CHAPTER XVI 

 THE PROGLACIAL GREAT LAKES 



REACTION OF THE WEST UPON THE EAST 



After Gilbert, in his capacity as geologist in charge of the Appalachian division of the 

 national survey, had reserved for his own study the "evidence of elevation and subsidence 

 existing in the topography of the entire district," as above noted, he curiously enough restricted 

 himself almost wholly to the area traversed by the abandoned shore lines of ancient lakes in 

 New York, Ohio, and Michigan, an area that lies well to the west of the Appalachian belt 

 proper. As to his reasons for this choice of field, it may be that he wished to complete certain 

 inquiries which he had opened 15 years before when reporting upon the Maumee Valley for 

 the Ohio survey; or perhaps he was led by a half-conscious homing instinct to return to the 

 neighborhood of his native city of Rochester and once more concern himself with the origin 

 and age of Niagara Falls, a subject that he had touched upon briefly in his first published 

 essay of 1867; but it is more probable that he turned to the shore lines of the ancient Laurentian 

 lakes in the hope of solving there a problem that had deeply interested him during his studies 

 of the Bonneville shore lines in Utah. This supposition is supported by two allusions in his 

 western reports to the possible tilting of lake shore fines in the Laurentian region, as follows: 



The first Wheeler survey report mentioned the then suspected deformation of the Bonne- 

 ville shore lines as of especial interest because, if proved to be true, it would show "undula- 

 tions of the solid earth, produced at so late a geological date that we may presume them iden- 

 tical with changes now transpiring"; and then after pointing out that records of such undula- 

 tions are found only where a broad water surface, like that of the ocean, affords a datum plane 

 of reference, he added: 



There are, however, a few interior lake systems, broad enough to tell us something of the warping of their 

 shores, and our continent contains two, at least, that are of value — that of the Laurentian lakes, and the one 

 that we are considering (III, 1875, 93). 



Similarly, in the Henry Mountains report, brief reference is made to the tilting of the 

 earth's crust as causing changes in the outlets of the Laurentian lakes (1877, 126). It therefore 

 seems likely that on transferring his field of work from the West to the East, Gilbert brought 

 with him the general problem of crustal warping as recorded in the shore fines of extinct lakes; 

 as he could no longer work upon the problem in the basin of Great Salt Lake, he took it up 

 with respect to the extinct lakes of the Laurentian region. It is true that his first field studies 

 in the East were concerned with river terraces in the southern Appalachians, but he soon 

 turned his attention to the ancient shore lines in the Northern States above mentioned, as 

 probably affording better "evidence of elevation and subsidence." He at once found the evi- 

 dence to be both plentiful and definite; for in the summer of 1883, his first season in search 

 of it, he discovered that the ancient lake shore lines in New York and Ohio do not lie level 

 but, as others had already learned for New York, gradually rose to the northeast. He thus 

 entered upon an important chapter in the physiographic geology of northeastern North America. 

 Many pages of that chapter will always bear the marks of his genius. Curiously enough the 

 crustal tilting that he so -promptly detected was precisely the opposite of that which he had 

 postulated 15 years before in the Maumee Valley report as necessary to hold up the waters of 

 the enlarged Lake Erie; the shore lines showed that the region of the lower St. Lawrence had 

 been lower, not higher than now in the late stages of the glacial period; and he was therefore 

 led to accept the presence of a broad glacial barrier — the retreating ice sheet — in the St. Law- 

 rence Valley as a means of explaining the higher stand of the lake waters, essentially as New- 

 berry had suggested in his Maumee footnote. He at first phrased the change of level as a 

 "local relative depression of the land" in the region of Lakes Erie and Ontario, but later 

 regarded it due to uplift of the St. Lawrence region. 



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