150 EUGENE WESLEY SHAW 



today extending up on the side of the valley and a few miles away 

 the full thickness of over ioo feet is present, is evidence that the 

 deposit was formerly ioo feet deep here as it is elsewhere. There 

 could scarcely be any other possibility except that the valley-side 

 deposit represents an older fill, and there is no foundation for such 

 an assumption. 



A theoretical consideration of the question of local ice dams 

 yields interesting results. The possibility of an initial ice jam 

 is not to be questioned. Moreover, the supposition that such a jam 

 might be large in a northward or iceward flowing stream in a 

 subglacial climate is reasonable and is supported by known con- 

 ditions on the McKenzie and other streams which work under 

 somewhat similar circumstances. 



But the ice dams in this case must have been several times as 

 high as the highest known and must have persisted through many 

 summers warm enough to melt back the thousands of feet of ice 

 in a continental ice sheet. Indeed if we assume that the Monon- 

 gahela carried the same amount of suspended matter which it 

 carries today (in all probability it did not carry so much), that all its 

 load of undissolved matter was dropped, and that immediately after 

 the reservoir became filled the dam went out, we get a minimum 

 estimate for the life of the dam of about 1,000 years. If only a 

 quarter of the material were dropped the time would be 4,000 years. 



During this time the run-off of the basin must have passed over 

 the dam, for if the dam had suddenly risen above the height of cols 

 in near-by divides, the lake immediately behind the dam would 

 not have been silted up. Moreover, considerable coarse material 

 is found just above the supposed dam, indicating that there were 

 strong currents and that only a small fraction of the suspended 

 matter was dropped. 



The hypothesis of an ice dam, therefore, involves the assump- 

 tion that the Monongahela, which since early glacial time has, 

 with a very low gradient, removed rock material to a depth of 200 

 feet for more than 100 miles, was for centuries unable to cut through 

 or undermine these blocks of -ice over which its gradient and eroding 

 power must have been that of a cascade or waterfall. The 

 assumed floor of the valley below the site of the dam is 60 feet below 



