereby to a great submarine denudation — that by this action the inlets in which the Thames and coeval gravels had accu-

avoided reference to any earlier stages of the Wealden denudation. Mr. Prestwich, in his ' Memoirs on the Lower London Tertiaries,' gave reasons for supposing that the first elevatory action over the Wealden dome preceded the Lower Tertiaries. Those also who agree with him as to the age of the Lenham and Paddlesworth beds, must admit that a considerable planing off of the chalk, exposing probably the subcretaceous strata, was renewed prior to those Lenham and Paddlesworth beds being deposited. I would again call attention to the physical fact, plain to any who will take the trouble to extract the contours from the ordnance map, that there are two distinct sets of disturbances traceable in the south-east of England : — one, the sharp rectilinear east and west upthrows, by which the denudation of the Weald valley has been induced, and which I consider to have affected the Thames gravel, both east and west of London, and to be connected with the similar upthrows of the Isles of Wight and Purbeck (all, therefore, of a date posterior to the first accumulation of the Thames gravel) ; and the other, a set of anterior origin, consisting of a series of concentric curves, the exterior of which is formed by the Cotteswold escarpment, and the next to that by the chalk escarpment from Pangbourne to Royston, while the inner ones constitute a series of smaller escarpments, of which the trumpet-mouthed gorges in the North Downs, referred to in the paper as river-mouths, successively form parts, the whole set centring near Canterbury. These curves are connected very distinctly with another, and apparently contemporaneous, set, which centre near the western extremity of the Isle of Wight, and are concentric half circles formed by the Lower-Tertiary outcrop of Hampshire and the chalk escarpments of the south-west. The rectilinear movements to which the denudation of the Weald valley is due, have destroyed this anterior curvilinear contour, which is perfect up to the points where these rectilinear upcasts occur, I formerly supposed that these curved groups were, equally with the rectilinear ones, of post- glacial origin ; but an examination of the glacial beds of Bedfordshire and of Northamptonshire showed these beds in such a position relatively to the chalk and oolite escarpments (which form successive concentric curves of the Canterbury group) as necessitated the inference that the curves were of preglacial origin. These curvilinear movements appear, however, to be clearly synchronous with the first denudation of the Tertiaries from the south of England, and with that planing off of the chalk over the Weald, to which allusion has been made. The two principal rectilinear flexures, by which the direction of the denudation of the Weald valley has been induced, are those of Portsdown Hill and the Guildford Hog's-back ridge, running east and west and parallel to each other, and indicated by lines in Map No. II. The upthrow of these I consider to have so acted on the sea expelled from the north of Kent by the synchronous upcast of the country from there over to France, as to have caused it to denude their regular parallelogrammatic trough formed by the chalk escarpments of the Weald. The two inner curves of the concentric series which centres at Canterbury are visible on the accompanying map, No. I., the innermost of the two being formed by the escarpment which skirts the Canterbury-heights gravel on the west, and extends down to the Weald escarpment above Smeeth. The northern extremity of this curve has now disappeared into the north sea. The other, and next outward curve, is formed by the escarpment which skirts the East-Essex gravel on the west, and extends down to the Weald trumpet- mouth above Maidstone. The two trumpet-mouths west of this are successively partial repetitions of these curves ; but the next, complete repetition is made by the chalk escarpment from Pangbourne to Royston ; and the next to this, and last, is the Cotteswold escarpment, prolonged by the oolitic escarpment through Warwickshire, and by the oolitic escarpment of Northamptonshire.