POSTGLACIAL DEVELOPMENT OF CONNECTING RIVERS OF GREAT LAKES. 473 



The front of the ice sheet had retreated a considerable distance northward in the basin of 

 Lake Huron before the waters of that basin fell to the level of the first or early stage of Lake 

 Algonquin. In some of the stages just preceding this lake the outlet of the expanded waters 

 was eastward along the northward-sloping escarpment near Syracuse, N. Y., and the pressure of 

 the solid, impervious ice mass against that slope was still controlling the level of the waters in 

 the basin of Lake Huron. At that time a strait, now occupied by St. Clair and Detroit 

 rivers, connected the two lake basins. The flow of the waters through the strait could hardly 

 have produced a perceptible current so long as the passage was wide and deep, but as the lake 

 waters fell away the strait grew narrower and shallower and the current stronger, until it became 

 a great river rather than a strait. 



On the wide, flat floor of this connecting valley, transverse to the axis, run three low ridges, 

 which, except where they have been cut through by the river, remain substantially as they were 

 when the ice left them. Two of these ridges are morainic and one is partly moraine and partly 

 bedrock. After the glacial lake waters fell St. Clair-Detroit River began to flow from the 

 Lake Huron to the Lake Erie basin, naturally along the lowest line hi the valley. It soon cut 

 through the soft materials of which two of the ridges are composed and cut into the third to 

 a depth of a few feet, below which it met more resistant materials. 



In its first work the river produced two elaborate systems of distributary channels, one 

 near the village of St. Clair on St. Clair River and the other near Trenton and Amherstburg on 

 Detroit River. Most of the channels are shallow and must have been of very temporary dura- 

 tion, but they furnish an interesting record of the transition from a strait to a well-established 

 river. 



ST. CLAIR RIVER. 



EARLY DISTRIBUTARIES. 



EROSION OF THE MAIN MORAINE OF THE PORT HURON MORAINIC STSTEM. 



The highest of the transverse ridges and therefore the first to be uncovered and eroded by 

 the newly established river is about 1 J miles north of St. Clair and is the southward water-laid 

 extension of the main moraine of the Port Huron morainic system, here a single, simple ridge. 

 Its crest is almost flat longitudinally, and is relatively broad and smooth. Figure 10 shows the 

 distributaries on this moraine, the old river channel, and the esker on the plain south of it. 



The moraine is 3 or 4 miles broad at its base, but it is low and its crest is broad and ill 

 defined. From the end of its land-laid part northwest of Port Huron it runs south, forming 

 the watershed between Pine River on the west and St. Clair River on the east. Near the north 

 edge of St. Clair Township it turns southeast and after crossing the river extends east and north- 

 east in Ontario. The highest points on the crest near the gap cut by the river have an altitude 

 of about 645 feet above sea level or about 65 feet above St. Clair River. Northwest and north 

 along the crest the altitude rises on the average not over 5 or 6 feet to the mile — in the first 

 4 or 5 miles only about 10 feet. Two or three gravelly knolls, probably outwash from the ice 

 front, rise 10 to 15 feet higher than the general crest, but are not considered in the altitudes given. 



On the Canadian side there were apparently no distributaries. A small creek flowing west 

 of south from the moraine, about a mile east of Courtright, may possibly have been a small, very 

 temporary distributary, but at the two points where it was visited by the writer it is very nar- 

 row and the evidence seemed clearly against this view. The first flow which had sufficient 

 velocity of current to cut into the moraine was confined to a space about 4 miles wide extending 

 southeast to northwest on the crest between Moore on the Canadian side and the southeast 

 corner of sec. 1, St. Clair Township. Thus the overflow was in a general way toward the south- 

 west over the western limb of the moraine. The gentle front slope here descends to the south- 

 west and that direction was taken by the distributary streams. The group as a whole shows a 

 shghtly radial arrangement conforming to the curved front of the moraine — the western members 

 running southwest and the eastern ones more nearly south. The present channels on the 



