212 James Croll — Boulder-clay of Caithness. 



it was si^cytv-n that, owing to the shallowness of the northern seas, 

 those, seas must have been at that period all blocked up with solid 

 ice, displacing the water and moving along the sea bottoms, the 

 same as on dry laad elevated above the sea level. In fact the 

 northern seas, including the German Ocean, being filled at the time 

 with glacier-ice, might be regarded as dry land. Ice of this sort, 

 moving along the bed of the German Ocean or North Sea, and over 

 Caithness, could not fail to push before it the shells and other 

 animal remains lying on the sea bottom, and to mis them up -with 

 the clay which now. xemains upon the land as the evidence of, its 

 progress. , , > _ •' , , ■ •■ 



About two years ago I h^d a conversation with Mr. Peach on the 

 subject. This gentleman, as is, well known, has long been faniiliar 

 with the Boulder-clay of Caithness. He felt convinced that the clay 

 of that country is the true lower till, and not a more recent deposit, 

 as Mr. Jamieson supposes. , He expressed to me his opinion that the 

 ■glaciation of Caithness had been effected by masses of land-ice 

 crossing the Moray Firth from the mountain ranges to the South- 

 east, and passing over Caithness in its course. The difficulty which 

 seems to beset this theory is, that a glacier entering the Firth would 

 not leave it and ascend over upon Caithness. It w;ould take the path 

 of least resistance and move into the North Sea, where it would find 

 a free passage into deeper water. Mr. Peacli's. theory is however 

 an important step in the right direction. It is a part of the truth, 

 but I believe not the whole truth. The following is submitted as a 

 solution of the question. 



The Proposed Theory. 



The opinion is now pretty well established that, during the severer 

 part of the Glacial period, Scotland was covered with one continuous 

 mantle of ice, of such a thickness as to bury under it the Ochils, 

 Sidlaw, Pentlands, Campsie, and other moderately high mountain 

 ranges. For example, Mr. Geikie and some of the , other officers of 

 the Geological Survey found that the great masses of the ice from 

 the North-west Highlands, came straight over the Ochils of Perth- 

 shire and the Lomonds of Fife. In fact, these mountain ridges were 

 not sufficiently high to deflect the icy stream either to the right hand 

 or to the left. The flattened and rounded tops of the Campsie, 

 Pentland, and Lammermoor ranges, bear testimony to the denuding 

 power of ice. And I have been informed that a member of the 

 Edinburgh Geological Society has lately found Boulder-clay on the 

 Pentlands at an elevation of 1,600 fee't. . 



The following facts, recorded by JNfr. Jamieson, will show how 

 prodigious must have been the thickness of the ice in the North 

 Highlands. 



'•All the facts," says Mr. Jamieson, "are in harmony with the notion 

 that the ice was of enormous thickness. Thus the detached moun- 

 tain of Schehallion in Perthshire, 3.500 feet higli, is marked near 

 the top as well as on its flanks, and this not by ice flowing down the 

 sides of the hill itself, but by ice pressing over it from the north. 



