RECESSION OF THE ICE-SHEET. 127 



Extensive and thick beds of gravel, sand, and clay or fine silt, called 

 stratified or modified drift, were deposited a|ong tlie avenues of drainage 

 from the glacial boundary, especially dui-ing its rapid final recession. The 

 dissolution of the ice, with accompanying rains, produced extraordinary 

 floods along all the rivers flowing away from the waning ice-sheet; and 

 these were heavily laden with detritus set free from the lower part of the 

 ice in which it had been held, and brought down by the rills and small and 

 large streams formed on the melting ice surface. Other portions of the 

 englacial drift were let down as an iipper deposit of till, which lies in a 

 loose, unstratified mass upon the subglaeial till or groui:d moraine. The 

 abundant deposition of di'ift, both stratified and unstratified, during the 

 final melting of the ice-sheet, was first brought into due prominence by 

 Prof James D. Dana,' who denominated this the Champlain epoch, deiiv- 

 ing the name fi'om its marine beds adjoining Lake Champlain. 



On the Atlantic Coast the Champlain subsidence of the land below its 

 present level is known, from fossiliferous maiine beds overlying the till, to 

 have been slight in northeastern Massachusetts, 150 to 230 feet in New 

 Hampshire and Maine, nothing or of small amount in Nova Scotia, but 

 considerable, with increase from east to west, along the lower St. Lawrence 

 Valley, being 375 feet opposite the Saguenay and 560 feet at Montreal, 

 but thence diminishing southward along Lake Champlain and westward in 

 the upper St. Lawrence and Otta'^^'a valleys. The country southwest of 

 Hudson Bay sank 300 to 500 feet; Labrador, 1,000 to 1,500 feet; and 

 western Greenland and Grinnell Land, 1,000 to 2,000 feet. Again, in 

 British Columliia and the Queen Charlotte Islands Dr. Dawson and otliers 

 find proofs of submergence, ranging up to 200 or 300 feet, while the glacial 

 conditions still endured. 



This closing stage of the Glacial period was immediately succeeded 

 by a time of great erosion of the ^•alley deposits of stratified drift, as soon 

 as the continued glacial recession beyond the drainage areas of the rivers 

 cut oft' the supi^ly of water and of drift that had been derived from the 

 melting ice. The resulting excavation of the glacial flood-plains has left 

 remnants of those deposits in conspicuous terraces along all our river val- 



' Am. .Jour. Sci. (3), Vol. V. p. 198, aud various papers iu Vol. X- 



