92 



PHYSICAL GEOLOGY 



posed to strong winds. They range in height from a foot or two 

 up to 300 or 400 feet, and in rare cases even more. The great 



majority do not exceed 20 feet. 

 In cross section they are typi- 

 cally as shown in Figure 82. 

 The longer and gentler side faces 



FIG. 80. Deposit of sand along 

 the side of a fence. 



FIG. 82. Cross section of a 

 sand dune. 



FIG. 81. Beginning of a dune on the 

 beach of Lake Michigan, near Dune 

 Park, Ind. Shows how sand accumu- 

 lates about obstructing vegetation. 



the dominant wind, wh le the shorter and steeper side 

 is the leeward slope. The windward slope is a roadway up 



which sand is rolled and dragged 

 to the crest, behind which it drops. 

 Its steepness varies with the 

 strength of the wind, and the size 

 and quantity of the sand. Strong winds are able to move 

 a small amount of fine sand up relatively steep slopes; 

 weak winds burdened with much coarse sand require a gentle 

 grade. The leeward slope represents the angle at which the 

 sand will rest, and rarely exceeds 24 or 26. Dune sand is 

 often distinctly, but irregularly, stratified (Fig. 83). Dunes 

 vary in ground plan from roundish mounds and hills to elongate 

 ridges. Examples of various types of dunes are shown on the 

 topographic maps of Plate I. 



In order to read topographic or contour-line maps, it is neces- 

 sary to know that contours are lines drawn on maps to express 



