acad>m< op sciences] PROGLACIAL GREAT LAKES 151 



At a point where the outflowing waters farther downstream crossed a small area of resistant 

 crystalline rocks, the channel assumes the form of a narrow, rocky gorge of steeper descent, 

 where the lake waters must have rushed in tumultuous cascades and rapids ; the Mohawk River 

 still cascades there, and on its northern bank is the manufacturing town of Little Falls. 



After the outlet channel of Lake Iroquois was found, Gilbert made one of the finest of 

 all his beautiful interpretations concerning the postglacial physiographic history of the region. 

 He saw that if the upheaval of the land in the northeast continued while the discharge of the 

 lake was maintained at the Rome outlet, the shore should progressively emerge from the lake 

 to the north of the outlet, and the lake should progressively submerge the shore to the west. 

 In other words, the successive attitudes of the lake margin to the north of an axial line of no 

 change drawn northwestward through the outlet should be marked by a series of lower and 

 lower shore lines, each of which would be left undisturbed by the withdrawing waters; and 

 the shore lines thus formed, all uniting at the outlet, should diverge northward, so as to be 

 separated by increasing vertical intervals with increase of distance from Rome. On the other 

 hand, the successive lake margins to the west of the axial line should rise upon the land and, 

 pushing the beach gravels before them more or less successfully, should tend to obhterate 

 the first formed shore lines, and therefore leave a simpler record than to the north. 



Later observations completely verified these expectations. To the north there are mul- 

 tiple and gradually diverging beaches, which at their greatest distance from Rome range over 

 a vertical interval of scores of feet; the development of these shore lines "on a magnificent 

 scale" was presumably favored by their situation at the eastern end of a large lake prevailingly 

 swept over by westerly winds. To the west of the Rome outlet the shores are marked by 

 simpler beaches, often by only a single beach, the structure of which shows an upward and 

 landward growth. A long stretch of this single beach is displayed in the drumlin district 

 between Syracuse and Rochester, where the beach usually consists of shoal-water bars forming 

 a long linear succession of short members, each slightly concave to the north, between the 

 clift ends of numerous drumlins; farther south "many other dm ml ins are intact; farther north 

 a plain slants away and drumlins are wanting for a considerable distance, as if the encroaching 

 waves of the slowly rising lake had cut the drumlins away there as they pushed the beach 

 bars farther and farther south. The beach would undoubtedly have been pushed farther 

 south still had not the ice barrier in the St. Lawrence Valley withdrawn sufficiently to open 

 a lower passage, whereupon the Rome outlet to the Mohawk was abandoned, the St. Lawrence 

 River came into being, Lake Iroquois was lowered to Lake Ontario, and the beach bars were 

 left high and dry. It all became astonishingly simple and clear as soon as Gilbert showed how 

 clear and simple it really is. 



A detail may be here added. A southward extension of the Ontario lowland permitted 

 the extension of a long bay branch of Lake Iroquois into the north-south trough now occupied by 

 Cayuga Lake, one of the several Finger Lakes of western New York; and it might be expected 

 that the Iroquois shore lines which are so well developed east and west of this bay branch, 

 should be found looping southward around the sides of the trough and thus marking the extent 

 of the bay. But as a matter of fact the shore lines are not visible in the farther part of the 

 trough. Gilbert explained their invisibility not by their absence, but by their submergence 

 beneath the Cayuga waters, which in consequence of the uplift of the land to the northeast 

 have risen increasingly southward. This is a good example of the interlocking of small items/ 

 by which Gilbert's studies on the proglacial lakes were so largely characterized. 



The single western beach Hs found in good force in the form of an offshore bar at Lewiston, 

 on the American side of the Niagara, as it flows northward from its gorge across the low land 

 that leads to Lake Ontario. The structure of the bar, well revealed in a gravel pit, is that 

 of overwashed layers of gravel that slant backward, away from the lake area and toward the 

 land border; that is, the bar was retrograded as the lake slowly rose. Gilbert's letters tell of the 

 satisfaction he had on coming upon this part of the beach and finding that the arrangement 

 of its layers gave independent confirmation of the inference as to retrogression that he had 

 made many miles away to the eastward. He had the pleasure of explaining the beach and 



