Mapping Geologic Structures

Below the surface of our planet, rocks experience substantial forces. These forces include compression (→←), extension (←→), and shearing (Yy). All of these forces deform rock. Not only can the forces bend rocks (creating a fold ), they can even break them (called a fault ). These forces help create the structure we observe in rocks.

Faults. Faults are breaks in rock layers where the movement has occurred. They are shown on a geologic map by black lines.   The lines are solid where the fault is evident. The lines are dashed where the fault is not visible at the surface and must be inferred by a geologist.   Most faults are roughly planes and, therefore, can be described using strike (direction) and dip (angle down from horizontal). Sometimes the side of the fault moved upward (U) or downward (D) relative to the other side is labeled.

Folds. Folds occur due to compression of ductile rocks. In sedimentary rocks, the axes of folds are also shown on geologic maps as lines. An anticline is drawn as a long line straddled by a two-headed arrow in which the arrowheads point away from one another. A syncline is drawn the same way with the arrowheads pointing toward one another. In rare cases, where sedimentary beds have been fully overturned by folding, a U-shaped symbol is used to give the strike and dip of the bed.

Strike and Dip

When Earth materials undergo stress, the result is strain or deformation. Strain may vary from slightly warping of a sedimentary bed and complex flow textures in a metamorphic rock to the rupture of solid rocks along a fault. Geologists often have to visualize geologic structures in three dimensions (3-D) to understand how rocks got into their present position. They usually have to work from the limited clues they see on Earth’s surface (or on geologic maps that depict the limited clues). Geologic maps show the rocks at Earth’s surface and near-surface, not deep underground. They depict the area where each rock unit is exposed, how each is oriented, and where faulting occurs.

Small strike-and-dip symbols show Rock orientation on a map within the colored area for a given rock. These symbols indicate the general orientation of the units and whether a bed is horizontal or tilted.

Consider the sandstone bed ➀  in the figure below. This sandstone was deposited horizontally but later was tilted to the angle you see in the figure. The original surfaces of the sandstone layer from its bedding planes.

Add a horizontal plane to show how the rocks were originally oriented.   This can be thought of as the surface of a lake ➁ .

The imaginary line where the horizontal plane intersects the rock’s bedding plane (the red arrow) is called the strike ➂ . The strike line has a compass direction on Earth’s surface (like northeast).

The dip angle indicates how steeply the sedimentary bed dips relative to the horizontal plane. The dip is measured downward from the horizontal plane (lake surface), as shown by the green arrow ➃ . This angle is expressed in degrees (for example, 30°).

The strike and dip describe the three-dimensional orientation of the sedimentary bed. This is recorded on a geologic map using the strike-and-dip symbol ➄ . The longer line is the strike direction, the shorter line is the dip direction, and the number is the dip angle in degrees (down from the horizontal).

Looking back at the Pine Grove map: