Chapter 17 Unraveling Geologic History

sion is indicated by an uneven boundary between layers or
layers that are cut off. Igneous rocks must be the result of
melting, intrusion, extrusion, and solidification, also known
as crystallization. If you are not familiar tilting, folding, faulting,
and metamorphism, ask your teacher to explain them.
Most profiles begin with deposition or solidification. (The
rock needs to be formed before it is changed.) If the bottom
layer is metamorphic rock, deposition or solidification is followed
by metamorphism.
Keys to Rock Types
Do not expect to see the same symbols on every map or profile.
However, a key should make the meaning of the symbols
clear. A key should accompany each profile. Pages 6 and 7 of
the Earth Science Reference Tables have charts to help you to
identify igneous, sedimentary, and metamorphic rocks.
The charts for sedimentary and metamorphic rocks contain
symbols that are sometimes used on maps and diagrams
of geologic profiles to indicate various kinds of rock. Many of
these symbols represent characteristic textures of the rocks
they symbolize. For example, sandstone is composed of gritty
particles, which is usually represented by a dotted pattern.
Shale tends to break into thin layers, which is usually shown
by a pattern of short horizontal line segments. A combination
of line segments and dots usually represent siltstone, a
sedimentary rock of intermediate-size particles. Limestone is
shown by a symbol that looks like stacked bricks because
limestone often breaks along joints and bedding surfaces into
large, somewhat rectangular blocks.
ACTIVITY 17-3 SYMBOLS AND ROCKS
HOW CAN WE INTERPRET GEOLOGIC PROFILES? 395
Copy map symbols from the Earth Science Reference Tables, or
profile diagrams and/or from geologic maps. Next to each symbol
tell how the symbol resembles the texture of the rock it represents.
Adding photographs or samples of the various kinds of rocks can
enrich this assignment.