POSTGLACIAL DEVELOPMENT OF CONNECTING BIVEKS OF GREAT LAKES. 495 



eastern bank in comparatively recent times. At the time of Early Lake Algonquin the river 

 could not have had this deep passage and have maintained the water at the level indicated by 

 the Rouge beach, which is now 13 feet above the river at the head of Grosse Isle. But if the 

 relatively deep channel at Limekiln crossing did not then exist, and if the river was held up 

 partly by the ledges of rock in the vicinity of Stony Island, and if the shallow passages west 

 of Stony Island and between Trenton and Grosse Isle were narrower than now, the former 

 high level of the river is explained. At that time the Sibley and Slocum Island channels were 

 no doubt active, but the channel east of Stony Island, now the main channel of the river, was 

 probably not open or was small. The bedrock in the river slopes downward east from Stony 

 Island and from the submerged rock ledges southeast of it. As the channel deepened and shifted 

 eastward, it was pressed over against the Canadian shore and more and more of the river was 

 drawn to it. The great curve which it made by cutting into the Canadian shore is exactly con- 

 tinued in the northeast-southwest line which forms the northern shore of Bois Blanc Island. 



It seems certain that this gradual eastward shifting of the channel, with deepening and 

 enlargement, has been the chief cause of the recent, post-Nipissing fall of the river and lake 

 waters to the north. It was here on the Limekiln crossing and on the rocky ledges about 

 Stony Island that the critical point lay. It was this obstruction which held up the waters of 

 the Lake Huron basin during the south-flowing stages of Lake Algonquin. 



Among the early distributaries one or two cut across to the south along a line passing a 

 little east of Bois Blanc Island, where the mam channel now is. This may have been the 

 Amherstburg distributary (p. 491), and it may have included the north branch of the Elliott 

 Point distributary. For a considerable time after the deepened channel between tbe Lime- 

 kiln crossing and the north end of Bois Blanc Island had been established, the channel east of 

 Bois Blanc remained undeveloped. Although much narrower than the passage west of the island 

 and narrower even than the present Trenton Channel, the channel east of Bois Blanc is the 

 deepest of all. That its development must have been relatively recent appears from its greater 

 depth and narrowness, from the steepness of its banks at Amherstburg, and from the abrupt 

 manner in which it departs from the great curve which the river had just previously developed. 

 It seems quite certain, therefore, that its enlargement took place after the development of the 

 great bend. 



When the first dredging was undertaken by the United States Government in the shallow 

 rapids at Limekiln crossing the greatest depth was 13 feet. On a fine running west from Lime- 

 kiln crossing the river was wide and shallow, with its deepest part in the newly excavated 

 bend. The opening of the Amherstburg passage, with its greater depth and shorter distance 

 must have increased the eroding power of the river at the Limekiln crossing, and with the 

 deepening of these parts the water was lowered more or less in the shallow rapids to the west 

 and in the Trenton Channel. 



It seems probable that at the time of Early Lake Algonquin the channels west of Grosse 

 Isle carried a volume of water as great or possibly greater than those east of it, but that the 

 more effective erosion on the east side has established the main channel in its present course. 



Several peculiarities of the present river are certainly very recent. The channel which 

 passes down the east side of Fighting Island is as deep as the main channel west of the island 

 but is much narrower. It appears newer than the main channel, especially from the fact that, 

 although deep, its submerged banks are very steep, much steeper than those of the main chan- 

 nel. This channel is a short cut, and the water in it flows a little more rapidly than in the 

 present main channel, and it will probably be gradually enlarged and eventually become the 

 main channel. 



When the river was somewhat higher than now the present Trenton Channel probably 

 received its water in a shallow sheet over the flat grounds of northern Grosse Isle and of the 

 vicinity of Wyandotte. The northern part of the Trenton Channel, which passes in front of 

 the city of Wyandotte, was not then so deep as now. Its submerged bank is very steep, sug- 

 gesting recent or present bank cutting. 



