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. East of this arch the surface is underlain 

 for a long distance by soft shales (Huron and Erie) 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- 



