James Croll — Bouldcr-clay of Caithness, 213 



On the top of another isolated hill, called Morven, about 3,000 feet 

 high, and situated a few miles to tlio north of the village of 

 Ballater, in the county of Aberdeen, I found granite boulders unlike 

 the rock of the hill, and apparently derived from the mountains to 

 the west. Again, on the highest water-sheds of the Ochils, at 

 altitudes of al)Out 2,000 feet, I found this summer (18()4), pieces of 

 mica schist full of garnets, which seem to have come from th©- 

 Grampian Hills to the north-west, showing that the transporting 

 agent had overflowed even the highest parts of the Ochy ridgo. . 

 And on the West Lomonds, in Fifeshire, at Clattering- well Quarry, . 

 1,450 feet high, I found ice-worn pebbles of Red Sandstone 'and 

 porphyry in the debris, covering the Carboniferous Limestone of 'the 

 top of the Bishop Hill. Facts like these meet us everywhere. Thus 

 on the Perthshire Hills, between Blair Athol and Dunkeld, I found 

 ice-worn surfaces of rocks on the tops of hills, at elevations of 2,200 

 feet, as if caused by ice pressing over them from the north-west, 

 and transporting boulders at even greater heights." Jo urn. Geol. 

 Soc, vol. xxi., p. 165. 



It is therefore evident that the great mass of ice entering the 

 North Sea tO' the East of Scotland, especially about the Firths of 

 Forth and Tay, could not have been less than from 1,000 to 2,000 

 feet in thickness. Its thickness, however, was probably much 

 greater than that. 



The grand question now to be considered is, what became of the 

 huge sheet of ice after it entered the North Sea ? Did it then break 

 up, and float away as icebergs ? This, hitherto, appears to have been 

 taken for granted. The average depth of the North Sea is not over 

 40 fathoms. It is therefore perfectly obvious that a glacier 1,000 or 

 2,000 feet in thickness could not possibly have floated in such a sea. 

 This depth of water is not sufficient to float a glacier 300 feet 

 thick. The ice must consequently have moved along on the bed of 

 the sea in one unbroken mass, the same as it would have done had 

 its bed been dry land. The North Sea gradually deepens, from 

 about 20 fathoms at the English Channel to about 50 fathoms at the 

 north of Scotland. The 50-fathom line crosses from about the 

 Moray Firth over to near the coast of Norway, and we must go to 

 the north and west of the Orkney and Shetland Islands before we 

 reach the 100-fathom line. The ice-sheet of Scotland could not, 

 therefore, have broken tip into icebergs in the North Sea. While it 

 remained in this region it would be land-ice. The sheet must have 

 found its way to the deep trough of the Atlantic, to the west of the 

 Orkney and Shetland Islands, before it could possibly have floated 

 away in the form of icebergs. 



No doubt the North Sea, for two reasons, is now much shallower 

 than it was during the period in question. (1.) There would, at the 

 time of the great extension of the ice on the northern hemisphere, be a 

 considerable submergence, resulting from the displacement of the 

 earth's centre of gravity (Phil. Mag. for April, 1866). (2.) The 

 North Sea is now probably filled up to a larger extent with drift 

 deposits than it was at the ice period. But, after making the most 



