GLACIAL AND POSTGLACIAL HISTORY OF GREAT LAKES REGION. 333 



PRESENT STABILITY OF THE LAND. 



In the northern part of the upper lakes many evidences, such as the newness of the modern 

 shore line and of the mouths or lower courses of the Nipigon and other northern rivers, strongly 

 suggest very recent or progressive emergence. In other parts of the lake region the evidence 

 seems to be against progressive change. Gilbert found what he believed to be evidence of pro- 

 gressive change in the records of the lake gages, and he estimated the rate of differential uplifting. 

 Recent investigations, however, have made changes in the data which he used and have neces- 

 sitated a modification of his results. 



Since the discharge returned to Port Huron, St. Clair River has cut down its bed by about 

 15 feet and Detroit River by half as much. This has greatly reduced the drowning of tribu- 

 taries caused by the return of the full volume to Port Huron after the abandonment of that outlet 

 during the time of the Nipissing Great Lakes. 



In the western parts of the Lake Erie and Lake Ontario basins recent raising of the water 

 level is indicated by drowned stream courses and in the Lake Erie basin by submerged stumps 

 and creek beds described by Moseley as occurring in Sandusky Bay. These facts seemed at 

 first to suggest that the tilting might still be in progress. But the drowning effects in these two 

 basins, at least to depths of 10 or 15 feet, are probably due to the return of the large volume of 

 discharge to Buffalo after the relatively long period of small discharge during the time of the 

 Nipissing Great Lakes rather than to very recent or progressive tilting of the land. Both at 

 Buffalo and on the St. Lawrence the outlet is on a rock sill and the downward cutting has been 

 slight. 



POSTGLACIAL MARINE WATERS IN THE OTTAWA AND ST. LAWRENCE VALLEYS AND IN 



THE LAKE ONTARIO BASIN. 



When the ice sheet withdrew from the basin of Lake Ontario and the northern slope of 

 the Adirondacks, the sea entered in its place and covered a large area. Its approximate limits 

 are indicated by clays, gravels, and sands, which contain fossil remains of marine organisms 

 such as are now living in the Gulf of St. Lawrence. The upper limit of the marine waters 

 was about 350 feet at Plattsburg, N. Y., and 523 feet at Covey Hill, Ontario. It was at least 460 

 feet at Welsh's siding near Smiths Falls, Ontario, and may have been much more, for at the 

 last-mentioned place the remains of a whale have been found in a gravel bed, definitely fixing 

 a minimum upper limit of marine submergence. The height of marine submergence at Mont- 

 real is reported to be about 625 feet. On the south side of St. Lawrence River a well-defined 

 beach marks what is taken' to be the upper limit of the marine waters and has been called by 

 Gilbert the Oswego beach. It declines gradually toward the southwest and passes under the 

 present level of Lake Ontario about at Oswego. 



The marine waters appear to have entered the basin of Lake Ontario some time in the later 

 part of the time of Lake Algonquin. Uplift appears to have been in progress when the sea 

 entered and to have soon ended the marine connection by raising the outlet above the sea. 

 At the end of Lake Algonquin the sea probably did not extend above Cornwall and Ottawa. 



