DRAINAGE SYSTEMS AND THE GLACIAL PERIOD. 201 



simply a broad, shallow valley through which there coursed 

 a single river of great magnitude, with tributary branches 

 occupying deep gorges. Professor J. W. Spencer has 

 shown with great probability that the old line of drainage 

 from Lake Erie passed through the lower j)art of the val- 

 ley of Grand River, in Canada, and entered Lake Ontario 

 at its western extremity, and that during the great Ice 

 age this became so completely obstructed with glacial de- 

 bris as to form an impenetrable dam, and to cause the 

 pent-up water to flow through the Niagara Valley, which 

 chanced to furnish the lowest opening. 



In speaking of the present area of Lake Erie, however, 

 as being then occupied by a river valley, we do not mean 

 to imply that it was not afterwards greatly modified by 

 glacial erosion ; for undoubtedly this was the case, what- 

 ever views we may have as to the relative efficiency of ice 

 and water in scooping out lake basins. 



In the case of Lake Erie, we need suppose no change 

 of level to account for the erosion of its basin, but only 

 that, since the strata in which it is situated were deposited, 

 time enough had elapsed for a great river to cut a gorge 

 extending from the western end of Lake Ontario through 

 to the present bed of Lake Erie, and that here a great en- 

 largement of the valley was occasioned by the existence 

 of deep beds of soft shale which could easily be worn away 

 by a ramifying system of tributary streams. Rivers act- 

 ing at present relative levels would be amply sufficient to 

 produce the results which are here manifest. 



But in the case of Lakes Ontario, Huron, Michigan, 

 and Superior, whose depths descend considerably below 

 the sea-level, we must suppose that they were, in the 

 main, eroded when the continent was so much elevated 

 that their bottoms were brought above tide-level. The 

 depth of Lake Ontario implies the existence of an outlet 

 more than four hundred feet lower than at present, 

 which, of course, could exist only when the general ele- 



