George M. Dmoson — Erratics at High Levels. 211 



that adopted by Principal Dawson to explain the transport of blocks 

 of sandstone from the Cumberland plains of Nova Scotia to the 

 summits of the Cobequid Hills, viz. that the ice-fields of successive 

 years may have raised these erratics and deposited them at higher 

 and higher levels, during a more or less rapid subsidence of the 

 land.^ This process is at least competent to effect the result, and 

 wherever coast-ice surrounds a sinking shore., the materials of the 

 beach must thus be gradually warped up, though but a small pro- 

 portion of the material from below could ever reach a great height. 



It is probably to action of this kind that Darwin wishes to appeal 

 in endeavouring to explain the positions of erratics in some parts of 

 England, though I only know his views by a reference to them by 

 Mr. Geikie, in a note on the same subject read by him before the 

 Geological Society of Glasgow in 1873, and in part reprinted in the 

 Great Ice Age ; in which it is endeavoured to explain the facts by 

 the forcing up in the mass of a glacier, owing to " frontal resist- 

 ance " of stones included in it. 



To my mind, many facts seem to show the impossibility of the 

 westward extension of a vast ice-sheet from the Laurentian Axis 

 across the plains. The most striking of these is perhaps the 

 existence of the great escarpment of soft Cretaceous rocks, which 

 runs parallel with the south-western base of the Laurentian, at an 

 average distance of 130 miles from it; the Winnipeg group of lakes 

 occupying the intervening low ground. There are gaps in this 

 escarpment, the most extensive being the valley of the Assineboine ; 

 but, in the main, it extends from near the Saskatchewan Eiver to the 

 forty -ninth parallel, with a bold north-eastern front, and a height in 

 some places of 900 feet above the low country occupied by the lakes. 

 That any mass of glacier-ice should have been forced across this 

 escarpment without destroying it, seems incredible ; though that 

 the La.urentian Glacier may have reached its base in places appears 

 not improbable. 



The wide valley occupied by the Winnipeg group of lakes and 

 Eed Eiver has doubtless, in the first instance, been formed by river- 

 erosion. A great stream at one time probably flowed southward in 

 it,' gradually cutting downward, and shifting its channel westward 

 on the sloping surface of the hard Laurentian rocks, at the expense 

 at first of the soft Cretaceous strata, and later of the Devonian 

 and Silurian limestones. The valley is pre-glacial, and, with the 

 escarpment, has been produced in this way precisely in the manner 

 explained by Professor Eamsay in describing the formation of the 

 Weald of Kent and Sussex.^ A like process may also account, to a 

 great extent, for the production of the valleys now occupied by the 

 Great Canadian lakes, and of those of Athabasca, Great Slave, and 

 Great Bear Lakes in the far North-west ; all of which hold a similar 

 position with regard to the Laurentian region, and overlying newer 

 and little-disturbed rocks. 



1 Acadian Geology, p. 65. 



2 Geology and Resources of the 49tli Parallel, p. 253. 



3 Physical Geology and Geography of Great Britain. 



