CHAPTER LI. 



THE CHAMPLAIN AND TERRACE PERIODS. 



AFTER a long period of elevation the northern portions of 

 Europe and North America were slowly depressed and the ice be- 

 gan to retreat. The floods from the melting glacier opened many 

 old drainage channels that had long been closed by glacial debris. 

 But as the depression continued, the streams became sluggish 

 and began to silt up their channels with clay, sand, gravel, etc. 

 At length over great areas drainage almost ceased, and wide, 

 shallow lakes and marshes and ponds were formed, their surplus 

 waters carried away by broad, sluggish streams. The waters and 

 waves of these shallow lakes dissolved and broke down much of 

 the hard pan and assorted it into beds of sand and clay. There 

 were several slight oscillations of the crust, giving rise to differ- 

 ent conditions, so that in some localities there are as many as 

 six or seven layers of different kinds of material lying over the 

 boulder clay. 



The Orange sand or bluff gravel, a thick deposit of glacial 

 material along the Mississippi river as far south as Louisiana, 

 may have been transported by glacial floods early in this period. 

 Later the Erie, Champlain and other clays were deposited, and 

 later still, over much of the glaciated area, reaching far down the 

 Mississippi, thick beds of fine clay or loess were formed. 



The ice seems to have retreated more rapidly in the west than 

 in the east, so that when the region now occupied by the great 

 lakes was free from ice, the natural outlet of the Ontario basin 

 was still closed by the glacier. These valleys, filled with water 

 from the melting ice, formed a great inland sea with shores of 

 ice on the east and north, discharging its surplus waters toward 

 the Gulf. An old beach ridge, nearly parallel with the shores of 

 Lake Erie, shows the level of this old lake to have been at one 



L. 8.-5K 



