Geologic maps are prepared by geologists to provide detailed information about the type and age of rocks exposed at the Earth's surface. They also contain information about the underlying structures formed when rocks respond to stresses below the surface - this process is called deformation. Therefore, geologic maps can be useful in identifying geologic structures and for locating natural resources such oil, gas, and groundwater. They can also help us identify potential areas prone to landslides.
Geologic structures are produced when rocks break or bend due to applied stresses within the Earth - they are said to deform. Plate tectonic processes and burial of sediments are two examples of processes that lead to deformation of rocks.
Breaks or ruptures along which movement has occurred are called faults. Faulting displaces the rocks on one side of the break called the fault plane relative to those on the other side. The two blocks associated with the fault plane were given their names by miners. The downward-sloping fault surface that you could stand on is referred to as the footwall. The other side is called the hanging wall because the overhang provided a place to hang a lantern while working in the mine. There are two general classes of faults, those having vertical movement and those having mostly lateral (horizontal) movement.
Vertical-motion faults are of two types: normal faults and reverse faults. Each is named by noting the sense of motion of the top block relative to the bottom block, regardless of which block actually moved. Simply assume that the footwall has not moved. Then determine which direction the hanging wall appears to have moved. If the hanging wall moves downward as if by the pull of gravity, the fault is a normal fault. If the hanging wall appears to have moved upward against the pull of gravity, the fault is a reverse fault.
Horizontal-motion faults are lateral faults. We can identify two types of lateral faults by standing on one side of the fault. A left-lateral is one where the block on the opposite side moved to the left. If the block on the other side of the fault moved to the right it's right-lateral. Lateral faults are also called strike-slip faults because they slip along the direction of strike (see definition below) on the fault plane.
It is fairly easy to envision rocks breaking as a force is applied to them, but, it is more difficult to imagine them folding like a wrinkled carpet. Yet when stress is applied to rocks, at sufficiently high temperatures and pressures, they do indeed wrinkle. Rocks deformed deep in the Earth's crust produce folds.
Folds basically are upwarped or downwarped layers or rock material. Antiforms form when rocks bend upward. If the oldest rocks are exposed in the middle after erosion the are called anticlines. Synforms are downwarped troughs. If the youngest rocks are exposed in the middle, then they are called synclines. Often, anticlines and synclines are adjacent and share a common limb. The axial plane is an imaginary plane that divides a fold as symmetrically as possible.
Generally, geologists can see how rocks or sediments are positioned where they crop out at the Earth's surface. A geologist makes observations and records information about the color, type of rock, presence and type of fossils or other structures she or he may find. Another very important piece of geologic data is the attitude or spatial orientation of the rocks. The attitude is recorded with a compass by measuring the strike and dip.
Strike - the orientation (compass direction) of a horizontal line on an inclined plane, such as inclined strata, a fault, a fracture or other surface. The strike is usually expressed relative to north (example, "north 45º east").
Dip - the inclination angle of a line on an inclined plane which is measured perpendicular to the strike.
You can see in the figure illustrating strike and dip that water poured onto an inclined surface always runs down the surface parallel to the dip. The inclination of the water line down from the horizontal plane is the dip angle. The "water-on-the-rock-method" for finding the direction and angle of dip is very useful. Because strike is perpendicular to dip, strike easily can be determined relative to the water line. The direction that the water runs down and inclined surface is the dip direction and must be expressed together with the dip angle (example, 10ºSW). The attitude of a planar surface is recorded with the strike given first followed by the dip angle and direction (N45ºW, 10ºSW).
CLICK HERE to print out some common symbols used geologic maps to represent rock outcrops and geologic structures.
