Cannonball Concretions in Eocene Sandstone
There are several locations where cannonball concretions have been exposed by erosion in the sandstone. Concretions are small to large rounded nodules that form in sedimentary rocks, typically in shale or sandstone. Concretions form slowly as minerals precipitate from ground water around a "nucleation site." A nucleation site can be a shell or bone, a piece of organic matter, or even a mineral grain. The cannonball concretion pictured below is in the Rock City area on the southside of Mount Diablo.
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concretion has been exposed by deep weathering in the soft sandstone.
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(close-up)
Cement in the sandstone often leaches out to produce a "honeycomb" weathering of
odd shapes |
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Minerals such as calcite, dolomite, or limonite precipitate and grow pervasively in the microscopic voids in between sedimentary grains. Over hundreds or even millions of years concretions can continue to grow. Sometimes when fossils are found inside, they can be well preserved. |
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Photos by Sue Schwartz, Park Staff.