Geopic of the week: Unconformity

 

brownstown and jackfork contact

Pictured above is the contact between two very different rocks: the Brownstown gravel above, and the Jackfork Sandstone below.  This is what geologists call an angular unconformity.  The contact represents a vast period of time.

The lower sandstone was deposited in the Pennsylvanian period, around 300 million years ago, when Arkansas was at the bottom of an abyss.  It was deposited in horizontal layers, but later, when the South American continent collided with the North American continent, it was deformed into mountains and raised above sea level.  For about the next 200 million years it eroded and subsided, until eventually, a shallow sea covered it again.  Rivers washed gravel into that sea, which became the Brownstown gravel.

This picture was taken near De Gray Lake State Park in south central Arkansas.

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