THE ATMOSPHERE AS A GEOLOGICAL AGENT. 



37 



bottoms, but the sand is often blown up out of the valley and lodged 

 on the bluffs above. 



Apart from these special classes of situations, any sandy region 

 the surface of which is dry is likely to have its surface material shifted 

 by the wind and piled up into dune ridges or hillocks (Fig. 4, PI. II). 

 Dunes probably reach their greatest development in the Sahara, where 

 some of them take the form of hillocks, and some the form of ridges. 

 Travelers in that region report that dune ridges are sometimes en- 

 countered the faces of which are so high and steep as to be difficult 

 of ascent, and that parties have been obliged to travel miles along 

 their bases before finding a break where crossing was practicable. 



Fig. 20.— Wind-ripples. (Cross, U. S. Geol. Surv.) 



Wind-ripples. — The surface of the dry sand over which the wind 

 has blown for a few hours is likely to be marked with ripples (Fig. 20) 

 similar to those made on a sandy bottom beneath shallow water, under 

 the influence of waves. Like ripple marks made by the water, wind- 

 ripples have one side (the lee) steeper than the other. While the 

 ripples are, as a rule, but a fraction of an inch high, they throw much 



