662 THE PLEISTOCENE PERIOD 



Such changes are probably but a meager index of the crustal warp- 

 ings of the period. Specific data on this point are less abundant 

 than could be desired, for the phenomena of erosion and deposition 

 which followed the elevation at the close of the Tertiary are not 

 readily differentiated from similar phenomena resulting from later 

 elevations. Nevertheless, evidence of Pleistocene changes of level, 

 as distinct from late Pliocene, are not wanting, especially near the 

 coasts and about the shores of the Great Lakes. 



From the evidence at hand, it appears that deformative move- 

 ments were widespread both in the western mountains and in the 

 area covered by the great ice-sheets. In general, the areas covered 

 by the ice-sheets have risen since the ice melted. It is a tenable 

 hypothesis that the rise, or some part of it, resulted from the melting 

 of the ice, and that it followed a depression caused by the weight 

 of the ice. The rise of the land has been greatest, on the average, 

 where the ice was thickest. This rise of the glacial centers is shown 

 in various ways, but especially by the raised beaches along the 

 coasts, and by the deformed shore lines of the interior lakes. Thus 

 the shore lines of Lake Agassiz are considerably higher at the north 

 than at the south, their inclination being as much as a foot to the 

 mile in the northern part of the basin. The shore lines of Lake 

 Iroquois (p. 639) decline from the northeast to the southwest at 

 the average rate of three and a half feet per mile. The beaches of 

 Lake Algonquin (Fig. 530) are 25 feet above the present lake at 

 Port Huron, and 635 feet above the lake at North Bay, Ontario. 

 The shore lines of the other lakes show comparable warping. 



There have been changes of level, though less extensive in 

 most places, in regions which were not glaciated. Thus along the 

 Atlantic coast south of the drift there have perhaps been complex 

 movements, but of no great range, in the course of the period. On 

 the whole, elevation (relative) appears to have exceeded depression, 

 but the latest movement (present) appears to have been one of sink- 

 ing, as the drowned ends of the valleys show. 



It is not improbable that movements of equal magnitude have 

 affected the interior regions of the continent, but, except about 

 the lakes, there is no datum plane like the sea-level to which these 

 changes may be readily referred. In a few places, local deformation 

 is notable. In New York and Ohio, the solution of underlying gyp- 

 sum and salt is suspected of being the occasion of some of the slight 

 deformations observed. 



