DIRK WIERSMA / SCIENCE PHOTO LIBRARY DIRK WIERSMA / SCIENCE PHOTO LIBRARY
Sedimentary rock concretion. Natural rock formation created by the localised cementation of particles of white sand during the formation of sedimentary rock. Concretions, also called nodules, are found between the bedding planes of some sedimentary rocks. They are harder than the rock they are found in. The particles of sand were bonded together by cementing materials, for example calcite or silica, that were precipitated out of circulating fluids. This example is 35 centimetres high and was found in the Harleville quarry near Paris, France.
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