Tag Archives: #geoblog

Geopic of the week: Arkansas Wavellite

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The above picture shows two examples of the mineral wavellite: an aluminum phosphate mineral prized by rock and mineral collectors and fairly common in the Ouachita Mountains.  The green sample on the right is the typical color of wavellite, whereas the blue sample is a rare form.  The green color is due to the presence of vanadium.  These samples were both collected from an abandoned quarry a couple of miles northwest of Mt. Ida, Arkansas.

It’s easy to see why mineral collectors would be interested in wavellite as it comes in a variety of attractive colors – rarely white, yellow and black in addition to the blue and green examples shown here. It also has a visually interesting growth habit; it grows in botryoidal (grape-like) spheres that internally consist of a radiating array of slender crystals that resembles an eye.  Examples of this eye-like structure can be seen by clicking on the “more views” link below.

Wavellite is also quite sparkly and Christmassy.  I even hear it’s the official mineral of the north pole.  Ho ho ho!

To see more views of Arkansas wavellite click here

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Geopic of the week: La Petite Roche (Little Rock)

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Pictured above is about all that remains exposed of the rock outcrop that is the namesake of the capitol of Arkansas, La Petite Roche or The Little Rock.  The outcrop was given its name by early french explorers to the area who first arrived in 1722.  Though unimpressive in stature, it is notable because it is the first exposure of solid rock that one sees when navigating up the Mississippi and Arkansas rivers, starting at the Gulf of Mexico.

The geologic explanation for this is simple;  The land between New Orleans and Little Rock Arkansas along that route is in the Mississippi River Valley, where the bedrock is buried beneath river-lain and wind-blown sediment.  The city of Little Rock straddles the boundary between the lowlands of the Mississippi River Valley and uplands of the Ouachita Mountains, which stretch west well into Oklahoma.  Therefore this little sandstone and siltstone outcrop marks that physiographic boundary. 

If you boated all the way from New Orleans to Little Rock without seeing a single rock you would probably be impressed by it too.

Geopic of the week: Prim boulders

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Prim boulder is a name given to curious, round, often nearly spherical sandstone boulders that are common in the area around the town of Prim in Cleburne county, Arkansas.  Though the town of Prim boasts a noteworthy abundance of these unusual specimens, they actually have been found within a 100 mile area from Newton to White county.  The one pictured here is still attached to the outcrop of sandstone:  Many are found laying around at the surface, the rock from which they came having long ago weathered away. 

Geologists believe these formed by precipitation of calcite and iron minerals from ground water.  The calcite and iron minerals precipitate in concentric bands and make that part of the sandstone more resistant to weathering so that the boulders remain after the rest of the sandstone has eroded away. 

If you are interested in seeing Prim boulders for yourself there are many on display around the community of Prim within easy sight from highway 263.  

Geopic of the week: Rosie boulders

Rosie boulders

Pictured above are gigantic sandstone boulders in a gravel pit near Rosie Arkansas.  While these house-sized boulders are not unusual, they’re location is: there is no nearby source for them.  Some geologists speculate that they were transported  to this location by a great tsunami generated during a major meteorite impact.  Perhaps the same meteorite that caused that tsunami created the 110 mile diameter Chicxulub crater on the Yucatan peninsula about 65 million years ago.

For more views of the Rosie boulders look here

Geopic of the week: Turbidite deposits in Baumgartner Quarry

The Baumgartner quarry is located a little South of Kirby Arkansas along highway 27 in the Ouachita Mountains.  It exposes approximately 160 m. (590 ft.) of the upper Jackfork Formation which consists of interbedded sandstone and shale deposited  about 300 million years ago when the area that is now the Ouachita Mountains was a deep ocean basin.  Deep oceanic deposits such as these are the kind petroleum geologists explore for oil.  Because these deposits are exposed at the surface in southern Arkansas, geo-scientists from all over come here to study our rocks and gain a greater understanding of the deeply buried rocks they look for oil in in places like the Gulf of Mexico.  This is a view looking west along strike.

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Geopic of the week: well exposed fault

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The picture above shows a normal fault exposed in an old railroad grade just north of Greers Ferry lake in the Ozark Mountains of central Arkansas.  Two different lithologies are juxtaposed against one another: siltstone on the left and sandstone on the right.   Exposures such as this one are relatively rare in the Ozarks where most of the bedrock is covered by loose sediment and vegetation.

This fault is extensive and roughly parallels the northern portion of Greers Ferry lake.  The fault gives the northern portion of the lake its linear shape (see below).

Greers Ferry lake