1869.] SEARLES WOOD BOULDER-CLAY. 103
such a mounting, in the form of the lower elevations intervening
between the Cheviots and the Northumberland coast, yet it does
not appear to me that the conditions of that part of the western
slope in which Shap Fell lies, offer any indications of such an upward
transit*, or that the position of the dividing ridge generally
would allow of the possibility of it.
We have therefore but to fall back upon the explanation of a floating-ice transit, permitted by an adequate submergence; and this explanation is strengthened by the absence of these boulders in the older or chalky part of the clay, since otherwise they ought to be present in this part equally as in the other.
Mr. Archibald Geikie, in the year 1863†, put forth his theory that the origin of the Scotch Boulder-clay was due to the extrusion, from the sea-foot of an ice-sheet enveloping the land, of the miscellaneous material which such a sheet would in its motion seaward have degraded‡. This hypothesis is that which for a long time I have entertained as the only true one of the origin of the Glacial clays of the east of England; and I venture to suggest that the application of it to the case now under consideration will not only harmonize with all the facts, both palaeontological and physical, but explain that otherwise inexplicable circumstance, the positions of the purple clay without chalk, to which attention has been called.
In the same outline map I have indicated, by tint shading, that area which I regard as sea during the deposit of all but the earliest part of the chalky clay, the unshaded part having been enveloped in an ice-sheet, which barred out the sea from all those depressions which, being below its level, would otherwise have been occupied by it, just as is the case at the present day along the west coast of Greenland according to Arctic voyagers§. The same shading that indicates sea indicates also the area over which the chalky clay occurs, except that, to curtail its dimensions, the map does not extend south-
- The whole of the great basin drained by the rivers Lowther and Eden (on
the side of which Shap Fell lies) is furrowed by a numerous set of small parallel ridges, with their prominences in roches moutonnees. These all run from S.S.E. to N.N.W., and a glance at the beautifully shaded contours of the Ordnance map shows at once, by a coup d'oeil, that a great ice-sheet has moved down this basin towards the Solway shore. Similar features are afforded by the contour-shading of the slopes on the other (or eastern) side of the dividing ridge north-west of Newcastle, where the motion has been similarly outward from the dividing ridge, being there from W.S.W. to E.N.E. Mr. T. M'K. Hughes, of the Geological Survey, kindly indicated for me such striations as he had observed on the rocks of this neighbourhood; and they agree generally with the ice-motion which the map-contours indicate.
† Trans, of the Geol. Soc. of Glasgow, vol. i. part 2.
‡ The Rev. J. B. Watson too, in his memoir on the Glacial beds of Arran (Trans. Royal Soc. of Edinburgh, vol. xxiii.), adopts the same hypothesis, and adds some valuable information as to the causes in operation at the ice-foot of the Norway glaciers, derived from personal observation. The Glacial conditions of the period of the chalky clay were, however, far in excess of any thing now obtaining in Norway, and more analogous to those on the west coast of Greenland, and along the skirts of the ice-bound land of the Antarctic continent.
§ See Dr. Sutherland in Quart. Journ. Geol. Soc. vol. ix. p. 301.
