Geologic Laws and Processes
Basic Geologic Laws
Several basic laws are fundamental to understanding the structure of earth materials.
The LAW OF ORIGINAL HORIZONTALITY states that a series of sedimentary layers will
generally be deposited in horizontal layers. You might think of this like snow falling
one day when it is not windy, and it blanketing the ground.
The LAW OF SUPERPOSITION states that if a series of sedimentary rocks has not been
overturned, the top most layer is always the youngest and lower most layer is always the
oldest. You might think of this like snow falling the next day and then the next day,
the bottom layer is older than the snow on the top layer.
A third law is important for igneous bodies and various other geologic features such
as folds, faults, joints, and unconformities (associated with erosion events). This law may
be illustrated by the interpretation of footprints in a puddle of mud. Imagine two footprints
crossing each other a man's and a woman's.
Who stepped in the puddle FIRST? How do you determine that?
Since the man's foot print is interupted or broken by that of the woman in her high heels,
we know that the man's print had to be there first in order for the woman's to break across the
This same type of deduction can be applied in geology. The LAW OF CROSS CUTTING RELATIONS says
that a feature that is broken (cross cut) by another is older than the one that cuts across. For
example, if one mass of igneous rock is cut across by another igneous body, the mass is older than
the body that cuts through it.
Earth materials have two main reactions to pressure. They either fold (bend) or fault (break).
After folding or faulting occur, EROSION is acting on the earth's surface. Sometimes erosion is
even acting as the folding or faulting are taking place. Folding can be a slow or a fast process, and
folds can be small or they can be very large.
Before we look at these geologic structures, let's look
a the ways we can depict parts of the earth and it's rock materials.
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