FAULTS are fractures or breaks in the earth's crust along which
movement has occurred. The term JOINT is used when the rocks fracture without movement.
Faults can be VERTICAL faults where the blocks of the crust move up or down, HORIZONTAL faults when the
blocks move sideways to each other, and COMPOUND faults where the movement is a combination of upward or
downward and sideways.
Two types of vertical faults are recognized.
The FOOT WALL BLOCK is the block which would be under
the feet of a person standing in a tunnel on the fault plane. The
HANGING WALL BLOCK would then be hanging overhead.
The UPTHROWN SIDE of
the fault is the side on which the movement has been up relative
to the other side. It is represented by an arrow pointing up on a
geologic section or block diagram or by a U on a map. The
DOWNTHROWN SIDE is the side which went relatively down and is
represented by such an arrow or the letter D.
GENERALIZATIONS: A normal fault is a fault in which the (hanging, foot) wall block has moved
upward relative to the (hanging, foot) wall block. A reverse fault is a fault in which the
(hanging, foot) wall block has moved downward relative to the (hanging, foot) wall block.
In vertical faults the (older, younger) rocks will be faulted up to the (older, younger)
rocks. This is an especially useful and important determination because when erosion occurs on the surface, the difference in the age of the exposed rock indicates
the relative movement of the fault. Theoretically, both blocks may have moved upward or both blocks may have moved downward or they may have gone opposite directions or only one may have moved. In reality, it is the
end result of the movement, though, that is what is important and what determines the type.
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