96 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 simi- 

 larly 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 shown 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 

 horizontal lines conforming to the surface of the ground, they wind smoothly 

 about smooth surfaces, recede into all re-entrant 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 

 along the surface 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 map- 

 ping 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 re- 

 appears 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 roads, 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 



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



