THE PLEISTOCENE OR GLACIAL PERIOD. 4S1 



the great ice-sheets. There have also been changes of level, though 

 probably less extensive, in the non-glaciated areas of the southern 

 and southeastern part of the continent. 



As already noted, some of the islands of southern California * seem 

 to have risen something like 1500 feet since the Pliocene. Other parts 

 of the California coast, and some of the adjacent islands, have been 

 subsiding during the same period. 2 Xear San Francisco, the surface 

 is thought to have ranged from 1800 feet below its present level, to 

 400 feet above. 2 Walcott has estimated that there has been eleva- 

 tion in the Inyo Mountains of California to the extent of 3000 feet 

 during the Pleistocene. 4 Along the northwestern coast of Oregon, i ; ~ 

 has been estimated that there has been a rise of at least 200 feet 

 during the Pleistocene. 5 Data concerning Pleistocene changes of level 

 in the west are not sufficiently numerous to permit the determination 

 of the axes of movement, if such there be. 



In general, the areas covered by the ice of the glacial period have 

 risen since the ice melted. It is a tenable hypothesis that the rise, 

 or some part of it, has resulted from the melting of the ice, and that 

 it followed a depression occasioned by the weight of the ice. The 

 rise of the land has, in general terms, been greatest where the ice was 

 thickest. 6 This rise of the glacial centers is shown in many ways, but 

 especially by the raised beaches along the coast lines, and by the deformed 

 shore lines of the interior lakes. Thus the shore lines of Lake Agassiz 7 

 are no longer horizontal, but are considerably higher at the north than 

 at the south. Their inclination is as much as a foot to the mile in 

 the northern part of the. basin. At the National Boundary, the shore 

 lines are 175 feet above those at the southern terminus of the lake, 

 and 200 miles north of the boundary they are 400 feet above the same 

 point. This deformation was largely accomplished before the lake 

 disappeared. 



1 W. S. T. Smith, Bull. Dept. of Geol., Univ. of Cal., Vol. II. Reviewed in Jour. 

 of Geol., Vol VIII, p. 780. 



2 Lawson, Bull. Dept. Geol., Univ. of Cal., Vol. I. Reviewed in Jour. Geol., Vol. II, 

 p. 235. 



3 Ashley, Jour, of Geol., Vol. Ill, p. 449. 

 * Jour, of Geol., Vol. V, p. 340. 



5 Diller, 17th Ann. Rept. U. S. Geol. Surv., Pt. I. 



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



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



