Geo-pic of the week: Olistoliths

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Pictured above is what geologists refer to as an olistolith.  The name comes from the Greek olistomai – to slide, and lithos – rock.  Olitstoliths are basically the geologic record of an ancient landslide.

The mass of rock at the base of the outcrop broke free from where it formed and slid downhill to this location.  After sliding (or tumbling) to its new location, sediment accumulated around the olistolith.  Eventually, it and the sediment became a new rock .  That’s what we’re looking at in this picture.

This olistolith is located on the north side of highway 412, west of Springdale, Arkansas.  It was deposited about 350 million years ago when that area was a gently sloping ocean shelf. 

Geopic of the week: Waterfalls

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Everyone knows that a waterfall is a place where a river or creek flows over a vertical drop-off, but did you know that there is a geologic reason why they form?  A waterfall, like Cedar Falls pictured above, forms where a hard, resistant rock such as sandstone overlies a soft, easily eroded rock like shale.  The difference in the rate each rock type weathers is what creates the waterfall.

When a stream passes over a single rock type, it erodes it evenly, carving a channel with a gradual slope.  However, when a stream’s course passes from a hard to a soft bedrock, it scours the soft rock at a faster rate.  As the supporting soft rock is eroded, the overlying harder rock progressively collapses, creating a vertical bluff over which the stream flows.  As this process continues an ever taller waterfall develops, and the location of the waterfall gradually migrates upstream.

Because we know how landforms such as waterfalls form, geologists can use tools, like aerial photographs and satellite images, to predict what kind of rock will be in an area before ever going there.

Geopic of the week: Rosselia trace fossils

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Rosselia – or Rosselia socialis – is a trace fossil that’s common to rocks deposited in a variety of shallow marine environments such as estuaries, tidal flats, lagoons, etc..  This picture was taken in a quarry in the Pennsylvanian Bloyd Formation, near Greers Ferry, Arkansas.   Rosselia is a funnel-shaped burrow with concentric cone-like layers, and a sandy plug near the center.  The picture shows a side view, or cross-section, of several burrows.

Like many trace fossils, Rosselia was made by a soft-bodied animal that was rarely if ever fossilized.  We only know it existed because of the burrows it left behind.  They may represent places an animal lived, fed, or perhaps both.

One theory suggests the burrows were occupied by worm-like animals that fed by filtering  nutrients from sediment, then excreting the sediment outward around their bodies in concentric muddy layers.  When new beds of sand were deposited, the animal would crawl to the top of the sand bed and make a new burrow; this behavior is clearly evident in burrows at the center of the photo.

 

Be sure to check out more pictures of Rosselia here!

Geopic of the week: Cone in cone structure

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Pictured above is a good example of cone in cone structure.  It’s a common feature in limestone, although it can form in other rock types as well.  This sample was collected from the DeQueen Limestone of the Gulf Coastal Plain of southern Arkansas. 

From the picture, you can see that cone in cone structure results in numerous nested cones, which resemble Bugles Corn Snacks (product placement).  The craters, visible above, are places where cones have worked themselves out of the rock.  In other places you can see the round bottom of a cone: those cones have yet to work free from the rock.

Thus far, no one is certain how cone in cone structure forms.  Several theories have been offered, but scientists have yet to arrive at a consensus.  However it forms, it’s fascinating to look at and wonder about!

Geopic of the week: Salt casts

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Salt casts are sedimentary structures that indicate a very saline environment of deposition.  This example was collected from the DeQueen Limestone in Howard County, Arkansas (peppermint for scale). About 146 million years ago, much of Arkansas south of the Ouachita Mountains was under near-shore marine conditions.

The cubes that stand in relief on the underside of this rock bed were once crystals of the mineral Halite – or common table salt.  These crystals grew in the mud of a tidal flat that was periodically flooded with very salty sea water.  The salt crystals were later dissolved, leaving cube-shaped molds that filled with mud, preserving the casts of the Halite crystals.

Ho Ho Ho, Merry Christmas! from the Arkansas Geological Survey

Geopic of the week: Starfish resting traces

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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.