204 



PLANT SUCCESSION 



example, erosion by the action of water may produce an area that is 

 entirely without plants (Fig. 89). This frequently takes place at the 

 upper end of a ravine or along a stream bank. Excellent examples 

 of this type of erosion may be seen in the "bad-land" regions of 

 North and South Dakcjta. In some regions ice is even more impor- 

 tant than liquid water as an agent of erosion. This is true especially 

 where there are glaciers. The ice mass of the glacier usually grinds 

 up and removes all plants and plant parts as well as the upper layers 

 of the substratum so that with the later retreat of the ice large areas 



Fig. 89. — A bare area of shale produced by the erosive action of water. 



of virgin soil or rock are laid bare. Sometimes when the water of a 

 pond or lake freezes the expansion that takes place causes the ice to 

 push against the shore and bring about erosion which results in bare 

 areas. Wind is also an important agent of erosion, especially in 

 regions of sandy soil where it may produce moving sand dunes or 

 may dig out depressions in the sand known as "blow-outs." Gravity 

 may produce a crumbling or slipping of land masses, bringing about 

 land slides which result in bare areas. 



Deposit is, in a sense, implied when we speak of erosion, since 

 materials removed from one place must eventually be deposited 

 somewhere else. However, as an agent for the production of bare 



