Reference work entry
A concretion is a type of nodule in sedimentary rocks with a composition different from that of the surrounding rocks. Nodules have different sizes and shapes. They are usually spheroidal, ovoid or irregular, and they can range in size from a few centimetres to several metres. Nodules can be distributed intermittently along a layer or spread through different layers. Common nodules include calcareous nodules, ferrous nodules and siliceous nodules (Fig. 44).
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