SURFACE GEOLOGY. 47 



It seems that in the period of the greatest submergence the larger 

 part of the summit of the watershed was under water, and was swept 

 by breakers and shore waves, by which some of the beds of sand and 

 gravel were formed, which are described under the head of Karnes ; and I 

 have supposed that a considerable portion of the materials composing 

 these kames, or eskers, was derived from icebergs stranding on the shoals 

 which now form the crest of the divide. At this time a sufficient depth 

 of water existed in the passes of the watershed to float icebergs of con- 

 siderable size, and as currents flowed through these passes, some of the 

 bowlders scattered over southern Ohio were probably transported by 

 them. When the water level had been somewhat depressed by the slow 

 elevation of the continent, these gaps became, as I have supposed, waste- 

 weirs, through which powerful streams of water continued to flow for a 

 long time, cutting the gaps deeper, and transporting great quantities of 

 gravel and bowlders, and depositing them in lines which lead down 

 toward the valley of the Ohio. 



The first of these passes through the watershed is that traversed 

 by the Miami Canal at the St. Mary's summit. This has a level 

 above the Lake of 367 feet, and connects the valleys of the Maumee 

 and Miami. 



The second is that called the Tymochtee summit, in Wyandot county, 

 which connects the valley of the Sandusky with that of the Scioto. 

 This has an altitude at present of 337 feet. 



The third pass is in Harrisville, Medina county, between the valleys 

 of Black river and the Styx, a tributary of the Tuscarawas, at a height of 

 336 feet. 



The fourth is the Akron summit, through which the Ohio Canal 

 passes, connecting the valley of the Cuyahoga with that of the Tusca- 

 rawas. The summit level of the Ohio Canal is 395 feet above the Lake. 



The fifth pass is that between the valleys of Grand river and the Ma- 

 honing, of which the summit is in Orwell, Ashtabula county, and has 

 an altitude of 363 feet above the Lake. 



Each of these gorges is now more or less filled with Drift ; but the 

 remarkable similarity of level which they present will strike the most 

 casual observer, and will not fail to suggest their reference to a common 

 producing cause. All the lines of drainage leading southward from 

 these passes are marked by deeply excavated channels, now more or 

 less perfectly filled by great accumulations of rolled and transported 

 material, such as would be the natural product of a copious flow of water, 

 continued through ages of time, and gradually diminishing and losing 

 its transporting power. Whoever has passed up the valley of the Miami 



