Soil and Bedrock Classification

Classification | Bedrock | Soil

Classification
Bedrock is classified as follows:

Soil is classified as follows:

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 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.

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.

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 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
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 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 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. The following table shows Unified Soil Size Classification:

The following table describes soil and clay characteristics:

The following figure shows grain sizes.