8o NEW YORK STATE MUSEUM 



subsidence of the waters the beds were exposed as a smooth plain, 

 now dissected by the river and its tributaries so as to give the 

 appearance of an interrupted series of fiat-topped ridges whose 

 level sky line marks the general terraced attitude. 



Forms of Landslides in Loose Sediments 

 Displacements of bedded clays and sands often occur on small 

 gradients. The materials as a whole possess less stability under 

 varying conditions of moisture content and climate than the 

 unsorted heterogeneous accumulations of rock weathering that are 

 commonly involved in slides in mountain regions. Their forms are 

 correspondingly varied and complex, in some instances embodying 

 very puzzling mechanical features. The gravity stress which is 

 the fundamental cause of dislocation may be transniitted long dis- 

 tances through the medium of a practically fluid stratum below the 

 zone of rupture, as has not infrequently happened in the Hudson 

 valley. Unlike the usual condition in mountain forms, there need 

 be no essential variations in the character of the material displaced 

 and the undisturbed beds. Any structural change that could be of 

 significance in the formation of such slides in the very nature of 

 the case is scarcely to be looked for, and the same is true also with 

 respect to a slipping surface. 



The conditions attendant upon the disturbances are generally 

 determinable by observation or by testing the ground in the 

 vicinity, from which some conclusion may be drawn as to the 

 causes leading up to the slides. The exact impetus or proximate 

 cause, however, can seldom be ascertained. Usually several factors 

 must be taken into consideration in determining the origin of 

 individual slides and their relative importance is diflicult to esti- 

 mate. The matter may be further complicated by the entrance of 

 some external influence into the situation either of natural develop- 

 ment or arising from the agency of man. 



Of the conditions which govern the form taken by the movement, 

 those of more immediate moment are the -nature of the beds, that 

 is, whether clay, sand or mixture of the two; the moisture content; 

 and the surface contour. The forms that have come under obser- 

 vation in the Hudson valley are as follows : 



1 Surface creep ; involving soilj sand and gravel, little active in 

 clays. 



2 Slumping and flows ; peculiar to clays and silts that have been 

 rendered mobile by water. 



