HOW THE GREAT LAKES WERE BUILT. 163 



Reversals of Ancient Rivers in Pennsylvania and New 

 York. The great changes in the water way of the Laurentian 

 River had their counterpart in the highlands to the south of the 

 lakes where the ancient streams were tributary to the Lau- 

 rentian, in place of to the modern Ohio and Susquehanna Rivers. 

 Among the more notable changes, the Alleghany (discovered by 

 Mr. J. F. Carll) flowed to the Erie basin, as did also the upper 

 Ohio (suggested by the writer, and further explained by Dr. P. 

 Max Foshay and Mr. F. Leverett). These and other streams now 

 reversed were tributaries of the Erigan River. In New York the 

 upper Susquehanna and some tributaries descended through the 

 "finger lakes" to the Laurentian River as it passed through the 

 Ontario basin. All the old streams coming from the high- 

 lands south of the lake basins flowed through broad, V-shaped 

 valleys, of ancient form, although of considerable depth. These 

 valleys became filled with drift which turned the waters of the 

 Ohio and Susquehanna Rivers to the south. This reversal in the 

 drainage has been further assisted by the recent northward tilt- 

 ing of the land, to be explained later. 



How the Ancient Valleys were Obstructed. The Lau- 

 rentian Valley and its tributaries were completed before the ice 

 age. Indeed, the high elevation of the continent during the cul- 

 mination of that period did not last long enough for the deepen- 

 ing of the channels of the main valley, as they could scarcely be 

 affected until a great canon had been excavated from the conti- 

 nental margin for eight hundred miles to the Ontario basin, 

 which was not the case. 



As for the modification of the ancient topography by glacial 

 action, it could have been only slight, and does not appear to have 

 been more than the sweeping of loose geological dust into the val- 

 leys, or on to the highlands to the south. The absence of any 

 great plow is shown by the direction of the scratches on the rock 

 surfaces, which lines are everywhere at great angles to the walls 

 and sides of the lake basins, and nowhere parallel to them, as 

 must have been the case if the valleys had been plowed out by 

 ice in any form. This crucial test and many other features had 

 not been applied fifteen years ago, when the writer commenced 

 these researches. Now this fancy of closet geologists has van- 

 ished before the application of facts. Yet the work of the ice age 

 was complex, and it is immaterial to the study of the lakes how 

 it was performed. In one way only does it come within the limit 

 of this subject, and that is in the phenomena of the ancient valleys 

 being filled by drift, whether stratified or not. It was this filling 

 of the old channels with drift that closed the ancient drainage of 

 the Laurentian Valley, which at a later date gave rise to the 

 lake basins. But the barriers of the lakes were further exagger- 



