150 GROVE KARL GILBERT— DAVIS [MEMO,Ks tvouxxi'; 



Gilbert spent from one to three months of seven successive summers, 1S85 to 1891, in west- 

 ern New York and neighboring areas; Canada was visited between Lakes Ontario and Huron 

 in the summer of 1887 at his personal expense, as survey funds could not be used for studies 

 beyond the national boundary. He gathered a great body of information on ancient shore 

 lines, much of which he never found time to state in his printed articles, which were as a rule 

 very brief. He reached the conclusion that "in a general way the old water-plane is nearly flat 

 at the west end of Lake Erie, rises to the northeast about one foot per mile in eastern Ohio, 

 and rises about two feet per mile in Pennsylvania and New York." But he was by no means 

 alone in this work. Other observers also engaged upon it proved the occurrence of crustal 

 warping in various parts of the Great Lakes region that Gilbert did not visit; yet no other 

 observer generalized the results obtained so widely or so wisely as he did. As it was impossi- 

 ble for bim to devote more than a fraction of his time to field study, he welcomed all the re- 

 sults gained by others and gave every encouragement and assistance to his fellow observers by 

 freely communicating his own ideas to them. Thus he wrote in 1888, to one of the most assidu- 

 ous students of the problem: " The ground that you and are studying I had hoped to 



study myself. As it now seems improbable that I shall be able to visit . . . this summer it 

 occurs to me that there may be some advantage in my communicating to you such working 

 hypotheses as I have framed"; and thereupen follow four pages of helpful notes. Another 

 letter to the same worker, written in 1890 when parts of his work in the Ontario basin remained 

 unpublished because he could not advance them further at that time, made a generous offer: 



I shall be very glad to communicate any data in my possession which may aid you in your investigations 

 or conclusions. If you can come to Washington before you publish, I shall not only be glad to go over the 

 entire subject with you, but shall esteem it a privilege to exchange views. 



Similarly, a few years later, he wrote to another observer in the same field, urging him to 

 go ahead on his part of the problem: 



I have little sympathy with the dog-in-the-manger policy of those who fear a deprivation of the fruits of 

 their labors. 



THE SHORE LINES AND OUTLET OF LAKE IROQUOIS 



A phase of the problem in which Gilbert's observations were critical and decisive concerned 

 the distinction between the shore lines of ice-dammed lakes and those of arms of the sea; for 

 while it had long been generally understood that the lower St. Lawrence Valley had been occu- 

 pied by marine waters after the withdrawal of the ice sheet from it, difference of opinion 

 existed as to the altitude of the uppermost marine limit. Gilbert resolved this uncertainty 

 by a careful examination of the northern slopes of the Adirondack Mountains, where he spent 

 two months in the summer of 1887, mostly on foot. No shore fines were found there at levels 

 corresponding to those which occurred "on a magnificent scale" around the western slopes; 

 hence, he concluded that while the shore lines of the western slope were in process of 

 formation, the northern slope as well as the whole breadth of the St. Lawrence Valley was 

 occupied by the retreating ice sheet. The northern slope was not only free from shore 

 fines; it was also free from the eroded channels of lake outlets — except that, as found 

 several years later, one high-level example ef such a channel, locally known as "the Gulf," 

 was found in the Potsdam sandstones on the northern side of the Adirondacks close to the 

 New York-Canadian boundary. 



In the absence of such channels at lower levels on the north of the Adirondacks, the outlet 

 of the falling proglacial lake must have been elsewhere. It was found at the lowest point 

 of the broad divide southwest of the Adirondacks, between the present drainage areas of the 

 Lake Ontario and the Mohawk-Hudson River. Part of the channel floor at the divide is 

 occupied by a small town bearing the name of the Eternal City of the Old World; this point 

 of discharge is therefore known as the Rome outlet. The shore lines diverged from that point 

 to tha north and the west; hence the lake water lay to the northwest of it; and for this lake 

 the name Iroquois was adopted. Eastward the channel, soon entered and followed by the little 

 Mohawk from the north, had a gradually descending bed, broadly eroded in shales and lime- 

 stones and inclosed by subparallel banks; hence the discharge of the lake was in that direction. 



