8 PHYSIOGRAPHY 



1. We are to remember that a contour indicates a certain height above sea- 

 level. In this illustration, the contour interval is 50 feet; therefore the con- 

 tours are drawn 50, 100, 150, 200 feet, and so on, above sea-level. The 50- 

 foot contour connects all points of the surface 50 feet above the sea, and the 

 100- foot contour connects all points 100 feet above the sea. In the space 

 between any two contours, the elevation is greater than that of the lower, and 

 less than that of the higher. Thus the contour at 150 feet falls just below the 

 edge of the terrace, while that at 200 feet lies above the terrace. All points 

 on the terrace are more than 150 but less than 200 feet above the sea. The 

 summit of the higher hill is stated to be 670 feet above the sea, and the contour 

 at 650 feet surrounds it. In this illustration nearly all the contours are num- 

 bered; but on most maps this is not possible. Instead, certain contours 

 say every fifth one are made heavier and numbered (PI. I) ; the height of 

 any contour not numbered may then be known by counting up or down from 

 one that is numbered. 



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

 zontal lines at a given height above the sea, they wind smoothly about smooth 

 surfaces. They run up into ravines, and project out in passing about promi- 

 nences. The relations of contour curves and angles to the form of the surface 

 can be traced in Fig. 4. 



3. Contours give some idea of the steepness of a 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. Therefore contours are far apart on gentle slopes, and near 

 together on steep ones. 



Drainage. On the maps of the United States Geological Survey, water- 

 courses are indicated by blue lines (PL I). If the streams flow all the year, 

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

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

 surface, the supposed course under the ground is shown by a broken blue 

 line. Lakes, marshes, and other bodies of water are also shown in blue. 



Culture. The works of man, such as roads, railroads, and towns, and also 

 boundaries of townships, counties, and states, are printed in black. 



Let us now apply these principles to Plate I (p. 4) . We notice 

 at the outset that one inch on the map corresponds to about two 

 miles, and that the contour interval is 100 feet. 



The contours (brown lines) are far apart at the left and close 

 together at the right. This means that the surface is rougher at 

 the east, and smoother at the west. The area at the east is higher 

 than that at the west, as the numbers on the contours show. The 

 500, 1,000, and 1,500-foot contours are heavier than the others. 

 Small areas only reach a height of 1,500 feet. At the north, the 



