SURFACE GEOLOGY. 65 
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As a matter of fact, bowlders are almost never found upon the true lake 
ridges, but are abundant on the slope between them, where they have 
been washed out and left specially conspicuous by the waves which car- 
ried away the softer material and piled it up in ridges. We seem, there- 
fore, compelled to reject the supposition that ice in any form took part 
in the creation of the lake ridges. But we have ample demonstration 
that causes now in operation are abundantly capable of producing just 
such effects. Any one who will visit the head of Lake Michigan will 
see lake ridges now forming, and he will also see there older beaches at 
higher levels, made in ancient times, precisely as the modern ones are 
built up. The present shore of Lake Erie also furnishes similar exam- 
ples, one of which I have before cited, viz., that which stretches from 
Cedar Point to the main land west of the mouth of the Huron, in Hrie 
county. This has closed the former outlet of Sandusky Bay, and will, 
eventually, cause the shallow water basin behind it to fill up with sedi- 
ment and become a marsh, or peat bog, like so many we find behind the 
older ridges. The Atlantic coast affords us innumerable examples of the 
formation of lake ridges at the present day. At Old Orchard beach, south 
of Portland, Maine, a continuous ridge of sand, from ten to twenty feet 
in height, completely encircles the bay, just back from the water’s edge; 
and on the sandy shores from New Jersey southward such shore ridges 
would be almost constantly in sight to any one who would follow the 
coast. Some part of the confusion of ideas which has prevailed in refer- 
ence to the lake ridges has arisen from the fact that they have been con- 
founded with terraces, into which they sometimesmerge. Thus, the north 
ridge, which stretches continuously from the Cuyahoga to Rocky river, 
is generally composed of loose material, water-worn gravel and sand, but 
in part of its course it is a terrace cut out of the Hrie shale, slightly cov- 
ered with a coating of gravel. In other cases, the lake ridges become 
terraces in the Erie clay, but they are shore lines all the same, and I find 
it hard to believe that any intelligent and unprejudiced observer, who 
will carefully examine the facts, will fail to subscribe to the view which 
I advanced many years since, that they are the product of shore waves 
along lines which mark periods of rest in the descent of the water level 
in the lake basins from the altitude 1t once reached, several hundred feet 
higher than now. 
THE CAUSES OF THE ARCTIC CLIMATE OF THE ICE PERIOD. 
The evidences of great alternations of climate during. the latest geo-. 
logical periods are such that few intelligent persons hesitate to accept 
them as proven. From the Tertiary deposits of the far north, Alaska,. 
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