282 G. F. WRIGHT POSTGLACIAL EROSION AND OXIDATION 



rents impinged on the sides of the deeply eroded rock channel. At Alle- 

 ghany City and -below Pittsburgh the same forces were at work, modified 

 by the entrance of the Monongahela Eiver from the south and Beaver 

 Creek from the north. By attention to these considerations these high- 

 level terraces can for the most part be eliminated from the evidence 

 imj^lying a great antiquity to the closing scenes of the Glacial period in 

 the Ohio Valley. 



It is difficult for the imagination fully to comprehend the conditions 

 attending the closing stages of the Glacial epoch in the upper Alleghany 

 Valley, for down this valley there escaped the drainage from north- 

 western Pennsylvania, western Xew York, and a good part of Ohio 

 drained by French Creek and the Beaver, producing a deep, torrential 

 stream whose surface was covered with bergs and smaller masses of 

 floating ice dropped from more than 100 miles of ice-front. Two differ- 

 ent forces were at work tending to deposit high-level gravel and at the 

 same time to keep the main channel well scoured and free from sediment. 

 I quote from a private communication from Professor \Yilliams, after 

 the completion with ample assistants of his survey of the region during 

 two seasons : 



"1°. The abnormal amount of berg: material from so long a front would not 

 go sailing quietly down the stream, but would go grounding on the margins, 

 heaving and tossing in midstream, choking up narrow passages to form tem- 

 poraiy ice dams, which would raise the level of the flood till the pressure was 

 suflicient to break the dam. when away the mass would go on a wave that 

 would send a part of the ice on high shelves, where it would strand above the 

 average level. I have seen in our little stram here in Woodstock [Vermont] 

 such a wave to carry debris 30 feet above stream level. This spring [1910] 

 the ice will leave a deposit of (average) inches deep up to 10 feet above the 

 average level and the greatest thickness of the ice pile was only 12 feet. 



''The overwash from the glacier carried both free gravels and gravels frozen 

 in masses of ice. The carrying power of ice has not been as fully considered 

 as it should be. Ordinary freshets in Vermont streams, with break-up of ice 

 in spring, show that ice 2 and 3 feet in diameter, frozen down to the stream 

 bottom, will lift and carry when the break-up comes stones up to one ton 

 weight. I have an instance of such a stone left, with gravel heaps, in a mow- 

 ing 50 feet from the water's edge, and every year the Vermont farmers have 

 to clear away gravels with cobbles weighing anywhere from a pound to 50 

 pounds. We can now see that the overwash of a glacier would carry every- 

 thing of small enough mass to be influenced by the force of the current. This 

 accumulation would be of sands and clean gravels of certain size and sands 

 and boulders frozen in ice. the latter being of many times the size. 



"A current of 1 foot per second will carry sands. The transporting force 

 of a current varies as the sixth power of the velocity. A current 3 feet per 

 second or 2 miles i)er hour will move stones 3 ounces in weight (hen's egg) ; 

 a torrent* of 20 miles per hour will carry fragments of 100 tons weight. 



* See Le Conte's Geology, pp. 19 and 20. 



