REPORT OF THE DIRECTOR I915 89 



between parts of the upper layer which lie at different levels and 

 thus out of balance. 



The physical variation of the beds, which is a necessary con- 

 dition in such disturbances, is probably not a very common feature 

 among clays although it seems prevalent over the Hudson valley 

 within the area covered by the sediments of former Lake Albany. 

 Similarity of absorptive powers and of tenacity between the upper 

 and lower beds removes the possibility of the formation of slides 

 of this kind. 



There is little doubt that there have been many occurrences of 

 such slides, and they are not at all the exceptional types which 

 might be inferred from the little notice that has been taken of them. 

 The writer personally has observed two occurrences, one at Stock- 

 port in 1908 and the other last year at Hudson. Only one previous 

 record has been made, that by W. B. Dwiglit who in 1866 gave a 

 description of a slide at Coxsackie, presenting sufficient details to 

 show its relations beyond peradventure. A rather vague allusion 

 to an occurrence of the same character is made by Mather in his 

 Geology of the First District. No doubt many have escaped atten- 

 tion, or have been considered as simple cases of subsidence. 



Geological and Engineering Aspects of the Landslides 



The influence of the various gravity movements described as a 

 factor in degradation is difficult to estimate, but none the less it has 

 considerable importance within an area like the Hudson valley. 

 There is good reason to believe that locally the work thus accom- 

 plished may predominate over that of all other leveling processes, 

 notwithstanding the supposedly rapid erosive action to which soft 

 sediments are exposed. As a matter of fact, erosion may be quite 

 stagnant on a surface that is protected by sod and vegetation : and 

 it is only when the earthy materials are directly exposed by rupture 

 of the protecting cover that they are appreciably degraded by the 

 runoff. 



The full effects of such disturbances are likely to escape notice; 

 for it is only the more extensive ones that manifest themselves on 

 casual inspection, and they are the exception of course. The incon- 

 spicuous forms — too slow or too interrupted in their movements 

 to attract attention — • are responsible for the largest share of level- 

 ing, since they are widely active with cumulative effect. Of large- 

 scale catastrophic slides, some ten or twelve are on record as 

 having taken place in the middle Hudson valley within the last 



