Rock Descriptions

From Noger (1988) and Carey and others (1994)

Limestone: Limestones are characterized by solution-enlarged joints and bedding planes that channel water into conduits. The majority of groundwater flows through the conduits and discharges at springs along major, permanent streams. Wells drilled in these areas may produce only a little water, or hundreds of gallons per minute, depending on the chance intersection of an enlarged joint or other opening. Little water moves through the unaltered bedrock. Groundwater flowing through fractures and solution openings is easily contaminated. These rocks are generally very hard, requiring blasting or heavy equipment for excavation, and the depth of soil coverage is highly variable. In some areas of Kentucky underlain by limestones, soils more than 30 feet thick have been reported.

Sandstone: These rocks are generally very hard, requiring blasting or heavy equipment for exavation. Sandstones tend to form thin soils and steep slopes. Groundwater flows through openings between sand grains and along fractures (widely spaced cracks).

Unconsolidated deposits: These deposits consist of noncemented clay, sand, and gravel and are found primarily in stream valleys. West of Lake Cumberland, these deposits occur both in stream valleys and upland areas. They are easily eroded during rainstorms. West of Lake Barkley, these deposits include loess, a fine-grained material deposited by wind. These deposits yield large volumes of water where aquifers are extensive. Areas of terrace deposits and alluvium in upper stream reaches may be too small to sustain high rates of production.

Fractured shales: Fresh exposures of fractured shale are hard and require heavy machinery for excavation. Although jointing and bedding planes in these brittle shales allow groundwater movement, there is little storage in the unfractured material. Wells in these rocks typically produce little water.

Clay shales: These shales are easily excavated and restrict groundwater movement. The high clay content can produce slippage and workability problems. Joints and bedding planes tend to heal or become clogged, and although clay minerals have large intergranular storage of water, there is little or no permeability to allow its movement. Wells in these rocks are generally dry.

Interbedded shales and limestones: This is bedrock composed of 80 percent or more shale and 20 percent or less limestone. Limestone layers are usually 2 inches or less thick. These rocks are easily excavated and generally restrict groundwater movement. Oversteepened banks and artificial cuts are subject to slippage. These formations have some limited potential as aquifers, but the high clay content generally blocks small conduits in the limestone. Wells in these rocks are generally dry.

Interbedded clay shales and sandstones: Where clay shales are dominant, successful water wells are difficult to drill. In areas where the unit is sandy, wells more commonly yield sufficient water for a domestic supply.

Interbedded limestones and shales: These rocks contain more than 20 percent limestone. Where limestone content exceeds 60 percent, wells may yield adequate water for a domestic supply.

Interbedded limestones, sandstones, and shales: These rocks consist of a vertical sequence of alternating limestones, sandstones, and shales.

Coals, sandstones, and shales: This unit consists of a vertical sequence of coals, sandstones, and shales that is generally horizontally discontinuous. Wells that penetrate a section composed of more than 50 percent sandstone have better than average yields, and almost all wells will produce enough water for domestic supplies. Many wells will produce sufficient supplies for small industries. Wells completed in coals, or obtaining flow from coals, are highly productive, but water quality may be marginal or poor. Wells completed in shales are commonly adequate for domestic supplies, depending upon the occurrence of weathered fractures in the shale.

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