Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/100

 Wealden denudation is represented. In the gravel (which is composed of chalk-flint, with a considerable percentage of quartzites) there occur fragments of other rocks, among which are some of chert, sandstone, and limestone, that may possibly belong to rocks within the Weald, though I am not aware that such an origin can be with any certainty affixed to them. Again, Mr. Prestwich mentions having found fragments of chert and ragstone that he refers to the Lower Greensand of Kent in the shingle of the cliff near Southwold, belonging to the Glacial formation.

Such occurrences as these, however, afford no ground for concluding more than that prior to the Glacial epoch such a planing off of the Wealden area had begun as to afford exposures of the beds beneath the chalk, from which some fragments might at the commencement of the glacial period have found their way into gravels then in course of formation. The evidence necessary, however, to justify any assumption that the Weald valley existed as a subaerial tract during the whole Glacial period must go very far beyond this. Whether we suppose this valley to have been occupied during the Glacial period with ice which streamed through the lateral valleys of the north-east side into the Thames area — or whether we suppose it to have had a milder climate, so that rivers of water instead of ice followed the same course — in either case great volumes of the wreck of the subcretaceous strata ought to have been brought into the glacial beds which approach so near to the Wealden area as do those of the south of Essex ; but these beds, especially the Boulder-clay, are conspicuous by their absence. If we consider through what various beds of stone the Medway and Darent valleys are cut, and what immense quantities of this stone must have been removed to form them, the absence or extreme paucity of such debris in the Glacial beds is significant ; but if we couple, as we have been asked to do, the denudation of the great valley of the Weald itself with the erosive action of the Wealden rivers, then this becomes still more significant, and the impossibility of the Weald having been under subaerial conditions during the prevalence of those excessively detrital agencies that we attribute to ice seems to me obvious — and the more especially when we remember the greater extent which the Lower Greensand formation must have occupied in the earlier stages of the Wealden denudation, all of which, with its great beds of stone in fragments, has gone somewhere.

Further, the Boulder-clay of the Essex heights is mainly composed of rolled chalk ; but it is not the soft chalk of Kent and Surrey, but the hard chalk of Yorkshire and Lincolnshire, termed " Rock " by the well-borers of those counties. The flank of the Lincolnshire chalk-wold for a long distance is occupied by a vast deposit of glacially degraded chalk, so pure as to be extensively quarried for lime, and so thick that the range of country formed out of it rivals in height the Wold itself. We thus see to what sort of detrital accumulation a range of chalk hills has given rise under the powerful action of glacier ice ; and it appears to me but reasonable to expect something of the kind to have occurred over the south-east of Eng-