198 GEOLOGY OF OHIO. 
shown by the general contour of the islands, their eastern sides being 
more sloped and cut away than the western, and not unfrequently 
masses of rock will be found broken and unworn on the western sides, 
showing that they were in the lee of the moving mass of ice. The 
direction of the movement is, however, still more distinctly shown on 
those portions of the glaciated surface which contain masses of flint. 
These have resisted the ice action to a greater degree than the surround- 
ing limestone, though they are found more or less worn or broken on 
their eastern sides, while a longer or shorter trail of limestone shows the 
protecting power of the flint. Beautiful examples of this kind are re- 
ported by Mr. Gilbert on West Sister Island, and a photograph of a block 
taken from that island will be copied to illustrate the chapter on the 
Drift which forms part of this volume. The margins of most of the 
islands have been more or less cut away by the action of the waves, so 
that the glacial markings are destroyed or removed; but in a few in- 
stances—as on the north side of Kelly’s Island and the south of Put-in- 
Bay—the sides of the rocky masses have been protected from wave 
action, and still exhibit their original form and character. Here we find 
evidence that the ice not only passed over every portion of the islands, 
but molded itself to their sides in such a way as to scar and furrow 
them quite as distinctly as the level surfaces. In one instance, a perpen- 
dicular wall, composed of layers of unequal hardness, has been fluted or 
beaded like a cornice, and even cut under, so as to present an overhang- 
ing shelf planed on its under as well as on its upper side. Such ex- 
amples afford positive proof that the cutting away of the limestone was 
effected by glacial and not by iceberg action; and it is impossible that 
any one should study the surfaces of these islands without becoming a 
convert to the glacial theory, for every phase of the excavations effected 
in those rocks over which glaciers have moved is repeated here in all the 
most striking details. 
The reason why the western portion of Lake Erie is so much more 
shallow than the eastern, and why that portion is studded with islands, 
is simply this: by a reference to the geological map of Ohio it will be 
seen that the line of the Cincinnati axis of upheaval passes through the 
western end of the Lake, and along this axis the rocks are raised up in a 
great fold, and the solid masses of the Devonian and Upper Silurian 
limestone come to the surface. Hast of this arch the surface is underlain 
for a long distance by soft shales (Huron and Hrie) of Devonian age. 
These have yielded readily to the erosive power of the glacier, and have 
been cut away to form the principal portion of the lake basin. When 
the moving ice mass reached the line of the Cincinnati arch it encoun- 
