Climate and Time in Their Geological Relations / A Theory of Secular Changes of the Earth's Climate

by James Croll

All I wish to maintain is that the transport of the blocks across the Pennine chain is easily accounted for if we admit, what is very probable, that the great ice-covering of Scotland overlapped the high grounds of the North of England. The phenomenon is the same in both places, and why not attribute it to the same cause?

There is another curious circumstance connected with the drift of England which seems to indicate the agency of an ice-covering.

As far back as 1819, Dr. Buckland, in his Memoir on the Quartz Rock of Lickey Hill,[274] directed attention to the fact, that on the Cotteswold Hills there are found pebbles of hard red chalk which must have come from the Wolds of Yorkshire and Lincolnshire. He pointed out also that the slaty and porphyritic pebbles probably came from Charnwood Forest, near Leicester. Professor Hull, of the Geological Survey, considers that “almost all the Northern Drift of this part of the country had been derived from the débris of the rocks of the Midland Counties.”[275] He came also to the conclusion that the slate fragments may have been derived from Charnwood Forest. In the Vale of Moreton he found erratic boulders from two feet to three feet in diameter. The same northern character of the drift of this district is remarked by Professor Ramsay and Mr. Aveline, in their Memoir of the Geology of parts of Gloucestershire. In Leicestershire and Northamptonshire the officers of the Geological Survey found in abundance drift which must have come from Lincolnshire and Yorkshire to the north-east.

Mr. Lucy, who has also lately directed attention to the fact that the Cotteswold Hills are sprinkled over with boulders from Charnwood Forest, states also that, on visiting the latter place, he found that many of the stones contained in it had come from Yorkshire, still further to the north-east.[276]

Mr. Searles Wood, jun., in his interesting paper on the Boulder Clay of the North of England,[277] states that enormous quantities of the chalk débris from the Yorkshire Wold are found in Leicester, Rutland, Warwick, Northampton, and other places to the south and south-west. Mr. Wood justly concludes that this chalk débris could not have been transported by water. “If we consider,” he says, “the soluble nature of chalk, it must be evident that none of this débris can have been detached from the parent mass, either by water-action, or by any other atmospheric agency than moving ice. The action of the sea, of rivers, or of the atmosphere, upon chalk, would take the form of dissolution, the degraded chalk being taken up in minute quantities by the water, and held in suspension by it, and in that form carried away; so that it seems obvious that this great volume of rolled chalk can have been produced in no other way than by the agency of moving ice; and for that agency to have operated to an extent adequate to produce a quantity that I estimate as exceeding a layer 200 feet thick over the entire Wold, nothing less than the complete envelopment of a large part of the Wold by ice for a long period would suffice.”

I have already assigned my reasons for disbelieving the opinion that such masses of drift could have been transported by floating ice; but if we refer it to land-ice, it is obvious that the ice could not have been in the form of local glaciers, but must have existed as a sheet moving in a south and south-west direction, from Yorkshire, across the central part of England. But how is this to harmonize with the theory of glaciation, which is advanced to explain the transport of the Shap boulders?

The explanation has, I think, been pointed out by a writer in the Glasgow Herald,[278] of the 26th November, 1870, in a review of Mr. Lucy’s paper.

In my paper on the Boulder Clay of Caithness, I had represented the ice entering the North Sea from the east coast of Scotland and England, as all passing round the north of Scotland. But the reviewer suggests that the ice entering at places to the south of, say, Flamborough Head, would be deflected southwards instead of northwards, and thus pass over England. “It is improbable, however,” says the writer, “that this joint ice-sheet would, as Mr. Croll supposes, all find its way round the north of Scotland into the deep sea. The southern uplands of Scotland, and probably also the mountains of Northumberland, propelled, during the coldest part of the glacial period, a land ice-sheet in an eastward direction. This sheet would be met by another streaming outward from the south-western part of Norway—in a diametrically opposite direction. In other words, an imaginary line might be drawn representing the course of some particular boulder in the moraine profonde from England met by a boulder from Norway, in the same straight line. With a dense ice-sheet to the north of this line, and an open plain to the south, it is clear that all the ice travelling east or west from points to the south of the starting-points of our two boulders would be ‘shed’ off to the south. There would be a point somewhere along the line, at which the ice would turn as on a pivot—this point being nearer England or Scandinavia, as the degree of pressure exercised by the respective ice-sheets should determine. There is very little doubt that the point in question would be nearer England. Further, the direction of the joint ice-sheet could not be due south unless the pressure of the component ice-sheets should be exactly equal. In the event of that from Scandinavia pressing with greater force, the direction would be to the south-west. This is the direction in which the drifts described by Mr. Lucy have travelled.”

I can perceive no physical objection to this modification of the theory. What the ice seeks is the path of least resistance, and along this path it will move, whether it may lie to the south or to the north. And it is not at all improbable that an outlet to the ice would be found along the natural hollow formed by the valleys of the Trent, Avon, and Severn. Ice moving in this direction would no doubt pass down the Bristol Channel and thence into the Atlantic.

Might not the shedding of the north of England ice-sheet to the north and south, somewhere not far from Stainmoor, account for the remarkable fact pointed out by Mr. Searles Wood, that the boulder clay, with Shap boulders, to the north of the Wold is destitute of chalk; while, on the other hand, the chalky boulder clay to the south of the Wold is destitute of Shap boulders? The ice which passed over Wastdale Crag moved to the E.N.E., and did not cross the chalk of the Wold; while the ice which bent round to the south by the Wold came from the district lying to the south of Wastdale Crag, and consequently did not carry with it any of the granite from that Crag. In fact, Mr. Searles Wood has himself represented on the map accompanying his Memoir this shedding of the ice north and south.