i6 



GEOLOGIC WORK OF ATMOSPHERE 



winds and abundant drifting sand, dunes are steep on the lee side 

 (be, Fig. 8), where the angle of slope rarely exceeds 25. Under the 

 same conditions, the windward slope is relatively gentle (ab). 



Fig. 8. Section of a dune showing, by the dotted line, the steep leeward (be) 

 and gentler windward (ab) slope. By reversal of the wind, the cross-section may 

 be altered to the form shown by the line ode. (Cornish.) 



If the winds are variable, so that the windward slope of one time 

 becomes the leeward slope of another, and vice versa, this form is 

 not preserved. By reversal of the wind, the section abc may be 

 changed to adc. Where the winds erode (scour) more than they 

 deposit, other profiles are developed. The erosion profiles may be 



Fig. 9. Dunes at Longport, coast of New Jersey, showing the irregular forms 

 developed by winds which erode. 



very irregular if the dunes are partially covered with vegetation 

 (Fig. 9). 



Topography of dune areas. From what has been said, it is clear 

 that the topography of dune regions varies widely, but it is always 

 distinctive. Where the dunes take the form of ridges (Fig. i, PL I), 

 the ridges may be of essentially uniform height and width for con- 



