492 PLEISTOCENE OF INDIANA AND MICHIGAN. 



out most of its drowned part to prevent its becoming choked with vegetation, although it 

 generally has a fringe of reeds or lily pads along its margin. The water is said to be 5 or 6 

 feet deep east of Amherstburg, increasing to 10 or 12 feet toward its mouth. 



RELATION OF EARLY DISTRIBUTARIES TO LAKE ROUGE. 



It is interesting to note the relation of the earlier distributaries to Lake Rouge. The 

 first two channels described above, having their heads on the higher ground southwest of Wyan- 

 dotte, are the only two on the American side which had been abandoned by the time the water 

 fell to the level of Lake Rouge. Indeed, it is not quite certain that the second or lower one, 

 which heads in the northeastern part of sec. 6, Monguagon Township, was then abandoned, 

 for the floor at its head is at about the same level as the beach. On the Canadian side the 

 only one abandoned at that time was the long channel east of Amherstburg having its head on 

 the high ground north of the railroad tracks. The largest channels developed at that time 

 were two corresponding to the present main channels on the two sides of Grosse Isle. Into 

 these the river appears to have sunk at a relatively rapid rate, until it uncovered the rock 

 ledge and bowlder beds which offered effective resistance and held the water at the level of 

 the first Rouge beach. 



Running northwest from the Sibley quarry there is a broad, low divide with its crest close 

 to 600 feet above sea level. Slight depressions in it having altitudes above 599 feet showed 

 no enlargement such as result where distributaries were established, but depressions below 

 this altitude did show such enlargements. 1 From this fact it is inferred that the new river 

 first began to cut distributary channels on the obstructing ridge when the water surface north 

 of the barrier had fallen to about 600 feet above sea level or 25 feet above the present river 

 level. 



The data for determining the level of the river surface at the time of the first cutting are not 

 so satisfactory for the ridges at Detroit and St.. Clair. But as nearly as can be stated the water 

 surface above the ridge at Detroit stood between 605 and 610 feet above sea level and at St. Clair 

 close to 640 feet. It seems certain, therefore, that as the waters fell in the basin of Lake Erie 

 and in the St. Clair-Detroit Valley the ridge at St. Clair was the first obstruction to be uncovered, 

 that at Detroit next, and that at Trenton last. The cutting at St. Clair and Detroit and in 

 the river bed above and below these points did not uncover any bedrock or other resistant 

 material, so that the river cut down at a relatively rapid rate. Near Trenton, however, the 

 river uncovered some resistant limestone and sandstone ledges, but chiefly a tough till thickly 

 set with bowlders, mostly crystalline erratics brought there by the ice sheet. This is the char- 

 acter of most of the material that has been removed from the bed of the river in the recent 

 deepening of the channel of the Limekiln crossing, where the original depth before dredging 

 was 13 feet. The effectiveness with which this material resisted the erosion of the river was 

 greater than present conditions would lead one to expect, for it is certain that the level of Lake 

 Erie fell considerably very soon after Detroit River first began to flow and has only recently 

 come back to its present level. The river must therefore have had more fall, and the rapids 

 in the river, instead of being 2 or 3 miles long as now, must have extended several miles out 

 into what is now Lake Erie. The current was probably more rapid at the crossing than it is 

 now. It was the obstruction at the crossing, however, that determined the level of the water 

 in the rivers above this point and in the basins of Lakes St. Clair and Huron. 



DEVELOPMENT OF THE DETROIT RIVER ESTUARY. 



DROWNING OF LAKE ERIE SHORE. 



The shores of Lake Erie, especially the western shores, are distinctly drowned by the 

 relatively recent backing up of the lake upon them. It seems certain, therefore, that the 

 marshy shores near the mouth of Detroit River and the shallow offshores have not been cut 



i At the time this area was being studied the marks of the topographic survey showing the altitude at every road corner were still fresh and 

 afforded accurate measurements at two or three critical points. 



