A map line connecting
points representing places on the earth's surface that have the same elevation. It thus
locates the intersection with the earth's surface of a plane at any arbitrary elevation
parallel to the datum plane. Contours represent
the vertical, or third, dimension on a map, which has only two dimensions. They show the
shape and size of physical features such as hills and valleys. A depression is indicated
by an ordinary contour line except that hachures, or short dashes, are used on one side
and point toward the center of the depression.
Source: Leet, L. Don. 1982.
Physical Geology, 6th Edition. Englewood Cliffs, NJ: Prentice-Hall