TOPOGRAPHIC MAPS 221 



the other bank. If the two lines are followed upstream, it 

 is found that they are both really parts of a single line, which 

 crosses the stream somewhere. Farther up the stream the 

 land is higher than this first line, and at a longer or shorter 

 interval, according to the slope, another line is found crossing 

 the stream. The contour lines consequently bend upstream. 

 Where there is no stream, a similar upward bend of contours 

 shows the presence of a. valley, as is indicated in Fig. 116. 



242. Reservoirs. If at any place a wall is built across 

 a valley with its top at the elevation of the contour nearest 

 to the stream, the water flowing down the valley will be 

 stopped by the wall. When the water has reached the top 

 of the wall, it will have overflowed the banks of the river 

 to the first contour level. A narrow reservoir will be formed, 

 with the water setting back upstream to the place where 

 the contour line crosses the stream. If the dam should be 

 built between the second contour lines from the river, the 

 reservoir would be deeper (as much deeper as the number of 

 feet in the contour interval), and the water would set back 

 to the crossing of the second contour. 



243. Topographic Maps of the United States. The 

 United States Geological Survey has undertaken the work 

 of making topographic maps of the whole country. Each 

 state shares the expense of printing the maps after the 

 government survey has been made. The scale of the maps 

 is about one mile to the inch. If all the maps were put 

 together to make a map of the United States, they would 

 measure about 250 feet from east to west. Each sheet of 

 the government maps represents a piece of country thirteen 

 miles wide (east to west) and seventeen miles long (north to 

 south). Contour lines are in brown ink; lines representing 

 man's work, such as boundary lines, roads, and railroads, are 

 in black; water is in blue. Houses on a country road 

 are shown by black dots; city and town streets are solid 

 black lines. (LABORATORY MANUAL, Exercise XXI.) 



