Soil and bedrock
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Soil and bedrock


Geologists divide bedrock into three classes.

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 controls the type, texture, and shape of rocks.

These variables complicate identification. 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 primarily of quartz, feldspar, and some dark minerals (usually mica). Granite has a crystalline texture and is usually even-grained (grains somewhat equal in size).
  • Basalt is a hard, generally fine-grained rock. It is dark-colored (green, gray, or black) with a glossy texture. Basalt is heavier than most rocks.

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 identified as:  foliated-gneiss (irregular banding), schist (regular banding), and slate (layered), while others are massive or granular and are called non-foliated-marble.

Sedimentary rocks

Sedimentary rocks are clastic or non-clastic.

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 mostly 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.

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 variations

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 wind-blown areas.

Soil identification

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.

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.

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 soil is:

  • 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 inches) and boulders (greater than 12 inches) 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 considered cohesionless.

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

Inches Millimeters U.S Standard Sieve Size Particle size
12 and above 256 and above   Boulder
3 to 12 75 to 256   Cobble
3/4 to 3 19 to 75   Coarse gravel
3/16 to 3/4 4.75 to 19 3/16 in. = 4 Fine gravel
3/32 to 3/16 2.4 to 4.75 3/32 in. = 10 Coarse sand
  0.42 to 2.4 0.42 mm = 40 Medium sand
  0.74 to 0.42 0.074 mm = 200 Fine sand
  0.005 to 0.074   Silt
  0.005 and below   Clay

Soil and clay characteristics

Characteristics Silt Clay
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 moist state) Plastic thread has little strength. Crumbles easily as it dries. Dries quickly. Plastic thread has good strength. Dries slowly.
Dispersion (settlement in water) 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.
Dried coat Easily crumbled in hands. Will not crumble in hands. Dry lumps can be broken but not powdered.
Bite test Gritty feeling between teeth. No gritty feeling between teeth.

Logging method

The core drill operation obtains subsurface data. To obtain accurate data, 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 water.

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 samples.

Logging procedure: before drilling

  1. Confirm landowner's permission to enter property if drilling on private property.
  2. Coordinate traffic control and necessary brush removal with TxDOT area office.
  3. Stake desired core drill hole sites and obtain ground elevations.
  4. Locate any subsurface utilities: power lines, gas lines, telephone cables, sewer pipes, etc.
  5. Locate water sources for drilling purposes near the job site, and secure permission to use them.
  6. Complete all steps before the core drill crew and rig arrive.

Logging procedure: during drilling

  1. Lay out core samples in succession, as obtained, and mark depth at each 5-foot intervals.
  2. Break open samples to expose fresh surfaces for accurate identification and classification.
  3. Identify, describe, and log the subsurface materials, and record all core hole data.
  4. Compare all core samples with previous core samples.
  5. Prepare any undisturbed samples for the laboratory by wrapping them in plastic wrap and labeling them for future identification.

Logging procedure: after drilling

  1. Cover all uncovered drill holes.
  2. Pick up debris and clean up the area in general.
  3. Repair any damaged property (fences, lawns, etc.)
  4. 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.

Logging field equipment

The logger needs the following equipment as aids to description of the materials:

  • Pocket knife to cut the samples for testing hardness and exposing fresh surfaces.
  • Millimeter scale to determine the size of the particles.
  • Tape measure for measuring recovery and rock quality designation (RQD) of bedrock core samples from each run.
  • Dilute hydrochloric acid to aid in recognizing calcium carbonate materials such as limestone, chalk or dolomite.
  • A 10x magnifying glass to better identify materials by enabling closer inspection.
  • Soil pocket penetrometer for measuring initial unconfined compressive strength of cohesive soils in the field.