REPORT OF THE DIRECTOR I915 95 



that they are now unequally distributed, forming dunes 40 or 50 

 feet high in places and very thin or absent in other sections. Their 

 contact with the clays normally shows no interruption in the suc- 

 cession, but in one or two localities has been found to evidence a 

 marked unconformity as the result of erosion. This relation seems 

 best explained by an interval of erosion after the beds were exposed 

 to the air and had been temporarily denuded of the protecting 

 sands, following which they were again covered by wind deposits. 



The beds are trenched to various depths by lateral streams which 

 head in the ranges of hills on either side of the valley and by a 

 network of streamlets which have their sources within the lower 

 lands. A few powerful tributaries like the Mohawk and Hoosic 

 carry drainage from far beyond the valley confines. 



The contours along the river are rather shai^p where the beds 

 are dissected almost to their base and show no marked change of 

 slope to the top of the terraces. The actual gradient may reach 

 35 or 40 degrees. This seems a high angle for such soft materials 

 in a region of fairly heavy rainfall, and is traceable to the effects 

 of landslides in continuously renewing the slope as the rains 

 remove the waste which has been brought down by previous slides. 

 In the interior the contours are less abrupt, the cross valleys 

 for the most part being shallow except where they mark a pre- 

 glacial drainage line. In the latter case their bottoms lie well 

 below the terraces and there is a steep rise to the level of the latter. 

 The Mohawk is exceptional among the larger streams in that it 

 occupies a postglacial rock channel from its outlet at Cohoes to 

 Schenectady, and falls with the Hudson gorge in a series of 

 cataracts. The clays along its course consequently are thin. 



Observations on the Hudson Valley Slides 

 Troy. Many evidences of slides are met with in the clay bluffs 

 that front the river in Troy and the stretch south of there to 

 Rensselaer. In fact, scarcely any of the banks in that section 

 have slopes that are the result of erosion directly; their steep 

 gradients, concavities and talus accumulations show the influence 

 of creep or flow and of occasional large slides. 



One of the earlier records of an extensive disturbance of the 

 kind relates to a slide that took place in Troy January i, 1837. 

 Mather, in his report on the geology of the first district,^ gives a 

 rather circumstantial account of it which reveals some interesting 



1 Albany, 1843, p. 37-38. 



