Dip and Strike on Geologic Maps


DIP is the acute angle that a rock surface makes with a horizontal plane. STRIKE is the direction of the line formed by the intersection of a rock surface with a horizontal plane.

example of dip and strike



Strike and dip are always perpendicular to each other on a map.

On a fold, the AXIS is the ridge or plane of sharpest folding. PLUNGE is the acute angle the axis of a folded rock mass makes with a horizontal plane. Think of arching a piece of paper that has its edges against the floor. The top of the arch will be parallel to the floor. This "fold" would NOT be plunging. If you would raise one of the open ends so that the corners and that end of the edges were no longer against the floor, the "fold" would be PLUNGING.

Agents of EROSION wear away the rocks at the earth's surface. So, an upfold may no longer be a rise in elevation, and a downfold may no longer be a low. Differential erosion results in the stronger rocks or the rocks that are more resistant to these erosion processes, end up higher because they take longer to wear away than the less resistant or weaker rocks. Folds are then worn down. The topography is the result of the interplay between the geologic structure, tectonic activity, gradation, and time. The geologic structure involves the rocks and their characteristics. Tectonic activity involves how the forces have effected those rocks such as the folding, faulting, uplift, and volcanism. Gradation is transportation and deposition of the material. Time is the stage in the whole process, because with time the surface is worn down, exposing new materials on the surface and completely changing the topography of the surface. The text demonstrates some of these changes and processes. Our interest here is to note that the folds are usually truncated. Look at figure 3.

eroded folds



Find the part of the figure that would be the cross section. The geologic structure at A is a(n) (anticline, syncline). B is a(n) (anticline, syncline). C is the axis of the (anticline, syncline).

geologic map symbols

The symbol that should be added to this axis line to show the structure would be (double arrows pointing to the line, double arrows pointing away from the line).

geologic map symbols added

The axis for D should run from point (g, h, i, k, m, n) to point (p, r, s, t, u, w). The symbol shown on the axis line should be (double arrows pointing to the line, double arrows pointing away from the line).


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