THE FEATURES IN DESERT LANDSCAPES 



215 



become softened into flowing curves, due to the protective mat 

 of vegetation. In arid lands those massive rocks which are 

 without structural planes of separation, partly as a consequence 

 of exfoliation, develop broad domes which are projected upon 

 the horizon as great semicircles, 

 broken in half it may be by 

 displacement. The same massive 

 rocks where intersected by vertical 

 joint planes yield, on the contrary, 

 sharp granite needles like those of 

 Harney Peak (plate 8 A). Similarly, 

 schistose or bedded rocks, when tilted 

 at a high angle, may yield forms 

 which are almost identical. Ex- 

 amples of such needles are found in 

 the Garden of the Gods in Colorado. 



At lower levels, where the flying 

 sand becomes effective as an erod- 

 ing agent, flat bedded rocks become 

 etched into shelves and cornices, and 

 if intersected by joints, the shelves 



and cornices are transformed into groups of castellated towers and 

 pinnacles of a high degree of ornamentation. These fantastic 

 erosion remnants are usually referred to as " chimneys" and may 

 be seen in numbers in the bad lands of Dakota, as they may in 

 Colorado either in Monument Park or in the new Monolithic 

 National Park (plate 8 B). 



Where wind erosion plays a smaller role in the sculpture, but 

 where after an uplift a river has made its way, horizontally bedded 

 rocks are apt to be carved into broad rock terraces, nowhere shown 

 upon so grand a scale as about the Grand Canon of the Colorado. 

 Each harder layer has here produced a floor or terrace which 

 ends in a vertical escarpment, and this is separated from the 

 next lower layer of more resistant rock by a slope of talus which 

 largely hides the softer intermediate beds. The great Desert of 

 Sahara is shaped in a series of rock terraces or steppes which 

 descend toward the interior of the basin. 



A single harder layer of resistant rock comes often to form 

 the flat capping of a plateau, and is then known as a mesa, or 



FIG. 227. Amphitheater at the 

 head of the Wed Beni Sur (after 

 Walther). 



