Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/201

1869.] SEARLES WOOD BOULDER-CLAY. 101 posited the clay which contains them, in any other way than by the occupation by an ice-sheet of the great vale that extends along the scarp of the Wold where the red chalk crops out.

Another important feature bearing upon the relationship of the chalky to the chalkless clay, is the absence in the one and the presence in the other of boulders of the well-known peculiar granite of Shap Fell in Westmoreland. During the several years that I have occupied myself in examining and mapping the Glacial beds over the east of England, I have never seen such a thing as a fragment of this granite either in the various sections examined or among the numerous boulders which, exposed by atmospheric agencies, have been collected from the fields and placed by the roadside or by farmhouses &c. Neither Mr. Rome nor myself ever observed one along the Holderness coast, where the chalky clay and the purple clay with some chalk occupy the cliff; but immediately that we passed these limits and entered, about Flamborough Head, upon the region of the purple clay without chalk, we found them in plenty; and Mr. Rome informs me that he has seen them along the whole coast north of Flamborough, where this purple clay without chalk alone occurs, as far as the mouth of the Tees; and it is to him that I am indebted for pointing out to me (which he did more than two years since) the restriction of these boulders to the clay without chalk.

Although neither he nor myself was able to find any of these boulders along the Holderness coast, they are nevertheless said to occur as far south as the Humber mouth*; and there is no reason to suppose that such may not be the case, though rarely — because the chalky clay and the purple clay with chalk are, on this coast, capped in two or three places by outliers of the purple clay without chalk, from which these boulders might be derived.

In order to render the explanation which I offer of these facts intelligible, a small outline map of the north of England (fig. 1, p. 102) accompanies this paper, and in it are carefully shaded the slopes of the great dividing ridge of the north of England. The position of Shap Fell being on the western side of this dividing ridge, it is clear that one of two events must have occurred to enable its boulders to pass over the higher ground which separates the Shap country from the eastern side, over which the purple clay containing these boulders is distributed. The one event would be their transit by land-ice moving, not as it usually does from higher to lower ground, but upwards against the acclivity, and over the dividing ridge into the eastern area; the other would be the submergence of the country to an extent sufficient to permit the floating over of masses of ice freighted with these boulders.

With respect to the first of these alternatives, although we know that ice impelled by the force of the rearward mass of the sheet will, when this mass derives sufficient force by a descent from elevations considerably higher than the intervening obstacle, rise and pass over that obstacle — and though we may find traces of


 * Quart. Journ. Geol. Soc. vol. xxiii. p. 44.