Geological
Mapping
Geologic Mapping and Interpretation
Geology means “study of earth,” which can be portrayed by geologic maps. Geologic maps use brightly colored
patterns to show the absolute ages of different rock formations. An example of a geologic map is below from the a great county in Kansas.
Image taken from: Kansas
Geological Survey Geologic Mapping webpage. Interpreting geologic maps can be easier knowing a few rules that we determined by using three-dimensional block models of rock layers and common sense. The information that follows is based on the geologic block model and mapping labs used in my Introduction to Earth Science course. There
are three principles that will be introduced in order to interpret maps and geologic structure. The Law of
Superposition is used to determine the relative positioning sequence of
rocks. Nicolaus Steno was given the
credit for this law. Basically this
rule means in an undeformed sequence of sedimentary rocks, each bed of
rock is older than the one above it and younger than the one below it. The Principle of Original
Horizontality states that all sedimentary rock sequences were originally laid down
horizontally. Steno was also credited
for this principle. The Principle of Cross-Cutting Relationships states that when a fault cuts through other
rocks we can assume that the fault is younger than the rocks affected. There is one exception to the superposition concept and that is when layers of rock were being turned upside down. This is not common but it is easily recognized and this is where folding and faulting come in place. Folds and Faults There are two main deformations that rocks can go through that can
make dating the rock more difficult. A
fold is a bent rock layer or series of layers that were originally horizontal
and subsequently deformed. Folds come
from compression and can lead to an anticline, where the rock folds up and the
rock in the middle is the oldest, or a syncline, where the rocks folds down and
the rock in the middle is the youngest.
There can also be faults that can occur. A fault is a break in the rock mass along which movement has
occurred. There are two types of faults
and they both deal with hanging walls and footwalls. A normal fault is when the hanging wall moves down relative to
the footwall and they move apart. A
tensional force causes this action.
There is also a reverse fault when the hanging wall moves up relative to
the footwall and they move together. A
compressional force causes this action.
These folds and faults create our mountains and hills such as the Alps,
Northern Rockies, and the Himalayas. To learn more about geology and geologic mapping go to geology.com. This site gives information about
the geologic time scale, different rock disasters, and links to help you buy your very own geologic maps. Return to the index. 