concretions - hand/display specimen of a sandstone concretion from the Palm Spring Formation
Concretions are a typical structural feature of some sedimentary rocks, particularly sandstones. Early in the burial history of a rock, before it becomes solidified, a cement will precipitate around a nucleus, often a fragment of calcium carbonate, filling the pore spaces as the sediment becomes consolidated into rock. The sediment grains then become cemented into a rough sphere around that fragment. Occasionally neighboring concretions grow against each other and are cemented together.
These specimens subtly show differential weathering, where resistant layers stand out. These layers likely represent drier periods, where calcium carbonate cement accumulated more slowly.
Typically, concretions in this formation are round, though unusual shapes form a small percentage of those that have weathered out. Occasionally several concretions grow together as in several of these groups. They can range from the size of a pea to that of a beach ball. These have weathered out of the sandstone of the Plio-Pleistocene Palm Spring Formation.
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