Pictured above is clay and sand of the Nacatoch Formation of southern Arkansas. The clay beds, which stand slightly in relief, have been churned, and the sand has numerous cylindrical structures of various sizes. These are hallmarks of bioturbation, or reworking of sediment by living things. The cylindrical structures are the preserved casts of roots. The disrupted clay beds are evidence of the burrowing of mud-loving critters.
Bioturbation structures are commonly preserved in rock and offer glimpses into the environment where sediment was deposited. Specifically, it tells us that this sediment was near the surface in a relatively calm near-shore marine environment long enough after deposition for living organisms to move in and set up shop. Clues like these are one of many tools geologists use to piece together the early history of the earth from the rock record.
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