30 



GEOLOGY. 



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

 are conveniently represented on contour maps, and since such maps will be used 

 frequently in the following pages, a general explanation of them is here intro- 

 duced. 



" 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. 



Fig. 13. — Diagram showing the outline of dunes in process of destruction. Seven 

 Mile Beach, N. J. (N. J. Geol. Surv.) 



Fig. 14. — Illustrates the protective effect of vegetation against wind erosion. 

 Dune Park, Ind. (Cowles ) 



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 



