32 GEOLOGY. 



1. A contour indicates approximately a certain height above sea-level. In 

 this illustration the contour interval is 50 feet; therefore the contours are drawn 

 at 50, 100, 150, 200 feet, and so on, above sea-level. Along the contour at 250 

 feet lie all points of the surface 250 feet above sea; and similarly with any other 

 contour. In the space between any two contours are found all elevations above 

 the lower and below the higher contour. Thus the contour at 150 feet falls 

 just below the edge of the terrace, while that at 200 feet lies above the terrace; 

 therefore all points on the terrace are show^n to be more than 150 but less than 

 200 feet above sea. The summit of the higher hill is stated to be 670 feet above 

 sea; accordingly the contour at 650 feet surrounds it. In this illustration nearly 

 all the contours are numbered. Where this is not possible, certain contours — 

 say every fifth one — are accentuated and numbered ; the heights of others may 

 then be ascertained by counting up or down from a numbered contour. 



2. Contours define the forms of slopes. Since contours are continuous hori- 

 zontal lines conforming to the surface of the ground, they wind smoothly about 

 smooth surfaces, recede into all reentrant angles of ravines, and project in passing 

 about prominences. The relations of contour curves and angles to forms of 

 the landscape can be traced in the map and sketch. 



3. Contours show the approximate grade of any slope. The vertical space 

 between two contours is the same, whether they lie along a cliff or on a gentle 

 slope; but to rise a given height on a gentle slope one must go farther than on a 

 steep slope, and therefore contours are far apart on gentle slopes and near together 

 on steep ones. 



For a flat or gently undulating country a small contour interval is used ; for a 

 steep or mountainous country a large interval is necessary. The smallest interval 

 used on the atlas sheets of the Geological Survey is 5 feet. This is used for regions 

 like the Mississippi delta and the Dismal Swamp. In mapping great mountain 

 masses, like those in Colorado, the interval may be 250 feet. For intermediate 

 relief contour intervals of 10, 20, 25, 50, and 100 feet are used. 



Drainage. — Watercourses are indicated by blue lines. If the streams flow 

 the year round the line is drawn unbroken, but if the channel is dry a part of 

 the year the line is broken or dotted. Where a stream sinks and reappears at 

 the surface^ the supposed underground course is shown by a broken blue line. 

 Lakes, marshes, and other bodies of water are also shown in blue, by appropriate 

 conventional signs. 



Culture. — The works of man, such as road, railroads, and towns, together 

 with boundaries of townships, counties and states, and artificial details, are 

 printed in black." ^ 



Topography of dune areas. — From what has been said, it is clear 

 that the topography of dune regions may vary widely, but it is always 

 distinctive. Where the dunes take the form of ridges (Fig. 1, PL II), the 

 ridges are often of essentially uniform height and width for consider- 

 able distances. If there are parallel ridges, they are often separated 



^ From folio preface, U. S. Geol. Surv. 



