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boundaries. The conventional signs used to represent these 
satures are shown and explained below. Variations appear on 
ome earlier maps, and additional features are represented on 
pine special maps. 
- 
All the water features are represented in bine, the smaller 
[reams and canals bv single blue lines and the larger streams. 
w C/ ^ *] 
ie lakes, and the sea by blue water lining or blue tint Inter¬ 
nment streams—those whose beds are dry for a large part of 
le year—are shown by lines of blue dots and dashes. 
* 4 / • 
Relief is shown by contour lines in brown, which on some 
laps are supplemented by shading showing the effect of light 
grown from the northwest across the area represented, for the 
urpose of giving the appearance of relief and thus aiding in 
re interpretation of the contour lines. A contour line repre- 
?nts an imaginary line on the ground (a contour) every part 
f which is at the same altitude above sea level. Such a line 
>uld be drawn at any altitude, but in practice only the con¬ 
curs at certain regular intervals of altitude are shown. The 
ne of the seacoast itself is a contour, the datum or zero of abi¬ 
de being mean sea level. The 20-foot contour would be the 
[ore line if the sea should rise 20 feet. Contour lines show 
e shape of the hills, mountains, and valleys, as well as their 
Successive contour lines that are far apart on the 
ap indicate a gentle slope; lines that are close together indi¬ 
te a steep slope; and lines that run together indicate a cliff. 
fThe manner in which contour lines express altitude, form, 
id grade is shown in the figure below. 
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