142 EUGENE WESLEY SHAW 



In 1883 Professor G. F. Wright presented evidence of a large 

 ice dam at Cincinnati, and shortly thereafter Professor I. C. White, 

 in a paper before the American Association for the Advancement 

 of Science, referred the terrace deposits of the Monongahela to 

 that dam. 



Chamberlin, in 1890 {Bull. U.S. Geol. Surv. No. 58, 13-38), 

 showed that the upper series described by Stevenson could not be 

 ascribed to the ice dam, because of their great range in altitude. 

 He also pointed out certain characters of Stevenson's lower series 

 which indicated that they were of fhaviatile, not lacustrine, origin. 

 These characters were: (1) the terraces slope with the present 

 streams; (2) the material capping the terraces is distinctly fluvial; 

 (3) they are rock platforms; (4) the form and distribution of the 

 terraces is of fluvial, not lacustrine, order; (5) the abandoned 

 channels must have been of stream origin. 



In 1896 Professor White expressed himself {Am. Geol., XVIII, 

 December, 1896, 368-79) as still convinced that the glacial 

 lake, Monongahela, did exist and was responsible for the terrace 

 deposits, but that the ice dam was probably not at Cincinnati, 

 but in the vicinity of Beaver, Pa. 



In the Masontown-Uniontown folio, published in 1902, M. R. 

 Campbell advances the theory that the deposits and abandoned 

 channels are due to local ice dams which formed in Kansan time. 

 He points out the fact that it is an extremely difficult and slow 

 process for a stream to cut off any of its meander in a rugged region 

 like Pennsylvania, and that it is impossible for a stream to establish 

 a totally new course unless the conditions under which it operates 

 are very different from those which normally affect the develop- 

 ment of streams. 



Again, as an objection to the view of Professor White, Mr. 

 Campbell states that while it would be possible for a stream to 

 change its course by superimposition if it were first caused to silt 

 up its valley and then permitted to cut down again, he finds that 

 part of the Carmichaels abandoned channel was not so silted up, 

 and he therefore concludes that the change of course was not due 

 to silting up and superimposition, but to local causes. Mr. Camp- 

 bell's idea is that ice jams formed in glacial time and that these 



