336 " ELIOT BLACKWELDER 



covered with forest and must be some generations or centuries old, 

 although nearly all of their topographic features and forms are still 

 preserved. Still others have been scarred by gullies to such a degree 

 that they are now recognizable only upon somewhat careful exam- 

 ination. There are many places in which landslides of two or three 

 distinct ages may be seen side by side on the same valley slope. 



Wind work in the Quaternary period. — The climate of the entire 

 region is moderately dry, and in the centers of the large basins it 

 is decidedly arid. Under such circumstances the wind is admittedly 



Fig. 50. — A rock-fall in the canyon of Darby Creek, Teton Range 



an important agent in shaping topographic forms. The character- 

 istics of its work are, however, greatly influenced by the varying 

 conditions in different parts of the district. Thus, in the hot, dry 

 central part of the Wind River and Green River basins, the erosive 

 action of the winds can be observed daily. Bunches of sagebrush 

 and greasewood are left perched on hillocks, where the silt has been 

 blown out from around them by the wind. Protruding ledges of 

 rock have been carved by the sand-blast into the usual fantastic 

 pillars, mushroom forms, and honeycomb surfaces. Many of the 

 smooth slopes along the sides of the valleys are swept so clean that 

 every lamina of rock may be measured or traced for yards and even 

 miles along the outcrop. The smooth tops of many mesas and 



