POST-CRETACEOUS HISTORY OF WESTERN WYOMING 337 



terraces out in the bad lands coincide with the more massive and 

 hence resistant layers of sandstone from which the wind has com- 

 pletely swept off the soft beds of sandy clay. 



Fig. 51. — Diagrams to illustrate the formation of an earth-flow such as the "Gros 

 Ventre Slide" of 1909-11: (A) before the action began; (B) after the slide took place. 

 (From Bull. Geol. Soc. Am.) 



On the other hand, wind erosion must necessarily have wrought 

 but slight effects upon those parts of the mountain sides that are 

 protected by forests, and but httle more upon their rocky summits. 

 The timber belt, constituting Merriam's' Hudsonian and Canadian 



' C. H. Merriam, "Life Zones and Crop Zones of the United States," Bull. 10, 

 U.S. Geol. Survey, 1894. 



