Geologists divide bedrock into three classes.1. Igneous Rocks
Igneous rocks are found in approximately 20 counties of the Llano Uplift,
South Central Texas, and the Trans-Pecos areas. These rocks are derived from
cooled and solidified molten rock material, called magma, that was pushed up
from the interior of the earth. Magma that cools beneath the surface forms
intrusive rocks and magma that reaches the surface forms extrusive rocks. The
rate of cooling, mineral composition, and mode of placement control the type,
texture, and shape of rocks.
All of these variables complicate identification so that a background in
mineralogy and petrology is necessary to identify each properly. The igneous
rocks that outcrop in Texas are generally described as intrusive (such as
granite) or extrusive (such as basalt):
- Granite is a hard, generally coarse-grained rock that is light-colored
(pink, red, or gray) and heavier than most rocks. It is composed chiefly of
quartz, feldspar, and some dark minerals (usually mica). Granite has a
crystalline texture and is usually even-grained (grains equal in size).
- Basalt is a very hard, generally fine-grained rock. It is dark-colored
(green, gray, or black) with a glossy texture. Basalt is heavier than most
rocks.
2. Metamorphic Rocks
Metamorphic rocks are formed by the alteration of preexisting rocks (igneous,
sedimentary, and other metamorphic rocks) by heat, pressure, or both. These
alterations develop new textures, structures, and minerals. Some metamorphic
rocks are characterized by a banded or layered appearance and called
foliated-gneiss (irregular banding), schist (regular banding), and slate
(layered)-and others are massive or granular and called non-foliated-marble.
3. Sedimentary rocks
Sedimentary rocks are clastic or non-clastic.
A. Clastic Rocks
Clastic rocks are formed from the accumulation of pre-existing rock fragments or
plant material in the case of lignite. Clastic sedimentary rocks are formed by
mechanical processes such as erosion from a land mass. This erosion breaks the
rock into fragments, which in turn are transported by either wind or water and
redeposited. Soluble minerals then cement the individual grains together.
Clastic sedimentary rocks are classified according to size. The unified soil
size classification chart shows particle sizes in millimeters and inches in
relation to the standard U.S. sieve sizes for clastic materials. Clastic
sedimentary rocks are formed by the cementation of individual grains of
respective particle sizes, and they include the following:
- Shale is composed of clay particles cemented together. Most shales in
Texas are of a marine origin. Depending on the chemical composition, some
shales may degrade quickly into clay when exposed to air and water.
- Siltstone and sandstone are composed of silt and sand-size particles
respectively. Sandstone is much more common than siltstone. Common cementing
agents for sandstone are carbonate and iron oxides. Occasionally silica
cement is encountered. The hardness of these rocks depends on the cementing
agent with iron cemented the softest and silica cemented the hardest.
- Conglomerate is composed of gravel-sized and larger particles. Most
conglomerates are found in West and Central Texas. The most common cementing
agent is carbonate. Silica is also encountered occasionally. Chert gravel in
conglomerates makes this among the hardest materials encountered in the
state.
- Limestone is an interesting clastic rock, composed of particles derived
either by precipitation of calcium carbonate from solution (oolites) or from
the carbonate shells of microscopic marine organisms. Limestone is
considered clastic because the separate grains are usually transported by
water before becoming cemented. It usually occurs as a white to light gray
or bluish-gray rock varying in hardness from soft to very hard. It
effervesces upon contact with dilute hydrochloric acid. Chalk is a soft
limestone. Dolomite is a modified form of limestone in which a portion of
the calcium has been replaced by magnesium. Dolomite effervesces only
slightly with dilute hydrochloric acid.
- Glauconite is a greenish mineral formed in marine environments. It is a
hydrous silicate of iron and potassium and commonly occurs as a weakly
cemented granular material.
- Lignite is composed of decayed or partly decayed plant material and is a
compact brownish-black initial form in the coal process. Lignite is found in
the gulf coastal region and east Texas. It is extremely light, especially
when dry.
B. Non-Clastic Rocks
Non-clastic rocks are formed by the chemical precipitation of minerals from
a solution. These chemical precipitants settle to the bottom of a body of water.
When first deposited, these sediments are loose and incoherent. In time, they
are slowly hardened by compaction, cementation, and re-crystallization.
Non-clastic sedimentary rocks are classified according to chemical composition,
and they include the following:
- Chert is a fine-grained crystalline silicate that varies in color and is
hard. It breaks smoothly and is a common constituent of gravels and
conglomerates. Flint is a gray to black variety of chert abundant in all
parts of Texas.
- Iron deposits vary in color according to their oxidation state (from
black, red, reddish brown, to yellow). They are soft and, in some cases, the
cementing agent for bedrock, especially sandstone. Iron oxide occurs as
hematite, siderite, and limonite in East Texas. In many areas of Texas,
finely disseminated iron oxide is responsible for red soil and bedrocks.
- Evaporites are a group of water-soluble salts that have been precipitated
upon the evaporation of water. They are similar in physical characteristics
in that they are white or light colored, generally soft, and do not react
with hydrochloric acid (except calcite). Halite and potash salts can be
detected by their saline taste and are most commonly found in west and
northwest Texas. Gypsum occurs extensively in west Texas.
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