1 8 GEOLOGIC WORK OF ATMOSPHERE 



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. 



"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 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 PI. I). 



"The manner in which contours express elevation, form, and grade is shown 

 in the preceding sketch and corresponding contour map, Fig. 10. 



"The sketch represents a river valley between two hills. In the foreground 

 is the sea, with a bay which is partly closed by a hooked sand-bar. On each side 

 of the valley is a terrace. From the terrace on the right a hill rises gradually, while 

 from that on the left the ground ascends steeply in a precipice. Contrasted with 

 this precipice is the gentle descent of the slope at the left. In the map each of 

 these features is indicated, directly beneath its position in the sketch, by contours. 

 The following explanation may make clearer the manner in which contours delineate 

 elevation, form, and grade: 



"i. 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; along the contour at 

 200 feet, all points that are 200 feet above sea; and so on. In the space between 

 any two contours are found elevations above the lower and below the higher con- 

 tour. 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 sur- 

 rounds 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 



