THE ATMOSPHERE AS A GEOLOGICAL AGENT. 29 



Along coasts^, dime ridges are often transverse to the wind, and 

 groups of dune hillocks are frequently elongate in the same direction. 

 Here the soui'cc of supply of the sand is itself an elongate belt, often 

 transverse to the dominant ^ind, and the resulting dunes often have 

 great length transverse to the ^^ind. T\Tiere the wind has strong 

 mastery over the sand^ the longitudinal tendency is seen, even along 

 coasts.^ 



The shapes of dunes in section, like the shapes in ground-plan, 

 depend on the relative strength and constancy of the winds and the 

 supply of sand. With constant winds and abimdant drifting sand, 

 dunes are steep on the lee side (he. Fig. 11), where the angle of slope 

 is the angle of rest for the sand. It rarely exceeds 23° or 2-^°? Under 

 the same conditions the windward slope is relatively gentle {ah, Fig. 

 11). If the winds be variable so that the windward slope of one period 



Fig. 11. — Section of a dune showing, by the dotted line, the steep leeward (6c) and 

 gentler windward {ah) slope. By reversal of the ^^^nd the cross-section may be 

 altered to the form shown by the line adc. (Cornish.) 



becomes the leeward slope of another, and A'ice versa, this form is not 

 preserA'ed. Thus, by reversal of the A^ind. the section ahc, Fig. 11, may 

 be changed to adc. It the ^inds and the supply of sand be equal, 

 on the average, from opposite directions, the slopes should, on the 

 average, be equal, though perhaps unequal after any particular storm. 

 The steep slopes of new-made dunes are lost after the sand has ceased 

 to be blo^^'n. At some points where the winds erode Tscour) more 

 than they deposit, new profiles are developed (Figs. 12 and 13). The 



^ erosion profiles may be very ir- 



X >v regular if the dunes are partially 



Fig. 12. — Cross-section, of a dune show- covered ^itll vegetation. The ef- 

 ing the profiles developed by scour of ^^^^ ^^ vegetation in restraining 

 the ^\-ind on both flanks. (Cornish.) . , . ^ . , • ^. T 



wmd erosion is shown m Fig. IF, 



where plants have preserA'ed a remnant of a dune. 



* Cornish, loc. cit., p. 294. 



''Diller states (17th Ann. Rept., F'. S. Geol. Surv., Pt. I, p. 450) that on the coast 

 of Oregon the slope of dunes is sometimes 40°. 



