Geology Reference
In-Depth Information
Figure 2.6. Geologic maps
(top) show three-dimensional
rock units (bottom cross-
section), such as limestone,
sandstone, and intrusive
rocks, and structural features
such as faults. The relative
ages of the units and
structural features are
commonly indicated in a
stratigraphic column
organized from youngest
(top of column) to oldest
(bottom of column).
attributes in a given area are sometimes combined into a
group. For example, a series of lava eruptions and ash
flows mapped as formations and members on a volcano
could be de
ned as a stratigraphic group.
Formations can be more complicated than the simple
case of the lava
flow. Imagine a lake basin that receives
run-off and sediments from the surrounding mountains.
As the streams empty into the lake, there is a decrease in
the speed of the water
flow and, hence, a decrease in their
ability to carry sediments. Coarser materials, such as
boulders, would be deposited close to the shore, with
progressively smaller rocks and sediments being depos-
ited outward from the shore. Over time, the sediments in
this sequence continue to accumulate and can eventually
be lithi
ed (turned into rock). If one were to see only the
part formed close to shore, the rock would be a conglom-
erate (a rock composed of large, rounded rock fragments);
farther from shore, the rock would be sandstone (rock
composed of sand-size particles); still farther from the
shore, the rock would be shale (rock composed of very
fine grains, such as clay). Thus, three different rocks are
found, and each could be treated as a different formation;
however, the boundaries between these different rocks
would be gradational (smaller-size materials away from
shore) and would represent the local environment of for-
mation (i.e., progressively less energy to carry the sedi-
ments away from the shoreline). In cases such as this, the
conglomerate, sandstone, and shale would be called facies
(parts) of the same basic formation.
Structural attributes of rock units are indicated by var-
ious map symbols to show faults, folds, and the
“
attitude
”
of the rock units. Attitude refers to orientation, such as
horizontal, vertical, or tilted.
Geologic maps also indicate the ages of the rock units
and the timing of deformation. These relations are por-
trayed as a stratigraphic column, in which the oldest
materials are at the bottom of the sequence and the young-
est are at the top. In geology, time is usually indicated
from the bottom to the top, to re
ect the principle of
superposition. This principle states that in any sequence
