Geologic studies of the State of Texas have indicated the presence of
rocks formed during every era and period of geologic time. These range
from the Precambrian granites of the Llano uplift to the Quaternary
gravels of the High Plains.
[Illustration: Plate 10
GENERALIZED GEOLOGIC MAP OF TEXAS
Modified from Geologic Map of Texas, 1933]
One of the best ways to become acquainted with the geology of Texas is
to study the _geologic map_ of the State (Pl. 10). A geologic map shows
the distribution and age of surface rocks and may also indicate what
kind of geologic structures are present. The types of rocks that crop
out at the surface may be shown by means of symbols, colors, or
patterns, and these are explained by a legend which accompanies the map.
On Plate 10, colors are used to show the distribution and geologic age
of the surface rocks of Texas. Reference to this map will give the
collector some idea of the age of the fossils that might be found in a
given area. Some special geologic maps may have the location of geologic
structures and formation contacts indicated by means of symbols, such as
dashed lines, arrows, and similar special markings. However, the map
included in this publication does not show any of these special
markings.