POSTGLACIAL DEVELOPMENT OF CONNECTING RIVEKS OF GREAT LAKES. 499 



CHANNEL MAKING DURING TIME OF LAKE ALGONQUIN. 



The later or main period of southward discharge of Lake Algonquin appears to have endured 

 for a relatively long time, during which St. Clair and Detroit rivers must have greatly eroded 

 their beds, probably enough to bring their level down to that now marked by the Nipissing 

 beach at the south end of Lake Huron and to correlative levels descending, as in earlier stages, 

 by steps toward the Trenton barrier and the basin of Lake Erie. During this relatively long 

 period of flow with large volume the connecting channels were probably cut down nearly to 

 their present depths, and the way was prepared for the deepening of the beds of the tributaries 

 in the next period of abandonment. 



SECOND OR LAST DEEPENING. 



The closing stages of Lake Algonquin were marked by the retreat of the ice and the opening 

 of a passage eastward at North Bay, Ontario, by which the upper lakes discharged through 

 Mattawa and Ottawa rivers to the sea. This event left St. Clair and Detroit rivers dry again, 

 and it was chiefly then that the tributaries accomphshed the deepening now shown in the 

 drowning of tributary beds that appear to have been overdeepened. They cut to a base-level 

 about 30 feet lower than that which had controlled them when the great channels were last 

 carrying large volumes. It is to be remembered that at the close of the Port Huron-Chicago 

 stage of Lake Algonquin the level of the waters in the south end of Lake Huron was about 15 

 feet above present lake level, somewhat less in the basin of Lake St. Clair, and still less in that 

 of Lake Rouge. Cole * found buried peat beds in the St. Clair delta 14 or 15 feet below the 

 present level of Lake St. Clair, so that 30 feet or perhaps a little more may be regarded as a 

 close measure of the drop in base-level which affected the tributaries when the great rivers last 

 ran dry. 



These facts might afford a basis for a rough estimate of the time which passed between 

 the second abandonment of the outlet at Port Huron and its reestablishment when North Bav 

 was elevated. This interval, whatever it might be, would also measure the time during which 

 the upper lakes discharged past North Bay to Ottawa River — the duration of the Nipissing 

 Great Lakes. But it would hardly measure the transitional or two-outlet phase of those lakes, 

 for in that phase more or less of the discharge had returned to the southward course and the 

 deepening grew less. It is possible that results having considerable value as a check on -other 

 means of estimation might be obtained in this way. 



During the time of the Nipissing Great Lakes the streams winch gathered in the abandoned 

 connecting channels appear to have reversed the direction of flow, going northward to Lake 

 Huron. It seems probable that all the tributaries, including even the Ecorse and the Riviere 

 aux Canards flowed northward to the basin of Lake Huron rather than to that of Lake Erie, 

 which was so much nearer. This stream may be called Algonac River. This direction of flow 

 seems the more probable when it is remembered that the original depth on the Limekiln cross- 

 ing before dredging was only 13 feet and that that depth had almost certainly been attained 

 only in very recent times. The cutting of the bank on the Canadian side at the Limekiln cross- 

 ing is still going on. When the bank at this place stood 2,000 feet farther west than now the 

 greatest depth on the crossing, supposing the water surface to remain as it is now with reference 

 to sea level, would be only 8 or 9 feet. In the Trenton Channel the present least depth is 13 

 feet, about 3,000 feet south of the railroad bridge. The bottom is marked "stony" on the 

 charts, not rocky, so that the depth here has probably been increased in recent time. It may 

 be inferred from these facts that just before the great volume returned the lowest bottom sur- 

 face on the barrier was at least 5 feet higher than now, and of course the water surface was at 

 first slightly higher. 



The only obstacle to the supposition that all these streams flowed northward at that time 

 is the present shallowness of the water at the mouths of the distributaries of the St. Clair delta. 



i Cole, L. J., Michigan Geo!. Survey, vol. 9, pt. 1, 1903. 



