1
Rock Units and Your Community
Think About It
Page U33
 If you start out at an
outcrop of bedrock and
see a particular kind of
rock, how far would you
have to walk until the
rock type changed?
Date
Page #
2
Rock Units and Your Community
Investigate
Pages U34-35
1a. What kinds of
sedimentary rocks
are shown on the
map and cross section?
1b. What environment
do you think was present
during the deposition
of the rocks in this area?
1c. How does the thickness
thickness of the rock
units compare to the
distance covered by the
rock units?
1d. Why do you think
that some of the
sedimentary layers in the
cross section are tilted?
1e. Are the units
continuous? Explain.
2a. What kinds of
igneous rocks are
shown on the map
and cross section?
Date
Page #
3
2b. Are the igneous
rocks shown intrusive
or extrusive?
2c. Are the igneous
Rocks in the cross
section visible on the
map? Explain.
2d. Compare the rock
units labeled
Pinckneyville Granite,
Paleozoic Granite,
and Jurassic-Triassic
intrusives. How do
their sizes vary?
2e. How are they
different? How are they
similar? How do they
differ from the
Pinckneyville Granite?
2f. Describe the
Relationship between the
Pinckneyville Granite,
Paleozoic Granite and
Jurassic-Triassic
Intrusives and the
surrounding sedimentary
rocks.
4
3a. What kinds of
metamorphic rocks are
shown on the map and
cross section?
3b. How does the
placement of the
metamorphic rock
units compare with
the surrounding
rock units?
5
Rock Units and Your Community
Digging Deeper
Pages U36-U37
Rocks
Date
Page #
are originally formed in large
volumes by a specific process
Rock unit
a body of rock that consists
dominantly of a certain rock type,
or a combination of types
Contact
the change from one rock unit to
another
Sedimentary
rock units
consist of conglomerate, sandstone
shale, or limestone that can be
exposed in the same area of the
land surface
Such units can be of the same age,
having been deposited in different
environments at the same time.
Usually have a width (lateral
extent) that is greater than their
thickness
Sedimentary
rock unit name
the first part is the name of some
place like a town, river or
mountain where the unit is found
6
The second part is the mane of the
rock type, or just the word
“Formation.”
Igneous rock units
form in two ways:
1. Cooling and crystallization of
magma below Earth’s surface to
form intrusive igneous rocks
2. Volcanic activity
Intrusive igneous
rock units
vary greatly in size and shape
Sills
sheets of igneous rock that
intruded (horizontally) along
layers of sedimentary rock
Dikes
sheets of igneous rock (vertically)
intruded along fractures that cut
through existing rock
7
Batholith
a large mass of intrusive igneous
rock with irregular shapes
Intrusive igneous
rock bodies
can be exposed by weathering
and erosion
Metamorphic rocks
vary greatly in composition
8
The shapes of the rock units are
more complicated than
sedimentary or igneous rocks
This is because metamorphism is
usually accompanied by largescale changes by processes like
folding and faulting
9
Rock Units and Your Community
Check Your Understanding
Page U37
1. Why is the shape
of metamorphic rock
units usually much
more complicated
than that of igneous
or sedimentary rock
units?
2. What is the change
from one rock unit to
another called?
3. What is the
difference between
a sill, a dike, and a
batholith?
4. What kinds of
rocks are laid down
in nearly horizontal
layers?
Date
Page #
10
Rock Units and Your Community
Understanding and Applying
Page U38
1. If only some of the
rocks are visible at
the Earth’s surface,
how do geologists
construct cross
sections?
2a. Can you tell what
rock types are present
in the area from the
size and shape of
the units? Explain.
2b. What rocks are
present on the Earth’s
surface?
2c. What rocks are
present below the
Earth’s surface?
2d. What is the relative
order of the rock units
in your community?
2e. Is one layer
always found
beneath another?
Date
Page #
11
2f. How do the sizes of
the rock units in your
community compare
with one another?
Which occupies the
most surface area?