CONCRETIONS
Concretions are post-depositional structures found in some sedimentary
successions, particularly in shales. They vary in mineralogy, size, and
shape, but often have slightly flattened spheroidal shapes. They
typically form by post-depositional mineralization around some nucleus (a rock
fragment or fossil fragment).
Concretions (left: 7.6 cm long; right: 6.2 cm across) - marlstone
(lime-clay) concretions from near-latest Pleistocene lake deposits (~11,000
years) in the Abitibi area of western Quebec, eastern Canada. The
specimen at left is remarkable for its stylized resemblance to agnostoid
trilobites.
Concretions - marlstone (lime-clay) concretions from Windsor, Connecticut, USA
(public display, Field Museum of Natural History, Chicago, Illinois, USA).
Concretions - marlstone (lime-clay) concretions from Sault Ste. Marie, eastern
Upper Peninsula of Michigan, USA (public display, Field Museum of Natural
History, Chicago, Illinois, USA).
Concretions - marlstone (lime-clay) concretions from Grand Junction, Colorado, USA
(public display, Field Museum of Natural History, Chicago, Illinois, USA).