REPORT OF THE DIRECTOR I915 8r 



3 Earth slides; materials of any sort, but not fluent; the move- 

 ment takes place on the face of slopes that are fairly steep and is 

 accomplished in one stage. 



4 Subsidence of surface through squeezing out of a wet clay 

 substratum on the plane of its bed. 



5 Subsidence of surface from unbalanced pressure upon confined 

 liquid substratum, leading to a reciprocal upward movement at a 

 distance. 



The forms named in the tfirst three classes are well known and 

 are mentioned by Howe in his classification of landslides under 

 the division " Movements of detritus." The last two, however, do 

 not seem to have come under general observation, at least they have 

 not been specifically referred to in the more important treatises on 

 landslides. The forms mentioned by Howe under " Movements of 

 detritus " are : soil or earth slips ; earth slides or soil slips ; mud 

 flows ; talus slumps ; and submarine flows. Of his last two classes 

 there seem to be no equivalents in the region under discussion, 

 although it is not unlikely that submarine flows may occur, inas- 

 much as the delta clays extend below tidewater in many places 

 under conditions which make their occurrence possible. 



I Surface creep. Creep is the gradual downward movement of 

 materials on slopes under the influence of gravity aided more or 

 less by climatic conditions. It is the commonest of all forms in 

 unconsolidated beds ; that is its occurrence is to be inferred from 

 effects frequently observed in the beds, although its progress is 

 too slow to be followed step by step. It takes place in gravels 

 and sands on light slopes, but not to the same extent in plastic 

 clays which, through their coherence and tenacity, are quite resist- 

 ant to its influence under ordinary conditions. Creeping operates 

 especially in the spring, just as the frost leaves the ground; it is 

 most effective at the surface, decreasing rapidly toward the interior 

 where frictional resistance at a greater or lesser depth overcomes 

 the influence of gravity. The action of frost is upon the moisture 

 held in the pores, causing an expansion which f6rces the particles 

 apart so that the body of material as a whole is enlarged. The 

 surface in consequence is lifted or heaved, a movement that is 

 translated into a downward one, on a slope as soon as the frost 

 bond is dissolved. Moisture alone acts in a somewhat similar 

 manner. 



Clay beds are affected by creep only in their passage from a dry 

 state to one of wetness. When thoroughly wet they give rise ^ to 

 other forms of movement, particularly slumps and flows. 



