GROUND-WATER 



43 



more easily. Under these circumstances, it sometimes slides down. 

 Such movements are known as slumping or sliding. If the slide is 

 large, it is sometimes called a landslide. Slumping is very com- 

 mon on the slopes of hills composed of clay or other loose matter 

 (Fig. 28). 



Many destructive landslides have been recorded, but a few facts 

 about one will illustrate the phenomena of all. On the 29th of 



Fig. 28. South face of Landslip Mountain, Colo The protruding mass in 

 the center has slumped down. (From photo. U. S. Geol. Surv.) 



April, 1903, there was a slide on Turtle Mountain, Province of 

 Alberta, Dominion of Canada. A huge mass of material nearly 

 half a mile square, and probably 400 to 500 feet deep, suddenly slid 

 down the steep face of the mountain, into the valley below. It 

 crossed the valley, which was half a mile wide, and rose a few hun- 

 dred feet on the other side. When it came to rest, the material 

 which had slidden down was spread over an area of a little more than 

 one square mile. The length of the slide was about two and a half 

 miles, and it is estimated that the time which it took was not more 

 than 100 seconds. The heavy rainfall of the preceding year had 

 filled the rock with water, and the earthquake tremors which 

 occurred shortly before the slide are believed to have hastened that 



