A bedrock geology map does not simply show surface exposures of rocks. It also contains information about the region’s structural geology. Through the use of a variety of symbols, the bedrock geology map describes folds, faults, and other three dimensional features of the rock units.
The most common symbol is the strike and dip symbol, which looks like this:
The long line represents the direction of strike, while the short line and associated number show the direction and angle of dip.
Strike represents the direction of a horizontal line contained within the surface of a bed. In both of the accompanying figures, a horizontal line strikes towards the upper left and lower right sides of the image. Dip represents the direction and angle the beds are sloping away from this horizontal line. The short dip line always points in the downhill direction.
A map will usually contain many strike and dip symbols. While mapping, geologists measure strike and dip in many locations using a surveying compass. Since rock bodies are often continuous, large scale structures like folds can be delineated by combining many measurements.
Folds
A fold consists of a layer of rock that has been bent around a certain axis:
In an anticline (left), beds dip away from the central axis. In a syncline (right), beds dip towards the central axis (think of a valley). Since beds slope in opposite directions on opposite sides of the fold axis, nearby rocks with similar strikes and opposite directions of dip are usually interpreted as being part of a fold.
Fold Symbols
A fold symbol can be thought of as a complex strike and dip symbol:
In a syncline, arrows point inward, indicating that the beds slope down towards the axis. In an anticline, arrows point outward (see accompanying image), indicating that beds slope away from the fold axis.
Anticlines are the most important folds to geologists because they can serve as oil traps.
Faults
Another important class of geologic structures mapped on a geologic map are faults. Faults occur when rocks break apart and move along a surface.
There are three types of faults:
Reverse Fault
Normal Fault
Strike-slip Fault
A reverse fault occurs when one body of rock is forced on top of another.
The line divides the two rock bodies involved in the fault. The hanging wall block is the overhanging, upthrown block, and the footwall block is the block that moved downward. The “teeth” of the reverse fault symbol are on the hanging wall.
In a normal fault, the hanging wall moves down relative to the footwall. The symbol for a normal fault is the same as that for a reverse fault, except that tics are used instead of teeth and they are placed on the downthrown side of the fault.
High angle faults are nearly vertical – they don’t have a hanging wall and a footwall. In this case, the direction of movement is marked with U and D, for upthrown and downthrown.
A strike-slip fault occurs when two plates slide past each other in the horizontal plane. It is marked by a bold line with arrows showing the direction of movement on each plate.
Images: “Monument valley at sunset, Utah, USA” by MBPROJEKT_Maciej_Bledowski via iStock; “Illustrations” by Top Energy Training