BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 

 Vol. 2, pp. 465-476, PL. 19 APRIL 30, 1891 



POST-PLEISTOCENE SUBSIDENCE VERSUS GLACIAL DAMS. 



BY J. W. SPENCER, M. A., PH. D., F. G. S. (l. AND A.). 

 {Read hejore ihe Society December 29, 1890.) 



CONTENTS. 



Page. 



General continental Oscillations 465 



Evidence of recent regional Emergence 466 



Interpretation of the Evidence 469 



Glacial Dams considered 471 



Conclusions 473 



General Continental Oscillations. 



The growing interest in the evolution of the continent now calls for more 

 accurate information than formerly, regarding the changes of level of land 

 and sea in recent geological times. As these oscillations constituted some 

 of the most important factors in the building of the Great Lakes, the study 

 of their history has contributed to our knowledge of the changing relations 

 of the continent and the sea. 



From investigation of the submerged channels along the American coast, 

 it has been shown that the continent Avas greatly elevated during some epoch 

 or epochs intervening between the middle Miocene and the early Pleistocene 

 periods. The elevation of the land was over 3,000 feet higher than now, 

 and probably reached for a short time to over 5,000 feet.* 



This elevated condition of the continent was followed by a depression of 

 the land to far below the present altitude before the upward movement 

 produced the now existing condition. There may have been more than one 

 episode of elevation and depression ; but the problem that we seek to solve 

 is. What was the maximum depression of the later Pleistocene times, after the 

 great beds of bowlder clay were formed; for the great elevation was shortly 

 before that period ? 



Most geologists are ready to accept the high continental elevation, but 

 there are differences of opinion respecting the amount of the subsidence. 



*"The High Continental Elevation preceding the Pleistocene Period," by J. W. Spencer: Bull. 

 Geol. Soc. Am., vol. 1, 1889, pp. 65-70. 



LXVIII— Bull. Geol. Soc. Am., Vol. 2, 1890. (465) 



