THE CONTOUR MAP 17 



>i It-ruble distances. If there are parallel ridges, they may be sep- 

 arated by trough-like depressions. Where dunes assume the form 

 of hillocks (Figs. 2 and 3, PL I) rather than ridges, the topography 

 is even more distinctive. In some regions depressions (basins) are 

 associated with the dune hillocks. In some places they are hardly 

 less notable than the dunes themselves. 



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 introduced. 



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

 (i) inequalities of surface, called relief, as plains, plateaus, valleys, hills, and 



Fi^r. 10. Sketch and map of the same area, to illustrate the representation of 

 topography by means of contour lines. (U. S. Geol. Surv.) 



