152 GROVE KARL GILBERT— DAVIS tMEMOIBS [ v J o A ™xi I ; 



its structure to a party of European and American geologists who made an excursion to Niagara 

 during a recess of the British association meeting at Toronto in the summer of 1897. 



But it was not only the shore lines of the extinct proglacial lakes that were shifted by the 

 tilting of the region; the shore lines of the existing lakes show the effects of similar changes, and 

 thus indicate that the tilting has been recently and is perhaps still in progress. The present 

 Lake Ontario, which lies wholly southwest of an oblique northwest-southeast line of no tilting 

 across its St. Lawrence outlet, has risen at its western end so as to drown certain valleys that 

 were eroded with respect to its surface as baselevel before the final rise took place. The 

 lower (northernmost) part of Niagara River is a fine instance of this kind; it is not a river 

 proper but a narrow embayment, deep enough to be navigable, which half drowns the valley 

 that the river had eroded across the lowland north of the Niagara escarpment before the south- 

 western end of the lake had risen to its present level in response to the tilting of its basin. A 

 little farther east Irondequoit Bay, on which Gilbert had a skating adventure in his boyhood 

 as already told, is a similar instance, although its cavity is not of so simple an origin as that of 

 the lower Niagara. Lake Erie repeats the same story; and the little embayments near its 

 western end, which Gilbert had acutely interpreted as drowned valleys in his Maumee Valley 

 report of 1871-73, are thus explained. The most important of them constitutes the harbor of 

 Toledo. The navigable depth of Detroit River, a matter of vast economic importance, is 

 similarly accounted for. How far the light of a true explanation throws its ihuminating rays ! 



A POPULAR ARTICLE ON THE GREAT LAKES 



The first published reports of Gilbert's progress in his Great Lakes studies were brief and 

 incomplete, but in 1888 he brought out a popular article ' which summarized them admirably. 

 It was seldom that he approached the general reading public in this manner; but his success 

 in this instance was such as to make one wish he had oftener done so. His manner of presen- 

 tation was graceful and pleasing, being intermediate between the sententiousness of some of 

 his formal reports and the jocosity of his familiar letters. The following extracts are therefore 

 presented as much in illustration of his easy style as in explanation of his results. 



After noting that "rivers are the mortal enemies of lakes," and recounting the manner in 

 which lakes are destroyed by their enemy rivers, he goes on : 



Nevertheless, in spite of this warfare of extermination, waged in all lands and through all time, there con- 

 tinue to be lakes, and so there must be in nature lake-producing as well as lake-destroying agencies. There 

 are indeed many such, but a few only need be appealed to to explain the great majority of lakes, and the chief 

 are upheaval and glaciation. 



These chief agencies are then briefly explained. As to the first, the — 



great natural process of uplift and downthrow tends to produce lake basins, and ... its tendency is opposed 

 by the great natural process of erosion and deposition by rivers. The two are so nearly balanced that the scale 

 is thrown to one side or the other by the accident of climate. Where much rain falls the rivers are powerful 

 and prevail, sawing gorges through ridges as fast as they rise, building up the floors of valleys as fast as they 

 sink. Where little rain falls the streams are weak, and the displacement of the earth's crust shapes the land 

 into lake basins. 



Hence lakes of this kind characterize recently disturbed arid regions. The effects of glacia- 

 tion are then taken up. 



A glacier is aptly called a river of ice. Like a river of water it has an upper surface sloping continuously 

 from source to goal, and like a river of water it rests on an uneven bed of its own shaping. When an aqueous 

 river is suddenly deprived of its supply of water, there remain along its channel a series of pools recording 

 the inequalities of erosion. When a great glacier is melted away the inequalities of its erosion are recorded in 

 a chain of lakes. 



The behavior of the ice sheet of the glacial period is briefly considered: 



Instead of flowing from a mountain down a sloping valley, it flowed radially from a central plateau of ice, 

 with little regard for the slopes of the land over which it passed. We do not yet know the center of dispersion, 

 but the ice entered our land as an invader from Canada . . . Twice the van was pushed far into the domain 



1 Changes of level of the Great Lakes. Forum, v, 1888, 417-428. 



