94 GEOLOGY 



relatively gentle (ab, Fig. 56). If the winds are variable so that 

 the windward slope of one period becomes the leeward slope of 

 another, and vice versa, this form is not preserved. Thus, by 

 reversal of the wind, the section abc, Fig. 56, may be changed to 

 adc. Where the winds erode (scour) more than they deposit, 

 other profiles are developed. The erosion profiles may be very 



Fig. 57. Dunes at Longport, coast of New Jersey, showing the irregular 

 forms developed by winds which erode. 



irregular if the dunes are partially covered with vegetation, as 

 shown in Fig. 57. 



The topographic map. Since dunes as well as other topographic features 

 are conveniently represented on contour maps, and since such maps \\ill 

 be used frequently in the following pages, a general explanation of them is 

 here introduced. 



"The features represented on the topographic map are of three distinct 

 kinds:. (1) inequalities of surface, called relief, as plains, plateaus, valleys, 

 hills, and mountains; (2) distribution of water, called drainage, as streams. 

 lakes, and swamps; (3) the works of man, called culture, as roads, railroads, 

 boundaries, villages, and cities. 



"Relief. All elevations are measured from mean sea-level. The heights 

 of many points are accurately determined, and those which are most important 

 are given on the map in figures. It is desirable, however, to give the elevation 

 of all parts of the area mapped, to delineate the horizontal outline, or contour. 

 of all slopes, and to indicate their grade or degree of steepness. This is done 

 by lines connecting points of equal elevation above mean sea-level, the lines 

 being drawn at regular vertical intervals. These lines are called contours, 



