912 GEOLOGY 



doubtless continued into the Pleistocene. Minor movements of 

 later date, such as those which affected the basins of Lakes Bonne- 

 ville and Lahontan during the Pleistocene have also been noted, 

 but such changes are probably but a meager index of the crust al 

 warpings of the period. Specific data on this point are less abun- 

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

 deposition which followed the elevation of the Ozarkian or Sierran 

 epoch are not readily differentiated from the similar phenomena 

 resulting from later elevation. Nevertheless evidence of Pleisto- 

 cene 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 wide-spread 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 on the average been greatest 

 wLere the ice was thickest. 1 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 2 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 3 (p. 883) decline from the northeast to the southwest 

 at the average rate of three and a half feet per mile. East of the 

 east end of Lake Ontario they are about 400 feet higher than at the 

 west end. The beaches of Lake Algonquin 4 (Fig. 588) 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 



1 DeGeer, Proc. Boston Soc. Nat. Hist., Vol. XXV, 1892. 



2 Upham, Mono. XXV, U. S. Geol. Surv. 



3 Gilbert, 18th Ann. Kept., U. S. Geol. Surv. 



4 Taylor, A Short History of the Great Lakes, published in ,SV////' 

 Indiana Geography; also Am. Jour. Sci., Vol. LXIX (1895), pp. <>'.), '-'I'.). 



