344 EARL R. SCHEFFEL 



mark a variety of fault plane, the heavily weighted and weakened 

 masses, impelled by gravity, breaking away and slowly dropping 

 and sUding from the parent body. The movement tends to churn 

 the mass rendering it more fluid so that later motion, still unobserv- 

 able, has apparently an element of flowage. As the slide progresses, 

 the face left bare by its movement widens, sometimes to many 

 feet. This face has usually a much steeper gradient than the 

 mass which previously covered it, offering a ready point of attack 

 by the elements and gravity, so that by repetitions of the sliding 

 process, one starting at the base may be responsible for a successive 

 series to the top of a hillside. 



As these slides are closely associated with the ground water, 

 new breaks often show this exuding from their surfaces, their 

 originally somewhat fissile faces soon slaking to a mud-plastered 

 effect. The ground water may be sufficiently abundant to form 

 a stream previously non-existent, which in turn may channel a 

 gully that will determine the drainage flow of the immediate area. 



In the same way that sHdes originate in the fresh shales, later 

 slides repeatedly affect the masses of earlier slides, the process 

 being active only when the material has ample water content. 



As an explanation of the common terrace form resulting from 

 slides, the following is suggested: Movement of the slide mass 

 favors its drainage, and when sufficiently drained movement ceases. 

 As drainage is most effective at the front, forward movement lags 

 or ceases at that point first. By pressure of the still undrained 

 rearward mass, the front in its resistance may be forced upward. 

 The result is a hummock with a bare and steeply sloping front, 

 a top of gentle slope which may be even roughly level or dip back 

 toward the hillside, giving favorable conditions for ponding of 

 water and further slide movements; at the rear, the bare steep 

 slope marking the upper border of the surface from which the 

 slide has broken away. 



Whether by weakening of their support due to slides on the 

 downslope side, or a crowding due to a faster movement on the 

 upslope side, trees under their influence usually tend to inchne 

 downhill. As the roots are anchored in unstable earth, the unbal- 

 anced condition of the tree further accentuates this tendency. 



