REPORT OF THE DIRECTOR I915 



87 



examples of such subsidence are often remarkable in their diagram- 

 matic resemblance to crustal displacements, a feature exemplified, 

 for instance, in plate 2, which shows the effects of a landslide that 

 was uncovered in the excavation for the present State Capitol at 

 Albany. 



The drag of the lower layer may suffice to displace the surface 

 blocks more or less horizontally, but if the overlying beds are tena- 

 cious, the main component of movement will be vertical. The 

 necessary conditions for the occurrence of slides of this type are 

 supplied in the Hudson valley; the weathered clays, which are up 

 to 40 feet thick, are relatively impervious to water, and tough, 

 whereas the underlying unweathered blue clay often takes up 

 water to change into a thin, slippery hiass that flows under com- 

 pressive strain. Just what change is brought about by weathering 











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Fig. 3 Clay block displaced by subsurface flowage 



to cause this difference in behavior of the clay is not yet apparent, 

 but there is no doubt of the existence of such variations as shown 

 by frequent instances of slides involving these precise conditions. 



If the surface beds are essentially the same as the lower stratum 

 they will settle down or " melt " into the latter, and the slip will 

 become a flow of the kind described in the previous class. 



The source of ground water which produces the zone of satura- 

 tion does not come directly from the surface; rather it is to be 

 looked for in a horizontal seepage along the bedding planes, since 

 plastic clays are almost impermeable to the circulation of water. 

 The sand partings invariably present in clays laid down in standing- 

 water provide the necessary channels for the horizontal movement 

 of water that may be admitted along the edges of the strata where 

 these overlap on a rock slope. The partings may be exceedingly 



