IV.—Post-Pliocene Submergence of the Isle of Wight

Extract

I Am tempted to offer a few notes with the conclusions I arrived at during some weeks residence this summer in the Isle of Wight, having been struck with the remarkably definite data which the island affords regarding the maximum depth of the submergence (or height of emergence) during the post-Pliocene period. The “plateau-gravels,” which are the main factors in the determination of the amount of submergence, have now been mapped by the Geological Survey, together with the other post-Tertiary deposits; but as regards their mode of formation the Survey (I believe) in its collective capacity refrains from expressing an opinion. I, for my part, have no hesitation in bespeaking for them a marine origin—(1) because of their wide distribution; (2) because of their elevated position, as shown by Mr. Codrington, Prof. Prestwich, and other writers, which is far beyond the reach of the rivers past or present; (3) because of their stratified structure and their composition. There remain the two other hypotheses, both equally out of the question—that they are lake deposits, or that they are of glacial origin. The absence of shells in the higher levels, reaching to 300–500 feet, is easily explained, and is no argument against marine conditions of deposition, as the percolation of subærial waters through such open and porous beds must necessarily have dissolved away the material of which marine organisms are mainly formed. As regards the geological age of the plateau-gravels of the Isle of Wight and Dorset, there can be little doubt that they are post-Tertiary, as they are found resting indiscriminately on all Tertiary strata of those parts. Prof. Prestwich classes them under the head of “Southern Drift,” and distinguishes them from his “Westleton Beds” on the ground mainly of difference in the composition of their constituent pebbles, and their separation from the Westleton Beds of the East of England and the Thames Basin by a barrier along the Wealden ridge. But even admitting this separation, the beds may be representative as regards age; and as for the absence or presence of certain pebbles (such as white quartz, etc), it can scarcely be expected that in any beds of conglomerate of wide extension there should be no variation in the composition of the constituent pebbles. Perhaps, on reconsideration, Prof. Prestwich might be inclined to admit that the plateau-gravel of the south and the Westleton gravel of the centre and east may be contemporaneous. With these preliminary observations, I proceed to the evidence of submergence and emergence afforded by these beds in the Isle of Wight.