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Carbonate Rock
Carbonate rocks are typically the compact remains of ancient marine sediments. Water may erode carbonate rocks to create large underground caves that collect liquid water in pools. These underground rivers and lakes form over thousands of years as groundwater seeps into the rock and gradually carves out spaces. Most carbonate rock aquifers are made of limestone, although some aquifers exists carved from marble and dolomite. Carbonate rock aquifers exist across the eastern United States as well as large areas of some Midwest states, such as Missouri.
Sandstone Rock
Sandstone is a porous rock that is also prone to fissures and other breakages; these two properties make sandstone a common source of water. While the sandstone itself is condensed, and only some water exists in the pores of the rock, large amounts may exist in the cracks and fissures between pieces of sandstone. Sandstone aquifers cover massive areas of the United States, with some aquifers crossing multiple states. They exist in large sizes across the Mountain States, the Appalachian Mountains and areas around Wisconsin and Minnesota.
Igneous and Metamorphic
The U.S. Geological Survey groups igneous and metamorphic rock aquifers together, because they have similar properties. These rocks typically have few pores to absorb water, and water primarily seeps into cracks between these rocks. Metamorphic rocks, formed from hardened lava, may have large bubbles and gaps that allow large amounts of water to collect. While these rocks do not absorb water as easily as carbonate rocks or other aquifer sources, the extensive range of these types of rocks still can produce large aquifers. In particular, igneous and metamorphic aquifers exist in the Pacific Northwest and the southern areas of the Appalachian Mountains.
Sand and Gravel
The final two groups of aquifer classifications by the USGS are unconsolidated and semiconsolidated sand and gravel aquifers. These aquifers consist of small pieces of sand, gravel and other loose rocks compacted but with spaces between the grains. Clay, carbonate rock or other rocks intersperse semiconsolidated sand aquifers, while unconsolidated sand and gravel aquifers are unconfined. These types of aquifers exist across almost the entire United States, particularly along the coasts and throughout the former glacial zones of the upper Midwest and Great Lakes states.