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
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
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
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.
Soil varies with parent material
(bedrock), climate, plant and animal life, slope of the land, and time. These
factors transform an original geologic deposit into a soil profile. The depth of
soil ranges from a few inches to hundreds of feet based on these factors. Some
sections of the state have no soil at all.
Residual and Sedimentary Soil
According to its geologic origin, soil is either residual or sedimentary.
Residual soil is formed in place. That is, it is a result of the weathering,
disintegration, and decomposition of the parent material. Sedimentary soil is
formed from materials that have been moved from where they originated by either
wind or water. These are commonly found in river flood plains and in arid
Soil is identified in the field by visual and mechanical tests. The criteria
for these are grain size, color, density or consistency, and moisture content.
For grain size, soil is either cohesive-clay, or cohesionless-silt, sand, or
gravel. Most soil consists of a mixture of these grains and organic material.
1. Cohesive Soil
Cohesive soil (clay) is composed of extremely small mineral grains shaped
like plates. Water is attracted between the plates by electrostatic forces to
varying degrees based on the chemical composition of the clay. Clay exhibits a
wide range of properties based on water content and chemical composition. When
dry, clay is hard and rigid due to the close attraction between the grains. When
clay is very wet, it exhibits an almost soupy consistency.
Clay occurs as both residual and sedimentary soil. Clay of a sedimentary
origin is initially deposited in a soup-like state. In upland areas, water
evaporation rapidly removes fresh clay deposits to produce fairly firm soil. In
coastal areas, this usually does not occur due to high ground-water levels. In
such an environment, the water is slowly squeezed from the clay by the weight of
subsequently deposited overlying soil. The result is typically very soft surface
clay that gradually increases in strength with depth.
2. Cohesionless Soil.
Cohesionless soil is composed of larger, more rounded particles than clay and
is subdivided based on grain size. The most commonly encountered cohesionless
Silt (passes a No. 200 sieve)
Sand (passes a No. 4 sieve and is retained on a No. 200 sieve)
Gravel (passes a 3-in. sieve and is retained on a No. 4 sieve)
Cobbles (3 to 12 in.) and boulders (>12 in.) are less commonly encountered.
The larger sizes of the particles cause them to interact by mechanical means.
Silt is fine enough that it exhibits some clay-like properties, but it is still
Pure cohesionless soil is free flowing when dry or completely saturated.
Moist silt and sand often exhibit an apparent cohesion due to negative pore
water pressures. This apparent cohesion is quite low but can still allow an
excavation face to stand unsupported for some time before collapsing.
Cohesionless soil is usually mainly composed of siliceous materials with
minor constituents of micas, feldspars, and carbonates. The most common
siliceous materials are quartz and chert. The table below offers classifications
of unified soil sizes.
Unified Soil Size Classification:
12 and above
256 and above
3 to 12
75 to 256
3/4 to 3
19 to 75
3/16 to 3/4
4.75 to 19
3/16 in. = 4
3/32 to 3/16
2.4 to 4.75
3/32 in. = 10
0.42 to 2.4
0.42 mm = 40
0.74 to 0.42
0.074 mm = 200
0.005 to 0.074
0.005 and below
Soil and Clay Characteristics:
Dilatancy (reaction to
shaking), movement of water in voids
Rapid reaction. Water
appears on surface when shaken. Squeezing soil causes water to disappear.
Sluggish and no reaction.
No water appears on surface when shaken.
Dry strength (cohesiveness
in dry state)
Low to medium reaction.
Powder easily rubs off surface of sample. Slakes readily in water.
High to very high
reaction. Powder does not rub off surface. Variable slake rate.
Toughness (plasticity in
Plastic thread has little
strength. Crumbles easily as it dries. Dries quickly.
Plastic thread has good
strength. Dries slowly.
Dispersion (settlement in
Settles out of suspension
in 15 to 60 minutes.
Settles in several hours
or days unless flocculation occurs.
Visual inspection and feel
Some grains barely
visible. Feels slightly gritty when rubbed between fingers. Dries quickly
and dusts off easily.
No individual grains
observed. Smooth greasy feel when rubbed between fingers.
Easily crumbled in hands.
Will not crumble in hands.
Dry lumps can be broken but not powdered.
The core drill operation obtains foundation data. In order to obtain data of
maximum accuracy, the logger must work closely with the driller, consulting on
changes in materials and coring operations while drilling. The logger must
recognize the reasons for adding extra water, drilling mud, or casing and should
note obstacles to drilling, such as caving, boulders, caverns, and any ground
In some cases a core sample cannot be recovered but the logger can watch the
color of the circulation water to see if any change takes place and analyze the
cuttings to see if the material correlates with the previous and subsequent core
Logging Procedure: Before Drilling:
permission to enter property if drilling on private property.
Stake desired core
drill hole sites and obtain ground elevations.
Locate any subsurface
power lines, gas lines, telephone cables, sewer pipes, etc.
Locate water sources
for drilling purposes near the job site, and secure permission to use
Complete all steps
before the core drill crew and rig arrive.
Logging Procedure: During Drilling:
Lay out core samples
in succession, as obtained, and mark depth by stakes at each 5-ft.
Break open samples to
expose fresh surfaces for accurate identification and classification.
and log the foundation materials, and record all test data.
Compare all core
samples with previous core samples.
undisturbed samples for the laboratory by wrapping them in plastic wrap
and labeling them for future identification.
Logging Procedure: After Drilling:
Cover all uncovered
Pick up debris and
clean up the area in general.
Repair any damaged
property (fences, lawns, etc.)
Deliver any samples
retained for testing.
Occasionally, core holes may need to be grouted or filled with
bentonite pellets if the possibility exists for contaminates to enter from the
surface or from subsurface aquifers. This is especially common in urban areas
with petroleum-contaminated soil.