CHAPTER XXVIII.
NORTH OF ENGLAND ICE-SHEET, AND TRANSPORT OF WASTDALE CRAG BLOCKS.[266]
Transport of Blocks; Theories of.—Evidence of Continental Ice.—Pennine Range probably striated on Summit.—Glacial Drift in Centre of England.—Mr. Lacy on Drift of Cotteswold Hills.—England probably crossed by Land-ice.—Mr. Jack’s Suggestion.—Shedding of Ice North and South.—South of England Ice-sheet.—Glaciation of West Somerset.—Why Ice-markings are so rare in South of England.—Form of Contortion produced by Land-ice.
Considerable difficulty has been felt in accounting for the transport of the Wastdale granite boulders across the Pennine chain to the east. Professors Harkness,[267] and Phillips,[268] Messrs. Searles Wood, jun.,[269] Mackintosh,[270] and I presume all who have written on the subject, agree that these blocks could not have been transported by land-ice. The agency of floating ice under some form or other is assumed by all.
We have in Scotland phenomena of an exactly similar nature. The summits of the Ochils, the Pentlands, and other mountain ranges in the east of Scotland, at elevations of from 1,500 to 2,000 feet, are not only ice-marked, but strewn over with boulders derived from rocks to the west and north-west. Many of them must have come from the Highlands distant some 50 or 60 miles. It is impossible that these stones could have been transported, or the summits of the hills striated, by means of ordinary glaciers. Neither can the phenomena be attributed to the agency of icebergs carried along by currents. For we should require to assume not merely a submergence of the land to the extent of 2,000 feet or so,—an assumption which might be permitted,—but also that the currents bearing the icebergs took their rise in the elevated mountains of the Highlands (a most unlikely place), and that these currents radiated in all directions from that place as a centre.
In short, the glacial phenomena of Scotland are wholly inexplicable upon any other theory than that, during at least a part of the glacial epoch, the entire island from sea to sea was covered with one continuous mass of ice of not less than 2,000 feet in thickness.
In my paper on the Boulder Clay of Caithness (see preceding chapter), I have shown that if the ice was 2,000 feet or so in thickness, it must, in its motion seawards, have followed the paths indicated by the curved lines in the chart accompanying that paper (See [Plate I.]). In so far as Scotland is concerned [and Scandinavia also], these lines represent pretty accurately not only the paths actually taken by the boulders, but also the general direction of the ice-markings on all the elevated mountain ridges. But if Scotland was covered to such an extent with ice, it is not at all probable that Westmoreland and the other mountainous districts of the North of England could have escaped being enveloped in a somewhat similar manner. Now if we admit the supposition of a continuous mass of ice covering the North of England, all our difficulties regarding the transport of the Wastdale blocks across the Pennine chain disappear. An inspection of the chart above referred to will show that these blocks followed the paths which they ought to have done upon the supposition that they were conveyed by continental ice.
That Wastdale Crag itself suffered abrasion by ice moving over it, in the direction indicated by the lines in the diagram, is obvious from what has been recorded by Dr. Nicholson and Mr. Mackintosh. They both found the Crag itself beautifully moutonnée up to its summit, and striated in a W.S.W. and E.N.E. direction. Mr. Mackintosh states that these scorings run obliquely up the sloping face of the crag. Ice scratches crossing valleys and running up the sloping faces of hills and over their summits are the sure marks of continental ice, which meet the eye everywhere in Scotland. Dr. Nicholson found in the drift covering the lower part of the crag, pebbles of the Coniston flags and grits from the west.[271]
The fact that in Westmoreland the direction of the ice-markings, as a general rule, corresponds with the direction of the main valleys, is no evidence whatever that the country was not at one period covered with a continuous sheet of ice; because, for long ages after the period of continental ice, the valleys would be occupied by glaciers, and these, of course, would necessarily leave the marks of their presence behind. This is just what we have everywhere in Scotland. It is on the summits of the hills and elevated ridges, where no glacier could possibly reach, that we find the sure evidence of continental ice. But that land-ice should have passed over the tops of hills 1,000 or 2,000 feet in height is a thing hitherto regarded by geologists as so unlikely that few of them ever think of searching in such places for ice-markings, or for transported stones. Although little has been recorded on this point, I hardly think it likely that there is in Scotland a hill under 2,000 feet wholly destitute of evidence that ice has gone over it. If there were hills in Scotland that should have escaped being overridden by ice, they were surely the Pentland Hills; but these, as was shown on a former occasion,[272] were completely buried under the mass of ice covering the flat surrounding country. I have no doubt whatever that if the summits of the Pennine range were carefully examined, say under the turf, evidence of ice-action, in the form of transported stones or scratches on the rock, would be found.[273]
Nor is the fact that the Wastdale boulders are not rounded and ice-marked, or found in the boulder clay, but lie on the surface, any evidence that they were not transported by land-ice. For it would not be the stones under the ice, but those falling on the upper surface of the sheet, that would stand the best chance of being carried over mountain ridges. But such blocks would not be crushed and ice-worn; and it is on the surface of the clay, and not imbedded in it, that we should expect to find them.
It is quite possible that the dispersion of the Wastdale boulders took place at various periods. During the period of local glaciers the blocks would be carried along the line of the valleys.