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 abundant drifting sand, dunes are steep on the lee side (_bc_, Fig. 11), where the angle of slope is the angle of rest for the sand. It rarely exceeds 23° or 24°.[14] Under the same conditions the windward slope is relatively gentle (_ab_, Fig. 11). If the winds be 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. 11, may be changed to _adc_. If the winds 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 blown. At some points where the winds erode (scour) more than they deposit, new profiles are developed (Figs. 12 and 13). The erosion profiles may be very irregular if the dunes are partially covered with vegetation. The effect of vegetation in restraining wind erosion is shown in Fig. 14, where plants have preserved a remnant of a dune. =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: (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. [Illustration: +Fig.+ 13.—Diagram showing the outline of dunes in process of destruction. Seven Mile Beach, N. J. (N. J. Geol. Surv.)] [Illustration: +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 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_, and the uniform vertical space between each two contours is called the _contour interval_. On the maps of the United States Geological Survey the contours and elevations are printed in brown (see Plate II).