academy of sciences] NIAGAEA AND THE GREAT LAKES 223 



This unpublished description is a good example of the richness of Gilbert's records. The 

 channel that he thus recognized is an important pathway. It is followed by a long-established 

 main road, the Erie Canal, now enlarged as the Barge Canal, the. four-track New York Central 

 Railway, the double-track West Shore Railway, and an electric railway for more local traffic. 

 It is therefore the route of hundreds of thousands of travelers every year. Its features are well 

 shown on the contoured topographic maps of the district and may be reviewed to great advan- 

 tage from the rear vestibule of an observation car, such as is commonly attached to through 

 express trains ; but probably not one in a hundred thousand of those who thus look at the chan- 

 nel see anything more than a pathway of travel, although it is of exceptional length. The 

 cross-spur channels of the plateau margin are much shorter; they are, geographically, small 

 affairs, but it would be difficult to name any other group of physiographic features of similar 

 size that possess a greater scientific significance; and tliis significance lies not so much in the 

 record that they preserve of a highly dramatic phase in the evolution of Niagara and the Great 

 Lakes as in the extraordinarily convincing testimony given by the accordant interrelations of 

 their detailed forms for the verity of certain curious conditions during a late phase of the Glacial 

 period. One might be tempted to compare Gilbert to a wizard in view of his capacity to bring 

 all these details into their true relations; but the comparison would be wrong, for the essence 

 of a wizard's performance lies in something that is intentionally concealed, while the essence of 

 Gilbert's presentation is its frank lucidity. 



THE NIAGARA ESCARPMENT 



Later in the same year, 1896, Gilbert made a characteristically accurate contribution to 

 the long-debated problem of glacial erosion. 3 It has already been noted that he took small part 

 in the old discussion as to the origin of the Great Lake basins by glacial erosion; it may now 

 be told that he took a very definite attitude with respect to the effect of glacial erosion in modi- 

 fying the preexistent escarpment of the Niagara upland, and further that it was in connection 

 with his observations on this matter that he discovered the sites of four temporary Niagara 

 falls in addition to the cataract that is still in existence. As to the glacial erosion of the upland, 

 his argument was in essence as follows : The north-facing scarp of the upland, trending east and 

 west where the Niagara River crosses it, was presumably as sinuous in preglacial time as non- 

 glaciated scarps of similar structure are to-day; but its outline is now simpler and its simplifica- 

 tion is due to ice scouring. Where the ice moving to the southwest advanced against the eastern 

 side of a salient, the irregularities in the resistant limestone which caps the escarpment were 

 exaggerated and small reentrants were worn back into furrows; but where the ice passed along 

 the western side of a salient, the escarpment was planed off to a nearly smooth front. The 

 weaker shales of the lowland on the north were scoured down and fluted in the direction of ice 

 motion; the measure of ice erosion there was estimated to exceed that on the upland limestone 

 in the ratio of 20 to 1. 



Regarding the first establishment of Niagara River: Apart from a well-known preglacial 

 and drift-filled reentrant west of the Niagara gorge, and in addition to the great gorge of the 

 actual Niagara which opens at Lewiston, Gilbert detected, as just stated, four postglacial notches 

 in the Niagara escarpment ; and he was thus led to believe that when, by reason of a change of 

 outlet, a great proglacial lake was divided into two smaller lakes separated by the Niagara 

 upland, the northern edge of the upland was so nearly level that frhe higher lake on the southwest 

 spilled over to the lower lake on the northeast at five different points. He wrote of this discovery 

 to a friend on November 24, 1896: 



One of the most interesting points brought out in the last week of field work is the fact that the water 

 drained from the Niagara escarpment at five points having nearly the same level. I think these drains were 

 simultaneous, beginning all at once during the lowering of a lacustrine water plane. Two of the discharges 

 . . . were of moderate volume and quickly ceast; a third . . . was more important, either in volume or in time 

 . . . The other two were at Lockport and at Lewiston. 



' Glacial sculpture in western New York. Bull. Oeol. Soc. Amer., i, 1899, 121-130. 



