Tag Archives: mississippian period

Geo-pic of the week: Basal Conglomerate

MissPennBoundary

Pictured above is a little piece of geologic history known as a basal conglomerate.  that’s a rock formed after a period of erosion that marks the boundary between two geologic time periods: in this case, the Mississippian (359-318 million years ago) and the Pennsylvanian (318-299 million years ago).

318 million years ago sea level subsided, bedrock was exposed, and the Mississippian Period came to an end.  When exposed to erosion at the earth’s surface, pieces break off from bedrock.  Flowing water in rivers, streams and oceans wears the edges of those rock fragments till they’re rounded.  Once ocean level rises and deposition resumes, the rounded gravel gets mixed with newly accumulating sediment and forms a rock which is made partly of fragments of the older bedrock.  Geologists call this type of rock a basal (at the base) conglomerate (containing round gravel) because it is the first bedrock signaling the beginning of a new period of geologic time.

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Geopic of the week: Starfish resting traces

Asteriacites edited

Also known as Asteriacites (just call them starfish traces), these impressions were left by marine invertebrate animals on the sandy floor of a sea that covered northern Arkansas during the late Mississippian period: about 320 million years ago.  These trace fossils were collected from the Batesville Formation, a mostly sandstone unit that outcrops in a thin east-west oriented belt across the Ozark Plateaus.  The sample above was found near the town of Leslie, Arkansas in Searcy County.  I can’t reveal the exact location, as that’s the only place I know of starfish resting traces in Arkansas.  These largely predatory animals, which move using a number of small arms on their points and have a mouth at the center of their bodies, are rarely preserved in the rock record.