A cannonball concretion is a cannonball-shaped mass of sedimentary rock material that was cemented together to form a structure harder than the surrounding sediments. The surrounding sediments slowly erode away, exposing the concretion.
Concretions form when cementing minerals, such as calcium carbonate, iron oxide, and silica, precipitate from ground water into the pores of the sediment. The minerals often precipitate around an organic central nucleus, such as a piece of shell or a leaf.
Concretion size, shape, and color vary depending on the cementing mineral, the sediment into which the mineral precipitates, the direction of ground-water flow, and other erosional factors. Concretions have been described as logs, pumpkins, and kettles. The Cannonball River in North Dakota was named, prior to Lewis and Clark’s expedition, for the cannonball-shaped concretions found in the vicinity of the river.
Today, concretions are objects of geologic interest in places like Theodore Roosevelt National Park and Fort Lincoln State Park. In communities where concretions are common, they often are used as lawn ornaments.
American Geological Institute, 1974, Dictionary of Geological Terms: Anchor Books, Anchor Press/Doubleday, Garden City, New York, 545 p.
Bluemle, John P., 2000, The Face of North Dakota, Third Edition: North Dakota Geological Survey, Educational Series 26, 206 p.
Katz, Bob, 1998, Concretions, Digital West Media, Inc., DesertUSA.com: accessed November 18, 2002, at URL http://www.desertusa.com/mag98/oct/papr/geo_conc.html.
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