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A concretion is a volume of sedimentary rock in which a mineral cement fills the porosity (i.e. the spaces between the sediment grains). Concretions are often ovoid or spherical in shape, though irregular shapes also occur. They form within layers of sedimentary strata that have already been deposited. Concretions usually form early in the burial history of the sediment, before the rest of the sediment is hardened into rock. This concretionary cement often makes the concretion harder and more resistant to weathering than the host strata.
Though the processes by which concretions form are poorly characterized, it is believed they form during the diagenesis of a deposit, usually shortly after the enclosing sediment has been buried. They are believed to occur when a considerable amount of cementing material precipitates locally around a nucleus, often organic, such as a leaf, tooth, piece of shell or fossil.
Concretions vary in shape, hardness and size, ranging from objects that require a magnifying lens to be clearly visible to huge bodies three meters in diameter and weighing several hundred pounds. The giant, red concretions occurring in Theodore Roosevelt National Park, in North Dakota, are almost 10 feet in diameter. Concretions are usually similar in color to the rock in which they are found. They are commonly composed of a carbonate mineral such as calcite; an amorphous or microcrystalline form of silica such as chert, flint, or jasper; or sometimes an iron oxide or hydroxide such as goethite. They can also be composed of other sedimentary minerals that include dolomite, ankerite, siderite, pyrite, barite and gypsum, to name a few.
Concretions are found in a wide variety of rocks, and are particularly common in shales, siltstones, and sandstones. They often outwardly resemble fossils or rocks that look as if they do not belong to the strata in which they were found. Occasionally, concretions contain a fossil either as its nucleus or as a component that was incorporated during its growth, but concretions are not fossils themselves. They appear in nodular patches, concentrated along bedding planes, protruding from weathered cliffsides, randomly distributed over mudhills or perched on soft pedestals.
Septarian concretions (or septarian nodules) are concretions containing angular cavities apparently caused by differential shrinkage of their interiors. The cavities usually contain crystals precipitated from circulating solutions, usually of the mineral calcite.
Descriptions dating from the 18th century attest to the fact that concretions have long been regarded as fascinating geologic curiosities. Because of the variety of unusual shapes, sizes and compositions, concretions have been variously interpreted to be dinosaur eggs, animal and plant fossils (called pseudofossils), extra-terrestrial debris or human artifacts. For this reason, fossil collectors commonly break open concretions in their search for fossil animal and plant specimens.
The word "concretion" is derived from the Latin con meaning 'together' and cresco meaning 'to grow'.
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