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FAQ — Definitions of Geologic Terms

compiled by Shari Kelley

What is limestone?
Limestone is a bedded sedimentary rock made up primarily of the mineral calcite (calcium carbonate). Limestone starts out as limy mud, calcareous sand, or fossil shell fragments deposited on the ocean floor. Limy mud can form through chemical precipitation of calcite out of oceanic water in a warm, shallow sea. In addition, many organisms like clams and oysters extract calcite out of oceanic water to make their shells, and when these organisms die, their shells accumulate on the sea floor. Sometimes the shells can be broken down to sand-sized particles by wave action. When the limy mud, calcareous sand, and shell fragments are buried and groundwater passes through the sediment, the calcite in the sediment often recrystallizes, thus forming a solid rock known as limestone.
What is sandstone?
Sandstone is a bedded sedimentary rock that often contains sand-sized particles of quartz, but sandstone can contain a wide variety of other minerals, depending upon the source rock from which it is derived.  Sandstones form when pre-existing rocks break down through the erosive action of water, ice, and wind into sand sized particles.  When the transporting agent (e.g., river, glacier, windstorm) loses energy, the sand grains are deposited.  As burial occurs, mineral-enriched groundwater passing through the sediment precipitates minerals in the pore spaces between sand grains, causing cementation of the rock by calcite, clay, quartz, or iron oxides.
What are some examples of weathering, erosion, and deposition in New Mexico?
Weathering is the mechanical (e.g., freeze-thaw cycles) or chemical (e.g. acid rain) breaking down of pre-existing rock, while erosion is the removal of the products of weathering by water, ice, or wind. Deposition occurs when the transporting medium (water, ice, or wind) loses energy or melts, dropping the material it was carrying.
While none of these natural processes are unique to New Mexico, weathering and erosion have sculpted many spectacularly unique landforms in our fair state. The stark landscape of the Bisti Badlands in northeastern New Mexico is caused by differential weathering and erosion of relatively hard sandstone and and soft shale. The hoodoos, erosional cones, and pedestal rocks that characterize Kasha-Katuwe Tent Rocks National Monument form when water and, to a lesser extent, wind erosion preferentially attacks the moderately-indurated sand and ash grains around the base of large blocks in gravel-rich beds. Eventually, the gravel clasts rest on pedestals, thus protecting the underlying sand and ash from further erosion. As time passes, the capstones are gradually undermined and the rocks topple, leaving an unprotected cone.
Running-water-related deposition is common along the Rio Grande valley in central New Mexico. In fact, as much as 15,000 feet of sand, gravel and mud gradually accumulated in the Rio Grande valley beneath the city of Albuquerque over the last 26 million years. The large volume of sediment accumulated here because the Albuquerque basin subsided (went down) and the Sandia Mountains went up during extension along the Rio Grande rift starting about 26 million years ago. Ice deposition due to glaciation is not currently active in the state of New Mexico, although some of the higher mountains in northern New Mexico were glaciated during the Ice Age. A dramatic example of deposition after windstorms is White Sands in south-central New Mexico.