From Ohio History Central
Concretions are solid, rounded masses of mineral matter that occur in sedimentary rock. Concretions have a different composition from the rock in which they are found. An example is a limestone concretion found in a bed of shale.
A concretion forms around a nucleus that commonly is a fragment of fossil shell, bone or plant material.
Concretions form after the material around them has been deposited. The concretion hardens into solid rock while the surrounding bed still is somewhat soft. This often is evident when a concretion is seen to have deformed surrounding layers of shale while the clays of the shale still were soft.
- Coogan, Allan H. Ohio’s Surface Rocks and Sediments; Chapter 3 in Fossils of Ohio, edited by Rodney M. Feldmann; Ohio Geological Survey, Bulletin 70, 1996.
- Pough, Frederick H. A Field Guide to Rocks and Minerals; Houghton Mifflin, Boston, MA; 1976.
- Sorrell, Charles A. Rocks and Minerals; Golden Press, New York, NY; 1973.