386 GEOLOGY. 



tant gravel deposits, and streams of valley gravels stretch far down 

 the drainage courses that then led away from the ice-edge. This is 

 notably true of the Allegheny and Ohio valleys. These old glacial 

 gravels are so related to the present trenches of these streams as to 

 seem to imply a channel erosion of 200 feet and more since their depo- 

 sition, 1 though this interpretation has been questioned. 



Fig. 511. — Section \ mile west of Thayer, showing masses of Aftonian gravel included 

 in the Kansan till. The most conspicuous mass is near the center of the figure. 

 The masses of gravel thus included are not cemented, and it is thought that they 

 must have been plowed up and included while in a frozen condition. The basal 

 part of the section at the right is Aftonian. (Calvin, la. Geol. Surv.) 



The products of the glacial waters of this stage in eastern Penn- 

 sylvania and New Jersey were commingled with non-glacial wash-prod- 

 ucts and will be discussed under the non-glacial formations (Columbia, 

 p. 447). 



The Natchez formation. — At Natchez, Mississippi, there is a section of assorted 

 material about 200 feet in thickness which is chiefly made up of derivatives from 



1 Leverett, Mono. XLI, U. S. Geol. Survey, p. 235. 



