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 channel 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 about the city of Sandusky. Here the grooves and scratches 
which indicate the direction of motion in the ice mass are about 8. 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 are 
