TERRACES AND VALLEYS IN WESTERN PENNSYLVANIA 151 



the top of the fill above the dam. The drop in water level must 

 have been as great or greater, and yet the dam must have withstood 

 the pressure and the wear year after year for thousands of years. 



Parker oxbow. — One of the most famous of the abandoned val- 

 leys is the old oxbow at Parker's Landing (see Fig. 3). It was 

 first described in detail by Chance {Second Geol. Surv. of Pa., 

 Rept. VV, 1880, 17-22). He calls attention to the disproportion- 

 ate size and breadth of the valleys of the two small streams 

 which now flow from the oxbow, and also to the fact that between 

 the heads of the streams there is low swampy ground. Glacial 

 gravels of probable Kansan age are found almost continuously 

 around the loop and in some places the deposit is over 50 feet 

 thick. Chance inferred that at the time of the earliest ice advance 

 this oxbow was occupied by the Allegheny River, and at a subse- 

 quent time the neck was severed. 



G. F. Wright held that this channel was formed and abandoned 

 before glaciation, and that the glacial material now found in the 

 oxbow was deposited there at a time when the Allegheny, being 

 overloaded with Kansan outwash, aggraded up to a position some- 

 what above the oxbow; that the gravel was carried into the ends 

 of the loop, but the river never reoccupied the entire loop. Wright 

 has long advocated the idea that the Allegheny was cut to about 50 

 feet below its present channel in pre-Glacial time, and that the 

 glacial valley train was thus about 350 feet thick, filling the inner 

 gorge and part of the broad valley above. 



Chamberlin and Gilbert studied the problem in 1889, and 

 their conclusions agree essentially with those of Chance, and are 

 found in Bulletin U.S. Geological Survey No. 58, 31. 



In 1894 Wright again published a paper (Am. Jour. Sci., 3d 

 ser., XL VII, 173-75) in which he holds to his previous conclusions. 



In 1900 E. H. Williams presented a paper at the Albany meeting 

 of the Geological Society of America (Bull. G.S.A., XII, 1900, 

 463) in which he agreed with Wright that the river has not 

 occupied the oxbow since the beginning of the Glacial period, but 

 he went so far as to hold that the river never did flow around the 

 so-called oxbow. He ascribes the feature to the work of two 

 small streams which "rise on opposite sides of a low col and de- 



