FoldsAnother kind of line shown on most geologic maps is a fold axis. In addition to being moved by faults, geologic units can also be bent and warped by the same forces into rounded wavelike shapes called folds. A line that follows the crest or trough of the fold is called the fold axis. This is marked on a geologic map with a line a little thicker than a depositional contact, but thinner than a fault (location 6).
Solid, dashed, or dotted lines
All thicknesses of lines are also modified by being solid, dashed, or dotted. Often contacts are obscured by soil, vegetation, or human construction. Those places where the line is precisely located it is shown as solid, but where it is uncertain it is dashed (location 7). The shorter the dash, the more uncertain the location. A dotted line is the most uncertain of all, because it is covered by a geologic unit, so no amount of searching at the surface could ever locate it (location 8). The lines on the map may also be modified by other symbols on the line (triangles, small tic marks, arrows, and more) which give more information about the line. For example, faults with triangles on them (location 9) show that the side with the triangles has been thrust up and over the side without the triangles (that kind of fault is called a reverse fault or a thrust fault). All the different symbols on the lines are explained in the map key (which is explained below).
