The contours of a concave (hollowed out) cone ([Fig. 9]) are close together at the center (top) and far apart at the outside (bottom).
| Fig. 7 | Fig. 8 | Fig. 9 |
The following additional points about contours should be remembered:
(a) A Water Shed or Spur, along with rain water divides, flowing away from it on both sides, is indicated by the higher contours bulging out toward the lower ones (F-H, [Fig. 6]).
(b) A Water Course or Valley, along which rain falling on both sides of it joins in one stream, is indicated by the lower contours curving in toward the higher ones (M-N, [Fig. 6]).
(c) The contours of different heights which unite and become a single line, represent a vertical cliff (K, [Fig. 6]).
(d) Two contours which cross each other represent an overhanging cliff.
(e) A closed contour without another contour in it, represents either in elevation or a depression, depending on whether its reference number is greater or smaller than that of the outer contour. A hilltop is shown when the closed contour is higher than the contour next to it; a depression is shown when the closed contour is lower than the one next to it.
If the student will first examine the drainage system, as shown by the courses of the streams on the map, he can readily locate all the valleys, as the streams must flow through valleys. Knowing the valleys, the ridges or hills can easily be placed, even without reference to the numbers on the contours.
For example: On the [Elementary Map], Woods Creek flows north and York Creek flows south. They rise very close to each other, and the ground between the points at which they rise must be higher ground, sloping north on one side and south on the other, as the streams flow north and south, respectively (see the ridge running west from Twin Hills).