Composed of a carbonate mineral such as calcite
A concretion is a compact mass of mineral matter, usually spherical or disk-shaped, embedded in a host rock of a different composition. This hard, round mass of sedimentary rock cement is carried into place by ground water. Concretions, the most varied-shaped rocks of the sedimentary world, 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 size, shape, hardness, and color, from objects that require a magnifying lens to be clearly visible to huge bodies 10 feet in diameter and weighing several hundred pounds.
They are commonly composed of a carbonate mineral such as calcite, but sometimes an iron oxide or hydroxide such as goethite or sometimes an amorphous or microcrystalline form of silica about a nucleus. But they can also be composed of other sedimentary minerals that include dolomite, ankerite, siderite, pyrite, barite and gypsum, to name a few.
The word "concretion" is derived from the Latin "con"-- meaning "together" -- and "cresco" -- meaning "to grow." These "grown together" rocks have a variety of origins that require geologists to integrate information from a variety of disciplines, including biology, chemistry, soil science, meteorology and geology itself. The origin of calcite concretions is not necessarily that of barite or gypsum concretions.
They appear in nodular patches, concentrated along bedding planes, protruding from weathered cliffsides, randomly distributed over mudhills or perched on soft pedestals.
Being more firmly cemented than the enclosing sandstone, concretions weather out in myriad shapes, that have been fancifully described as cannonballs, pumpkins, dinosaur limbs or bric-a-brac.
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.
Pumpkin Patch Concretions
This natural cementing of sand particles to form strange globular masses occurs across an area of the Colorado Desert in southeastern California. These concretions vary from realistic rounded, flattened, or cylindrical forms to imaginative shapes with imposing names like botryoidal (bunch of grapes), fusiform (like a spindle), and ameboid (of the microscopic creature). They may form a mosaic on the bare ground, more firmly cemented than the surrounding parent rock, all of which is Diablo sandstone of ancestral Colorado River delta origin.
Adjoining Anza-Borrego Desert State Park, local ensembles of concretions are widespread in thick, hardened channel sandstones of the Diablo Formation. Good examples can be seen in the upper reaches of Arroyo Tapiado (Spanish word for "Concretion Wash"), in the depths of Arroyo Seco del Diablo, Cannonball Wash and the famous Pumpkin Patch.
The Pumpkin Patch, an area located within the Ocotillo Wells Off-Highway Vehicle Area (video Link), is so-called because these strange concretions resemble pumpkins in shape, size and distribution. This unique landscape is the result of both precipitation and diagensis to form the concretions, as well as wind and water continuously eroding the surface soil to uncover the sandstone globes. On the nearby ridges, new pumpkin-size concretions continue to grow and be exposed.
Theodore Roosevelt NP Concretions
The giant, red concretions occurring in Theodore Roosevelt National Park, in North Dakota, are almost 10 feet in diameter! The cementing agent in these concretions is a combination of carbonate and silica. They are red in color because of iron-bearing minerals, including hematite and goethite, that are also present in the surrounding sediment, a fine-grained sandstone that eroded away at this outcrop to expose the concretions.
Kettle Point Concretions
Another major concretion outcrop occurs along 500 feet of Lake Huron at Kettle Point in southern Ontario. These concretions, composed of calcite, are referred to as "kettles" because of their resemblance to the bottom of a large cooking pot. These concretions are typically spheres to oblate spheroids, ranging in size from 1 to 5 feet in diameter. The outer surfaces of these concretions may also be covered by shallow indentations similar to the surface of a golf ball.
-- Bob Katz
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