Tag Archives: Ouachita Mountains

Geo-pic of the week: Key beds, and strike and dip

Kirby quarry edited

 

In this photo we are looking at rock beds, tilted till they are nearly vertical, and exposed in three levels in a quarry near Kirby, Arkansas, Ouachita Mountains.  Like a humongous 3-step staircase, each ascending level of the outcrop provides a deeper view into the rock formation.  An outcrop like this one illuminates a couple of basic but important concepts of geology: key beds, and strike and dip.

The key beds or beds that can be traced across the outcrop, such as the one marked with red dots above, appear to shift to the right as your eyes ascend the steps.  These are not faults!  It’s an optical illusion.  If our view were aligned parallel with the sides of the beds, they would appear aligned, but our view is actually diagonal to the bedding.  To illustrate this, here is the same picture with a drawing of the key bed as if it were jutting out of the outcrop. 

 

Kirby quarry edited3

 

This is why geologists measure the orientation of rock beds – known as the bed’s strike and dip.  Knowing how a bed is oriented in one place can help you to predict where it will be in another, perhaps inaccessible, place such as deep in the subsurface.  If that bed is full of oil, gas, or other precious commodity, predicting where it is becomes very important.

Geo-pic of the week: Graptolites

 

Graptolites

This is a picture of shale, collected from the Womble Formation, near Lake Ouachita State Park, Arkansas.  The photo shows examples of the, now extinct, Graptolites: fossilized colonies of tiny marine animals.

There were many types of Graptolites.  Some were attached to the sea floor, like corals, while others floated in the water, like plankton.  The feather-shaped fossils pictured here are actually the nests in which the animals lived.  Each tooth-like tube, on the edges of the nests, housed a tiny animal.  Several of these nests would be linked together into a larger colony.

At one time the oceans were full of Graptolites, but by about 300 million years ago they died out for unknown reasons.  Because they were abundant, widespread, and continually evolving, Graptolites are important fossils for dating ancient marine rocks. 

To download a copy of our self-guided tour of Lake Ouachita geology, click here http://www.geology.ar.gov/pdf/Lake%20Ouachita%20Geologic%20Float.pdf

Geo-pic of the week: Spring boxes

 

Spring cool storage colorized

Before the invention of electric refrigerators, blocks of ice in insulated wooden cabinets called “iceboxes” kept food from spoiling quickly in warm climates.  This required access to ice, which had to be hauled in from cold climates by boat, and wasn’t always available, especially in remote places.   The picture above shows a cool water spring that was modified long ago into a primitive kind of refrigerator. The structure is made of concrete.  When it was in use, it would have had a door to keep cool in and keep animals and insects out, as much as possible. 

Just like caves, cool water springs in Arkansas stay close to 56 degrees in the summer – the ambient ground temperature.  Anyone that’s spent a summer in Arkansas knows it gets oppressively hot.  Having a place you could store milk, eggs, and other perishables would certainly have come in handy.  You still come across these old structures if you spend a lot of time out in the woods around the state.  This one was photographed near Hot Springs, Arkansas, in the Ouachita Mountains.

Geo-pic of the week: Freshly exposed anticline

Mcloed St. Doctored

This is an anticline exposed on Mc Leod Street, southwest of Hot Springs, Garland County, Arkansas.   It’s not unique as, anticlines are common in the Ouachita’s and other mountain ranges throughout the world.  Most often though, these structures are large scale and cover expanses of land that can’t be viewed from a human vantage point.  When they do form on a scale that’s small enough for human observation, we typically don’t have the benefit of a freshly blasted exposure like this one. 

In fact, many times geologists must infer that folds like this exist in places deep underground that no one has or will ever see.  That’s why, if you see a geologist on the side of the road, taking something like this in, as in the picture above, just let him have his little moment.  The exposure is of deep marine sedimentary deposits of the Stanley Formation.

Geopic of the week: Asymmetrical anticline with intrusion

Angela's anticline with igneous dike

This photo is of an asymmetrical anticline in the Stanley Formation.  It’s asymmetrical because the right limb of the fold is dipping at a steeper angle than the left limb.  This type of fold is common in the Ouachita Mountains, however, this one has a small igneous intrusion on the left limb (lower left, dark gray).  The intrusion consists of a dike, which split several of the lower beds at nearly a right angle, and a sill emplaced parallel to the bedding.

From this picture, and basic geologic principals, we can tell the history of these rocks.  Sediment was first deposited in horizontal layers (principal of horizontality).  Later, the layers cemented to form solid rock – the layers must have been firm before they were deformed because they maintained their shape.   Next, tectonic forces in the earth bent the rock into an anticline and, after it was folded, the igneous intrusion was forced into the rock. We know the intrusion was last because it cut across the rock layers and the fold (principal of cross-cutting relationships).

One of the most challenging aspects of geology is interpreting a lot from a little information.  It’s also part of what makes it so interesting!

Geopic of the week: Plunging folds at Gulpha Gorge

Gulpha Gorge_adjusted

Imagine you took a stack of ribbons, compressed it till it buckled into bows, and then tilted the whole stack on its side.  That pretty much sums up what you can see in this picture of plunging, folded bedrock at Gulpha Gorge Campground, north of Hot Springs, in the Ouachita Mountains of Arkansas.  The bedrock of the Ouachitas was buckled and tilted about 200 million years ago when the South American and North American continents collided – part of the incredible process geologists call plate tectonics.

This is just a couple of wee folds that are exposed at the surface because the bedrock at Gulpha Gorge is novaculite – a really hard rock that doesn’t erode away easily.  However, if we could strip the vegetation and civilization away in central Arkansas, we would see that pretty much all the rocks in the region are folded and tilted in similar ways.  Some of the folds cover many square miles and can be seen from space on a clear day, and others are no bigger than a speed-bump. 

Geopic of the week: Arkansas gold scam

002

Pictured above is one of the “gold” mines of Arkansas.  This is one of many prospect holes dug in the Ouachita Mountains around 1886 when investors fell victim to the first documented Arkansas gold scam.  It’s located in the Charlton Campground just west of Mt. Ida in the Ouachita Mountains.  The reality is no gold in commercially minable quantities has ever been found in Arkansas. 

Scams involving gold and other precious metals are not unique to the state, but they have been a recurring problem, as recently as the mid 1980s.  The scams, in a nutshell, consist of staking out a claim on a piece of land, obtaining falsified assay reports that show inflated values of precious metals, and then duping investors into buying parts of the claim.

Though it’s hard to imagine falling victim to such a scheme, the con-men have historically been quite successful.  As rumors spread, more and more people rush to get in on the bonanza.  By the time the dust starts to settle, the original instigators are gone and so is everyone’s money.