Limestone is a sedimentary rock composed largely of
the minerals calcite and/or aragonite, which are different crystal forms
of calcium carbonate (CaCO3).
Like most other sedimentary rocks, limestones are composed of grains;
however, most grains in limestone are skeletal fragments of marine organisms such as coral or foraminifera. Other carbonate grains comprising
limestones are ooids, peloids, intraclasts, and extraclasts. Some limestones do not consist of grains
at all, and are formed completely by the chemical precipitation of calcite or aragonite, i.e.travertine.
The solubility of limestone in water and weak acid
solutions leads to karst landscapes. Regions overlying
limestone bedrock tend to have fewer visible groundwater sources (ponds and streams), as
surface water easily drains downward through joints in the limestone. While draining,
water and organic acid from the soil slowly (over thousands or millions of years) enlarges these cracks,
dissolving the calcium carbonate and carrying it away in solution. Most cave systems are through limestone
Limestone often contains variable amounts
ofsilicain the form ofchert(chalcedony,flint,jasper, etc.) or siliceous skeletal fragment (sponge spicules,
diatoms, radiolarians), and varying amounts ofclay,siltandsand(terrestrialdetritus) carried in by rivers. The primary source of the calcite
in limestone is most commonly marine organisms. These organisms secrete shells made of aragonite or
calcite, and leave these shells behind after the organisms die. Some of these organisms can construct mounds
of rock known as reefs, building upon past generations. Below about 3,000 meters, water pressure and
temperature conditions cause the dissolution of calcite to increase nonlinearly, so limestone typically does
not form in deeper waters.