CRITERIA REQUISITE TO A GLACIAI AGE 79 



that has depressed its channel deeply into a plain of glacial 

 or glacial-like gravels develops its bluffs on either hand, and 

 from time to time forces them back by under-cutting at its aggres- 

 sive bends. This action for the time is likely to develop nearly 

 vertical bluffs ; at least steep bluffs. While the cutting is in pro- 

 gress, the top of the bluff falls into the river and its surface 

 material, with whatever human relics may lie on it, is liable to be 

 strewn along the stream bed under the bluff and beyond. When 

 the stream shifts its course and the cutting ceases, the top still 

 continues to fall or slide or be washed to the bottom. In this 

 way the upper part of the bluff recedes, while the lower part is 

 built forward until a slope of stability is reached. In the pro- 

 cess some of the surface material is quite certain to become 

 deeply buried if the bluff be high. This has been amply pointed 

 out and illustrated by Holmes.' 



As already remarked in other connections, deposits are sub- 

 ject to slumps, creep, and other displacements that introduce 

 chances of error that must be carefully eliminated. 



So also loess bluffs are notably subject to creep in successive 

 large masses, which preserve their internal integrity, and hence 

 are liable to be regarded as undisturbed. This process is grandly 

 displayed at Vicksburg, where the displacements range through 

 some 200 feet. 



VII. RECOMPOSED SECONDARIES. 



Misleading as the inverted drifts may be, they are less illu- 

 sory than some recomposed secondaries. The leading class 

 here consists of subcBvial aggradatio?i accumulations. Nearly 

 every valley is subject to oscillations in the relative activity of ero- 

 sion and of transportation within its own limits Sometimes the 

 derivation of material in one part of the valley exceeds trans- 

 portation in another part, and deposition is the result. The con- 

 sequence is wash in one part and lodgment in another. These 

 oscillations may be due to changes in the precipitation, in the 

 vegetal clothing, in the relative stages of erosion, in the relations 

 to other streams and to other causes. In these cases the mate- 

 rial is derived mainly from the steeper slopes or steeper ravines, 



'Holmes, Journal of Geology, Vol. I, No. i, 1893. 



