254 W. N. THAYER 
time of this emergence is uncertain, but post-Devonian at all 
events, as deposits containing Devonian fossils have been found in 
places. These sediments were probably 5,000 feet thick over the 
province at the time of the emergence, and no doubt they also 
covered a part of the Laurentian Plateau and the Adirondack 
Mountains. 
From the time of the emergence until the Pleistocene epoch 
erosion appears to have been uninterrupted, and the region was 
denuded of 2,000 to 3,000 feet of rock. This almost completely 
uncovered the pre-Cambrian rocks of the plateau, and would have 
exposed the pre-Cambrian in the St. Lawrence Valley had not 
down-faulting on a large scale preserved a variable thickness of the 
sedimentaries.* 
During or immediately preceding the Pleistocene epoch there 
was a widespread downwarping of central and eastern Canada, 
which caused the former southward-flowing streams to change 
their direction of flow to the north. As the ice retreated, some of — 
the water found an outlet to the sea via the St. Lawrence River and 
distributed along its course a considerable amount of glacial débris. 
When the ice had retreated far enough, the sea advanced up the 
St. Lawrence Valley, assorted the glacial material by wave-action, 
and deposited upon it a thin bed of marine formation. With the 
final disappearance of the ice there was a general elevation of the 
north country, and marine beds are now found in certain places 
at elevations exceeding 600 feet.’ 
The Champlain-Hudson Valley was the scene of a similar 
sequence of events, as witness the character of the bedrock on the 
valley floor, the assorted glacial débris, and the postglacial marine 
beaches standing 300 or more feet above sea-level. 
«E. M. Kindle and L. D. Burling, op. cit., p. 20. 
2 J. W. Goldthwait, Geol. Survey Canada, Guide Book, No. 3, pp. 118-26. 
3W. J. Miller, V.Y. State Mus., Bull. 168. 
