CEAMPLAIN EPOCH. 555 



produce flooding, it is probable that there were two flooded periods. 

 These have doubtless been often confounded with one another. 



II. Champlain Epoch. 



During the Glacial epoch, as just seen, the whole northern portion 

 of the continent was elevated 1,000 to 2,000 feet above the present con- 

 dition ; the northern ice-sheet had advanced southward to 40° latitude, 

 with still farther southward projections favored by local conditions ; 

 and an arctic rigor of climate prevailed over the United States even to 

 the shores of the Gulf. At the end of this epoch an opposite or down- 

 ward movement of land-surface over the same region, probably in- 

 creased by the weight of accumulating ice, commenced and continued 

 until a depression of 500 to 1,000 feet below the present level was at- 

 tained. This downward movement marks the beginning of the Cham- 

 plain epoch. As a necessary consequence, large portions of the now 

 land were submerged ; it was therefore a time of inland seas. Another 

 result, or at least a concomitant, was a moderation of the climate, a 

 melting of the glaciers, and a final retreat of the ice-sheet northward. 

 It was therefore a time of flooded lakes and rivers. Lastly, over these 

 inland seas and great lakes loosened masses of ice floated as icebergs. 

 It was therefore pre-eminently a time of iceberg action. 



Evidences of Subsidence. — The evidences of the condition of things 

 described above are found in old sea-margins, old lake-margins, old 

 river-terraces, and old flood-plain deposits. 



1. Sea-Margins. — Old sea-margins, containing shells and other re- 

 mains of living species, are found all along the Northern Atlantic coast, 

 becoming higher as we pass northward. In Southern New England the 

 highest beaches are 40 to 50 feet ; about Boston they are 75 to 100 

 feet ; in Maine they are 200 feet and upward ; on the Gulf of St. Law- 

 rence they are 470 feet ; in Labrador 1,500 feet (Upham). In arctic 

 regions they are in some places 1,000 feet (Dana). The beaches may be 

 traced up both sides of the St. Lawrence River, and thence around Lake 

 Champlain, where the highest is 393 feet above tide-level.* LTpon the 

 beaches about Lake Champlain have been found abundance of marine 

 shells, and also the skeleton of a stranded whale. Evidently there was 

 here a great inland sea connected with the ocean through the Gulf of 

 St. Lawrence ; and over this sea icebergs must have floated. This con- 

 dition of things has given name to the epoch. In the subsequent re- 

 elevation of the continent, this salt lake (as it must have been at first) 

 was gradually rinsed out and freshened by river-water discharged 

 through the lake and into the St. Lawrence River, as already explained 

 on a previous page (p. 81.) All the crust-oscillations characteristic ol 



* Dana, Manual, p. 550. 



