Geopic of the week: Lepidodendron

the one

Pictured here is not an alligator or lizard skin fossil, for which these are commonly mistaken.  It’s an impression of the trunk of a now extinct tree-sized plant, known as a Lepidodendron.  It was collected from a stream bed in north Arkansas.  The diamond-shaped patterns are sockets where leaves once attached to the trunk.  The holes that are just visible within the “diamonds”, are pores through which the plant inhaled carbon dioxide.

Lepidodendron were common in Arkansas during the Carboniferous period (359-299 million years ago).  The Carboniferous (or coal-bearing) period is known for lush vegetation.  Many of the earth’s important coal deposits were formed from the remains of the rich forests that dominated the land during that time.

For more views of Lepidodendrons click here

Geopic of the week: Asterosoma

Astrosoma radiciforme closeup

Above is a commonly misunderstood geologic feature called an AsterosomaAlmost everyone, upon first seeing an Asterosoma, thinks it’s either a fossilized plant, flower, or some kind of fossilized animal – usually an octopus.

Asterosomas are actually trace fossils left behind by ancient marine animals (most likely worms or shrimp) that burrowed through mud in a delta or tidal-flat.  This one was found in the Carboniferous section of north Arkansas and is roughly 300 million years old.  These trace fossils are called Asterosoma because of their star-like shape.

In cross-sectional view, multiple Asterosomas sometimes overlie one another connected by a central vertical tube – like a garland of Asterosomas.  This suggests that, as new sediment was periodically washed into the environment, the animal may have burrowed its way back to the top of the mud and wallowed out another home for itself.  The animal itself was too soft-bodied to be preserved in the rock record.

To see more views of Asterosoma click here

Geopic of the week: Arkansas Novaculite

DSCN1076

This is a picture of several tools, both modern and ancient, manufactured from the famous Arkansas Novaculite.  Novaculite is an extremely hard sedimentary rock made almost purely of silica (99.9%).  It is similar to chert except that it is harder and even more silica-rich; it is possible that novaculite is formed from the metamorphosis of chert, though its genesis is uncertain.

Novaculite (from Latin, meaning razor stone) has been prized since prehistoric times as a material for making stone tools like knives and arrowheads.  Since at least the early 1800s novaculite has been utilized as an abrasive material for sharpening knives and other tools.

The manufacture of whetstones, or sharpening stones, from novaculite continues to be a thriving business in the Hot Springs, Arkansas area today.

Geopic of the week: Angular Unconformity

angular unconformity

The above picture at first glance doesn’t look like much but from a geological perspective these rocks convey a lot of information about the history of the earth.  This is what geologists call an angular unconformity.  An unconformity is simply a gap in the rock record; it represents a period of time during which either erosion was taking place or there was no sediment being deposited. 

We call this kind of unconformity angular because the lower rock beds are tilted at a different angle than the upper beds.  We know from the differences between the fossils in each of these rocks that there was about an eighty million year gap between deposition of the lower and upper rock sequences.  During that 80 million years the lower formation was exposed above sea level, eroded, and tilted by tectonic forces in the earths crust.  Eventually deposition resumed and the upper unit was deposited on the eroded and deformed lower unit.

It’s amazing what you can learn from a few rocks!  If you’d like to see these rocks in person, float the Buffalo National River from Tyler Bend to Gilbert.

Geopic of the week: “Quicksand Structures”

 

Sandstone pipe1

Pictured above is approximately 450 million year old (Ordovician) St. Peter Sandstone photographed near the White River, north of Mountain View, Arkansas.  At the center of the picture is a feature not uncommon in this formation: a “quicksand structure”.  Though not well understood, this kind of structure is believed to be a preserved conduit through which ground water once flowed to the surface in a boiling spring. 

A boiling spring is where ground water rises up through unconsolidated sediment such as sand, keeping the sediment in continuous motion so that it appears to be boiling.  Because ground water contains dissolved minerals, the tunnel through which the water is rising can eventually become hardened by precipitated minerals.  Today, as the sandstone that formed from that sediment is being eroded away, the conduit stands in relief  because it’s a little harder than the surrounding sandstone due to that mineralization. 

Clues like these “quicksand structures” offer us a glimpse into what the environment was like when the rock was being deposited.