CENOzoic (interior). 297 
lakes, whose former margins, now often far inland, they represent. 
In the Great Lake region, they are known to occur at various 
altitudes, from 1500 feet, or more above, down to the sea-level. 
Most of the scattered surface erratics belong to the Beach or 
Terrace series, having been chiefly derived from the older boul- 
der clay. All the Plistocene formations have been deformed 
by the continental oscillations and warpings subsequent to their 
deposition. The warping or differential elevation of the earth's 
crust about the Great Lakes, since they formed one body of water 
(Lake Warren), varies from almost zero to five feet or more per 
mile. The effects of the Plistocene warpings have been to de- 
form the old valleys and cause changes in the drainage (besides 
those from drift obstructions) and to bring to the surface rocky 
barriers, which, in part, close the older Cenozoic valleys of the 
Great Lake region, and convert them into lake basins. This 
phase is still one of the newer studies of Plistocene Geology. In 
the beaches. Mammoth, Wapiti, and Beaver remains, have been 
found. Fresh water shells are positively known to occur only 
in the lower beaches about the different lakes, and shell-life of 
any kind appears to be absent from all the higher shore deposits. 
However, in passing from the lakes to the St. Lawrence Valley, 
marine remains are found at increasing altitudes, in proceeding 
eastward — those at Montreal occur at 520 feet above the sea» 
and others at still higher elevations farther seaward. 
Other lake deposits are found beyond the region of the Great 
Lakes, as those of the Red River Valley of the North (Lake 
Agassiz). Between the Wasatch and Sierra Nevada Mountains, 
alluvial and marly beds, belonging, in whole or part, to the 
Plistocene System, are of wide extent. These beds were deposited 
in lakes now extinct or shrunken, of which the two best known 
and largest are Lake Bonneville (of which Salt Lake is a 
remnant) and Lake Lahontan (shrunken to Cai-son Lake). The 
terraces show recent differential changes of level, as in the Great 
Lake region. 
