A concretion is a volume of sedimentary rock in which a mineral cement fills the spaces between the sediment grains. Concretions are often ovoid or spherical in shape, although irregular shapes also occur. The word 'concretion' is derived from the Latin con meaning 'together' and crescere meaning 'to grow'. Concretions form within layers of sedimentary strata that have already been deposited. They 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 stratum. They quite often form by the precipitation of a considerable amount of cementing material around a nucleus, often organic, such as a leaf, tooth, piece of shell or fossil. For this reason, fossil collectors commonly break open concretions in their search for fossil animal and plant specimens.
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 thousand pounds. The giant, red concretions occurring in Theodore Roosevelt National Park, in North Dakota, are almost 3 m (10 ft) in diameter. Spheroidal concretions, as large as 9 m (30 ft) in diameter, have been found eroding out of the Qasr El Sagha Formation within the Faiyum depression of Egypt. Concretions are usually similar in color to the rock in which they are found. Concretions occur in a wide variety of shapes, including spheres, disks, tubes, and grape-like or soap bubble-like aggregates.
They are commonly composed of a carbonate mineral such as calcite, a form of silica such as chert, flint, or jasper; or an iron oxide or hydroxide such as goethite and hematite. They can also be composed of other minerals that include dolomite, ankerite, siderite, pyrite, marcasite, barite and gypsum.
Although concretions often consist of a single dominant mineral, other minerals can be present depending on the environmental conditions which created them. For example, carbonate concretions, which form in response to the reduction of sulfates by bacteria, often contain minor, percentages of pyrite. Other concretions, which formed as a result of microbial sulfate reduction, consist of a mixture of calcite, barite, and pyrite.
Concretions are found in a variety of rocks, but 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 stratum 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 mud hills or perched on soft pedestals.
Small hematite concretions ("blueberries") have been observed on Mars.
Concretions are commonly misunderstood geologic structures. Often mistaken for fossil eggs, turtle shells, or bones, they are actually not fossils at all but a very common geologic phenomenon in all types of sedimentary rock; including sandstone which is made up of compacted sand grains, shale which is made up of compacted mud, siltstone which is made up of a fine grained silt, and limestone which is made up of calcium carbonate precipitated by many marine invertebrates.
These concretions are not from this local area, but were brought here for display. Since concretions are found in bedrock the opportunities to see them in Michigan is rare. These have been brought here from a local stone quarry. The mine safety rules restrict the access to mines and quarries unless you have proper safety training. Also any concretions found in a quarry would soon be sent to the crusher for processing. Most likely they were formed in limestone.
Logging requirements are as follows:
At the posted waypoint you will find one concretion. 1.) What is the diameter of this concretion?
2.) This specium had other concretions growing on it's surface. Tell me how many there were/are.
3.) No longer required but always appreciated, post with your log a photo of you and your GPS with the tennis courts in the background.