THE CHAM PLAIN STAGE 511 



undergoing small and slow oscillatory movements, not having fully 

 reached isostatic repose. 



That the land northward from Boston was lower than now while the 

 ice-sheet was being melted away is proved by the occurrence of fossil 

 mollusks of far northern range, including Yoldia (Leda) arctica Gray, 

 which is now found living only in the Arctic seas, preferring localities 

 that receive muddy streams from glaciers and from the Greenland ice- 

 sheet. This species is plentiful in the stratified clays resting on the till 

 in the Saint Lawrence Valley and in New Brunswick and Maine, extend- 

 ing southward to Portsmouth, New Hampshire. But it is known that 

 the land was elevated from this depression to about its present height 

 before the sea here became warm and before the southern mollusks, which 

 exist as colonies in the Gulf of Saint Lawrence, migrated thither, for 

 these southern species are not included in the extensive lists of the fossil 

 fauna found in the beds overlying the till. 



In the Saint Lawrence Valley the marine deposits reach to the south- 

 ern end of Lake Champlain, whence the beds and this stage ending the 

 Ice Age are named, to Ogdensburg and Brockville, and at least to Pem- 

 broke and Allumette Island, in the Ottawa Eiver, about 75 miles above 

 the city of Ottawa. The isthmus of Chiegnecto, connecting Nova Scotia 

 with New Brunswick, was submerged, and the sea extended 50 to 100 

 miles up the valleys of the chief rivers of Maine and New Brunswick. 

 The uplift from the Champlain sealevel was 10 to 25 feet in the vicinity 

 of Boston and northeastward to Cape Ann; about 150 feet near Ports- 

 mouth, New Hampshire; from 150 to about 300 feet along the coast of 

 Maine and southern New Brunswick; about 40 feet on the northwestern 

 shore of Nova Scotia, thence increasing westward to 200 feet in the Bay 

 of Chaleurs, 375 feet in the Saint Lawrence, Valley opposite the Saguenay, 

 and about 560 feet at Montreal; 150 to 400 or 500 feet, increasing from 

 south to north, along the basin of Lake Champlain; about 275 feet at 

 Ogdensburg, and 450 feet near Ottawa. The differential elevation was 

 practically completed, as we have seen from the boreal character of the 

 Champlain marine molluscan fauna, shortly after the departure of the 

 ice-sheet. With the areas of the glacial lakes in the interior of the conti- 

 nent, this coastal region gives testimony of a wavelike elevation of the 

 formerly ice-burdened portion of the earth's crust, proportionate with the 

 glacial melting and closely following the retreat of the ice from its bound- 

 aries of greatest extent inward to the areas on which its waning remnants 

 lingered the latest. 



From the Champlain submergence our Atlantic coast was raised some- 



