GEOLOGY OF THE ISLANDS IN LAKE ERIE. 



Although forming part of two counties, the islands in Lake Erie con- 

 stitute a group which it has been more convenient to study together, as, 

 topographically, they are dependent on a common cause, and, geologically, 

 are so closely connected as to be best considered in one view. 



The number of islands in the west end of Lake Erie is considerable, 

 and they are scattered over an extensive area. All the larger ones, how- 

 ever, are so closely approximated as to be visible from a single stand- 

 point. The largest of all these islands is Point Pele, of which the area 

 is about 11,000 acres ; the next largest, Kelly's Island, contains about 

 3,000 acres; Put-in-Bay Island, 1,500 acres. North and Middle Bass 

 Islands, Sugar Island, Middle Island, Rattlesnake Island, Ballast Island, 

 Gibraltar, Green Island, and Starve Island, are all much smaller. Mid- 

 dle Island and Point Pele Island lie north of the Canadian line. All 

 these islands are formed out of the solid limestone rock, apparently by 

 glacial action, and are separated by channels of no great depth, of which 

 the rock bottoms (when they are not covered with Drift clay), like the 

 islands themselves, every where bear the inscription of the ice masses 

 which once moved over them. 



A deep cjtemnel connects Lake Huron with Lake Erie, now for the 

 most part concealed by Drift clays with which it is filled. Just what the 

 outlines and depth of this channel are, has not yet been correctly ascer- 

 tained ; but the borings made for oil at Enniskillen and Bothwell, in 

 Canada West, show that the clay which occupies it has in some places a 

 depth of two hundred feet. Borings made at Detroit show that a mass 

 of Drift material underlies the city to the depth of more than one hun- 

 dred feet below the surface of Detroit River. This deep channel appar- 

 ently connects with the basin of Lake Erie north of the islands that 

 have been mentioned, and south of it all the western portion of the Lake 

 is comparatively shallow. Here and there masses of limestone project 

 above the surface, and form, beside the group of islands already men- 

 tioned, the East, West, and Middle Sisters, the Hen and Chickens, etc. 

 The surfaces of all these islands are plowed and furrowed, and afford, per- 

 haps, the most conspicuous examples of glacial markings to be found in 

 the country. 



Most of these glacial furrows have a bearing nearly coincident with 

 the longer axis of Lake Erie, showing that the ice masses by which they 

 were formed moved in that line. The evidence is no less conclusive that 

 the motion was from the east end of the Lake toward the west. This is 



