184 GEOLOGY OF OHIO. 



with islands, and that a series of these islands stretches northward from 

 Sandusky and forms a barrier which must have offered serious opposition 

 to the westward movement of the glacier. The origin of the islands in 

 Lake Erie is more fully discussed in the report on Ottawa county than 

 it can be here, and it is there shown that they are the remnants, or rather 

 most projecting portion of the barrier to which I have alluded, and that 

 this barrier was formed by the arch of the strata, known as the Cincin- 

 nati axis, and described in detail in the first volume of the report. The 

 effect of this ridge, thrown across the lake basin and struck obliquely 

 by the moving ice mass, was to deflect that slightly to the south, and 

 to cause it to cut the deep notch in the lake shore at the mouth of the 

 Huron. The excavation of this point was also facilitated by the com- 

 parative softness of the Huron shale which underlies this portion of the 

 county. Sandusky Bay is' unquestionably one of the channels cut in the 

 Cincinnati arch by the glacier moving westward, and it corresponds 

 topographically with the channels between the islands from the north, 

 all of which are shallow and are cut by the ice out of the solid rock. It 

 is possible that the location of the Sandusky Bay ehannel was deter- 

 mined by the course of Sandusky river in former times. As is shown 

 elsewhere in this report, we have abundant proof that Lake Erie was 

 once a valley traversed by a river which now passes Detroit and flows 

 over the falls at Niagara. At that time Sandusky river was a tributary, 

 joining the main stream somewhere north and east of its present out- 

 let, and it may have formed a valley in this part of its course, which 

 was broadened and deepened by the subsequent glacier. The inscrip- 

 tion made by the great Lake Erie glacier is very distinctly shown in 

 many localities in Erie county, but especially on the Corniferous lime- 

 stone in and aboat the city of Sandusky. Here the grooves and scratches 

 which indicate the direction of motion in the ice mass are about S. 80° 

 W., or nearly coincident with the major axis of the Lake. All the 

 chief furrows correspond closely in bearing with those so conspicuous on 

 the islands, and were evidently formed by the same ice mass. Another 

 set of scratches are, however, seen upon the rock in some places. These 

 have a north and south bearing, and were produced, as I have supposed, 

 by the great glacier that excavated the basin of Lake Huron. 



The Drift deposits which overlie the glaciated surface in most parts of 

 the State have been removed from the greater part .of Erie county. The 

 bowlder clay is, however, found covering the rock surface in the south- 

 ern part of the county. This is, as usual, a blue, or where exposed and 

 its iron oxidized, reddish - yellow, unstratified clay, thickly set with 

 angular fragments of shale taken from the lake basin.. With these ar« 



