78 GEOLOGY OF OHIO. 



Laurentian mountains, and, as they increased in size, gradually crept down 

 on to and began to excavate the plateau which bordered them on the west 

 and south. The excavation of our lake basins was begun, and, perhaps 

 in large part, effected in this epoch. 



5th. As the cold increased, and reached its maximum degree, a great 

 ice-sheet was formed by the enormously increased and partially coales- 

 cing local glaciers of the former epoch. This many-lobed ice-sheet, or 

 compound glacier, moved radiatingly from the south, south-west, ' and 

 western slopes of the Canadian highlands ; its Ohio lobe reaching as far 

 south as Cincinnati. The effect of this glacier upon Lake Erie and Lake 

 Ontario would be to broaden their basins by impinging against and grind- 

 ing away, with inconceivable power, their southern margins. To the 

 action of this agent we must ascribe the peculiar outline of the profile 

 sections drawn from the Laurentian hills across the basin of Lake Ontario 

 to the Alleghanies, and across that of Lake Erie to the highlands of Ohio, 

 viz., a long, gradual slope from the north to the bottom of the depression, 

 and then an abrupt ascent over the massive and immovable obstacle 

 against which the ice was banked, until, by the vis a tergo, it overtopped 

 the barrier. In New York that barrier was a shoulder of the Alleghanies, 

 too high and too rugged to be buried under a continuous ice-sheet; but its 

 whole front was worn away for a hundred miles or more, and it was deeply 

 creased where now we see the peculiarly elongated lakes of New York, 

 and cut through, in certain gaps, to the valley of the Delaware. In Ohio 

 the erosion was easier, and carried further south. The barrier was also 

 lower, and was finally overtopped by one great lobe of ice which flowed 

 on to the south and west until its edge reached the Ohio river. 



The extent of the erosion produced in the epoch under consideration 

 will be best appreciated by one who will stand on the cut edges of the 

 great series of rocks exposed on the southern slopes of Lake Erie and 

 Lake Ontario, and in imagination fill the vast vacuity which separates 

 him from the base of the Laurentian hills. 



6th. With the amelioration of the climate the wide-spread ice-sheets 

 of the period of intensest cold became again local glaciers, which com- 

 pleted the already begun work of cutting out the lake basins. At first 

 the glacier which had before flowed over the watershed in Ohio was so 

 far reduced as to be unable to overtop its summit, but deflected by it, it 

 flowed along its base, spending its energies in cutting the shallow ba- 

 sin in which Lake Erie now lies. 



A farther elevation of temperature curtailed the glacier still more, and 

 Lake Erie became a water basin, while local glaciers, left from the ice- 



