18 | 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 baelost 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 Hrie 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- 
