BEACHES AND DELTAS ABOUT THE ST. LAWRENCE. 2(35 



charge from the upper lakes by way of Lake Nipissing was then consid- 

 erably lower than either their earlier Chicago outlet or their present outlet 

 by Detroit and Lake Erie. 



Quebec, the Eastern Provinces, the Northeast Territory, and Labrador. — 

 Attending the retreat of the ice-sheet from New England, Quebec, and the 

 eastern provinces, many glacial lakes of small size and short duration were 

 formed on areas declining toward the north or northwest, as in the valley of 

 the Coutoocook river in New Hampshire;* on the western flanks of the 

 Green Mountain range in Vermont, where Mr. C. L. Whittle informs me 

 that delta deposits of such origin occur up to heights of fully 2,000 feet; on 

 head streams of the River St. John in northern Maine ; and in southern 

 Quebec, between the Atlantic-St. Lawrence water-shed and the receding 

 ice-front. Fewer and still smaller glacial lakes, usually leaving no well- 

 marked records of their existence, doubtless also attended the glacial retreat 

 in New Brunswick, Nova Scotia, Newfoundland, and Labrador. But soon 

 the ocean-washed ice-border was melted back from the Gulf of St. LaAvrence 

 and along the broad St. Lawrence valley to Quebec and Montreal, admit- 

 ting the sea to the area of Lake Champlain, which, with the Hudson valley, 

 had been occupied during the recession of the ice by a long and narrow 

 glacial kke, extending from near New York city to near Montreal, caused 

 by the southward elevation and northward depression of the laud.t 



North of the St. Lawrence the receding ice opposed no barrier to 

 drainage from large areas until it withdrew across the height of land dividing 

 the St. Lawrence waters from those tributary to James's and Hudson's bays, 

 when upon the country around Lake Mistassini and upon many other tracts 

 glacial lakes of considerable size must have been formed. In the exploration 

 of that region traces of these former lakes, especially of their channels cross- 

 ing the water-shed, should be carefully looked for, as not the least important 

 of our records of the ice age. 



Extent and Thickness of the Ice-Sheet. 



The relation of the glacial lakes to the ice-sheet leads us to inquire what 

 were the extent and thickness of the ic^, its centers of outflow, the manner 

 of its final departure, and the areas probably occupied by its latest rem- 

 nants. In connection with these inquiries, we learn much from the delta 

 deposits of the glacial lakes, especially Lake Agassiz, concerning the out- 

 lines of the ice in its recession and the amount of its englacial drift, which 

 w^as contained in the ice-sheet and was set free during its final melting. 



The extreme southern limit of the glacial drift, and the division between 



* Geology of New Hampshire, vol. IH, 1878, pp. 103-120. 

 t Bull. Geol. Soc. Am., vol. 1, p. 566. 



