148 EUGENE WESLEY SHAW 



deposits rising, thinning, . and narrowing upstream, and consisting 

 of mixed coarse and fine material of local origin, the proportion 

 of fine being somewhat greater than in the valley train. The 

 larger the tributary the more gradually does the deposit rise and 

 thin, for the larger streams have less fall, and there is less difference 

 between the old gradient, with which the streams were more than 

 able to carry their loads, and the adjusted gradients, with which 

 the streams did neither cut nor fill. 



To illustrate, certain facts indicate that in pre-Glacial time the 

 lower 50 miles of the Monongahela had a fall of about one foot 



Fig. 2. — Longitudinal section of deposit on a stream tributary to one which is 

 overloaded with glacial debris. A distinguishing character of such deposits is that 

 they are definitely limited upstream by the convergence of the old profile in use 

 when the stream was cutting down, and the adjusted profile with which the stream 

 is just able to carry the load delivered to it by headwaters and side streams. 



per mile, that the adjusted gradient was about 9 inches per mile, 

 and that the valley train athwart the mouth of the river was about 

 no feet thick. At this point the Monongahela fill should have 

 been no feet thick, and this thickness should have decreased 

 upstream by 1 foot minus 9 inches, or 3 inches per mile, and at 

 50 miles the formation should have been thinner by 130 inches, 

 or 12^ feet. The results obtained by actual observation in the 

 field accord very closely with these figures. The deposit is nearly 

 100 feet thick at the West Virginia line. 



As another example, the Clarion gravels thin almost 50 per 

 cent in 10 miles. Originally, as indicated by the base of the 

 gravels, the stream had a fall of about 125 feet in the lower 10 

 miles of its course. The- adjusting of the gradient reduced this 

 to 60 feet. The difference between these figures, or 65 feet, plus 

 the thickness of the deposit at the upper end of the 10 miles, or 

 50 feet, is 115 feet, which is the thickness of the deposit at the lower 

 end of the valley. 



