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An the rock platforms you will see Concretions. These typically form as deposition is forming sedimentary rock. A solid core called a "nucleus" is deposited in the sediment. This is often composed of organic material, such as a leaf, tooth, piece of shell or fossil. A mineral solution (that is, a mineral dissolved in water)then precipitates around the nucleus and cements a covering.
The hard case is exposed when the rock is eroded.
The mineral solution is usually part of the host rock. In sandstones it is commonly calcium carbonate. Some concretions have iron oxides as the cement for their cases.
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