"SLIDES" IN THE CONEMAUGH FORMATION 345 



When the forward frontal movement ceases or slows the upward 

 crowding of this part may restore any trees upon it to a normal 

 position or even to one inclining uphill. Occasionally trees may 

 be seen inclining uphill on rather steep slopes. The slides produ- 

 cing this result are possibly of greater extent, with less differential 

 and more rapid motion, and a counteracting tendency due to the 

 inertia of the tree, most effective in its upper portion. Leaning trees 

 are riot as numerous as one might expect. This is largely due to the 

 fact that they hinder slide movement. Thickly wooded hillsides 

 rarely show such movement. 



The vegetation on a slide frequently continues growing as 

 though undisturbed. Between successive seasons of movement the 

 breakage surfaces may become sod covered. Hillsides showing an 

 almost kaleidoscopic change of surface may nevertheless be covered 

 most of the time with a mat of vegetation. 



Other things being equal, the thicker a shale the more subject it 

 is to slides, although a thin shale underlain by particularly impervi- 

 ous material may be as troublesome as a thicker layer less favor- 

 ably situated. 



The intervening layers of other rock between shale beds fre- 

 quently furnish a hindrance to the progression of slide movements 

 uphill, as well as serving to initiate independent shde movements. 

 Such layers, undermined by slides, will in time break off in blocks 

 of size determined mostly by the joint and bedding planes. 



2. Fluid. — This, the second type of slide, may be considered 

 as an extreme stage of the first. In this type the proportion of 

 water is so great and so thoroughly mingled with the shale mass, 

 that the whole goes downhill in usually visible motion, like a thick 

 fluid. On reaching a surface of sufficiently gentle slope, the heavier 

 materials spread out and settle, leaving the water to drain away 

 from the edge and over the surface like distributaries. These 

 shdes, like the third type, are not of great significance. 



3. Dry. — The dry, or third type of slides, are those with a true 

 sHding or rolling movement downward. On slopes barren of vege- 

 tation, movements in the material and drying may so loosen the 

 surface covering, especially of shale flakes, that portions of this 

 will move down in the manner stated unless checked by obstacles 



