82 NEW YORK STATE MUSEUM . 



The influence of creep is diminished by sod, as the interwoven 

 root fibers give coherence to the superficial layer of soil and earth 

 where the tendency to creep is greatest. There are no peculiar 

 features in its workings in the region under discussion, and the sub- 

 ject need not be further treated at this time. A very compre- 

 hensive description of the phenomena of creep has been given by 

 Kerr,^ whose paper does not appear to have been generally noticed 

 by geologists. 



2 Slumping and flows. These are peculiar to clay beds and 

 silts which are wet or saturated with water. They are superficial 

 in their extent, seldom reaching more than a few feet below the 

 soil, the depth varying with the nature of the materials, angle of 

 slope, rainfall etc. 



Slumping is a word rather loosely used for sudden movements of 

 earthy materials on slopes, whether in the nature of flows or slides. 

 It is here applied more strictly to the local disturbance or down- 

 ward movement of clayey materials more rapid than creep, but not 

 of such volume or continuity as involved by flow. It does not 

 require so much moisture as the latter, but takes place when the 

 clay is in a weakened though not thoroughly mobile state. It pro- 

 ceeds more slowly than flow and is apt to be intermittent, ceasing 

 with the hardening of the clay to resume motion again when the 

 latter has been softened once more by moisture. Its eft'ect upon a 

 clay bank is to produce a lunate scar that lengthens downward with 

 progress, the resultant gradient of the exposed beds being steeper 

 than that of the original slope. The displaced clay gathers in a 

 formless mass at the base where it may give rise temporarily to a 

 reversed slope, thus enclosing a longitudinal depression in which the 

 drainage is imponded. 



Slumping may be observed in almost every clay bank of the 

 Hudson valley. It becomes a troublesome feature when buildings 

 are placed on clays without any provision for its control, as has 

 often been done. In the city of Albany a few years since a whole 

 row of houses had to be abandoned and dismantled on account of 

 the subsidence of a bank of clay bordering a ravine. The move- 

 ment was not extensive or continuous, but renewed itself from 

 time to time. Sod is a deterrent, but not preventive of slumping, 

 and in time it leads to the rupture of the sod through removal of the 

 supporting layer. After the sod is broken and displaced it pro- 

 ceeds more rapidly. • • ■ 



1 Frost in Arrangement of- Superficial Materials. Am. Jour. Sci. XXI, 

 ser. 3, p. 345. i88i. 



