REPORT OF THE DIRECTOR I915 85 



various ways but have a common cause, the oversteepening of the 

 slope under the conditions existing at the time. This oversteepen- 

 ing may come about by undermining of the base through artificial 

 or natural means, by a local softening of the mass near the base, 

 and by the development of vertical cracks and fissures in beds of 

 clay which weaken their structure. Some external impulse, like the 

 jar from an earthquake or a heavy explosion, may serve to start 

 the movement, but such impulse is not essential. 



Slides of bedded clays possess some features not shared by those 

 of other soft materials. One feature is their marked tendency to 

 hold together and move en masse, which arises from their natural 

 tenacity when dry. The dislocations consequently occur in blocks 

 which may be of large size and develop tremendous momentum in 

 their course down a slope. Examples of this feature are illustrated 

 by several damaging slides that have taken place in the vicinity of 

 Troy, where the terraced clay beds rise steeply to a level 200 feet 

 or more above the main section of the city. Portions of the upper 

 beds, with some surface gravels and sands, break away from the 

 face of the terrace and slide down the slope, sweeping outward 

 toward the river with overwhelming violence. Particulars of these 

 disastrous disturbances are given on a later page. 



As the cause of earth slides, undermining by stream erosion may 

 be inferred to be commonly effective in such a region as the Plud- 

 son valley; its operation scarcely requires explanation. The same 

 may be said of undermining by the hand of man which has been 

 responsible for disastrous slides in clay banks. One such occurred 

 a few years since in a clay pit at Haverstraw, causing the demoli- 

 tion of several houses and the loss of many lives. 



A condition of temporary instability or oversteepening in a bank 

 may result from the infiltration of water into the lower beds, 

 thereby diminishing their strength and ability to sustain their load. 

 This infiltration may take place in a horizontal direction through 

 the presence of sandy layers which overlap on a sloping rock sur- 

 face. It is probable that some of the Troy slides have been caused 

 in this way, for they seem to have been accompanied in certain 

 cases by more or less wet clay, the horizon of which is indicated by 

 a shelflike projection left on the face of the cliff above the base. 



Qay beds in drying out after heavy rains or melting of, the winter 

 frost are apt to develop cracks which on the edge of a decided slope 

 may so weaken the structure as to lead to their dislodgement. Sub- 

 sequent rains which cause the filling of the joints by water may 

 facilitate the movement through the hydrostatic pressure exerted 



