REPORT OF THE DIRECTOR I915 83 



The occurrence of slumps is subject to seasonable variation, being 

 most frequent in spring with the melting of the winter snows and 

 the heavy rainfall of the spring months. The same movement may 

 renew itself from year to year if not completed in one period. 



Flow refers to movements of wet clay, so mobile as to run with 

 the freedom almost of a liquid. It occurs therefore on slight slopes, 

 or even in a horizontal bed if there is pressure upon an underlying 

 mass to force the clays to spread. The movement is between 

 different layers or parts of the loose sediments, seldom of the 

 latter along a rock surface ; flows on a bed of rock have never been 

 recorded in the Hudson valley so far as the writer has been able 

 to ascertain, and apparently they are rare elsewhere in the glaciated 

 region. 



Fig. I Effects of slumping in bank of clay and sand 



In the Hudson valley flows are restricted practically to the sur- 

 faces of the clay terraces fronting the river and may be considered 

 simply an accelerated phase of slumping. They are relatively 

 small. On top of the terraces where movements of larger volume 

 might be expected, the surface clays are firm and coherent, so much 

 so that they do not readily soften in the presence of water. The 

 movements then take the forms referred to under classes 4 and 5. 



Extraordinary examples of flow have been described from the 

 St Lawrence valley where the bottom is floored by the silty Leda 

 clay, which shows a marked tendency to movements of this kind 

 6 



