"SLIDES" IN THE CONEMAUGH FORMATION 343 



hold nearly vertical positions until effectively weathered, which 

 may take from one to several seasons. Under the combined 

 influence of the oxidizing action of the air, water, and temperature 

 changes, especially freezing when water laden, the shales are 

 shattered and comminuted until a slippery claylike mass is pro- 

 duced. Deep freezing is temporarily effective in stopping slide 

 movements, though the result when thawing is a hastening of 

 the process, as the mass more readily breaks away after the aid 

 given by the expansional and contractional movements of freezing 

 and thawing. 



The transverse strength of a weathered shale mass is much less 

 than that of the parent body, and when such a mass is sufficiently 

 weighted with water it tends to break away from the latter and so 

 originates the- slide. With the breaking away a comparatively 

 fresh surface of the shale is again exposed, so that the process is 

 free to continue indefinitely at the same point or at least until an 

 equilibrium between gravity and tenacity of materials is attained. 



Weathered shale on drying becomes hard, and shows no tendency 

 toward movement except in loose surface material. 



TYPES AND DYNAMICS 



Slides in the Conemaugh may somewhat arbitrarily be divided 

 into three classes, (i) primary, (2) fluid, and (3) dry. These 

 shade into each other or may combine. 



I. Primary.- — These are slides in which the moving mass is 

 highly saturated with water and of the consistency of a thick mud. 

 The primary slide is the first to form and decidedly the most 

 important in totahty of effects. The movements are rarely 

 observed though it is beheved movements fast enough to be visible 

 are of not infrequent occurrence. Results may often be noted 

 daily in active sHdes. A new movement of this sort is heralded 

 by a break in the sod, the break tending, with occasionally marked 

 variations, to follow a contour line. The position of this break 

 is further affected by the relative toughness of the sod. Paths, since 

 sod is weak or lacking, offer fines encouraging breaks. A single 

 break is rarely of great linear extent, but the whole face of a hill- 

 side may be at times seamed with them. These breaks really 



