100 million years ago, during the Late Cretaceous Period, a preponderance of igneous activity occurred in the continental region now known as Arkansas. In fact, all of the igneous rocks discovered in the state were emplaced around that time. Some of them are well known, such as Magnet Cove, located east of Hot Springs, or the diamond-bearing intrusion near Murfreesboro. There are also lots of smaller igneous intrusions like the one shown in the picture above.
Small igneous intrusions are found throughout the Ouachita Mountains. There are so many small intrusions that new ones are regularly discovered. Weathering at the earth’s surface has typically destroyed the original rock’s characteristics and what remains is mostly soft clay because the minerals that make up the intrusion are unstable under surface conditions.
If you happen to notice an unusual-looking body of rock that cuts across the strata of a road cut or other rock outcrop when you’re exploring the Ouachita Mountains, it’s likely that you have seen a Cretaceous igneous dike.
The picture above shows a boulder of Hot Springs Sandstone with well-developed sigmoidal veins. Sigmoidal veins – sometimes called tension gashes – form in rock by shear stress. That’s stress that causes adjacent parts of a rock to slide past one another. In the above picture the yellow arrows indicate the approximate orientation of the stresses that were applied to this boulder to create the sigmoidal veins.
Sigmoidal veins, at their inception, are shaped like parallel lines that bulge toward the center and taper at the ends. They originate due to tension created between the two opposing forces acting on the rock. Essentially the rock tears to alleviate this tension. If the shearing continues long enough, these openings in the rock begin to rotate. The eventual shape, seen above, is like the letter S. The ends of each S point opposite of the direction of the force that created them. Therefore, sigmoidal veins can indicate the forces at work on bedrock when it was buried underground.
The veins pictured here are at the edge of a parking lot next to the Arlington Hotel in Hot Springs Arkansas. After they developed the veins were in filled with quartz. The Hot Springs Sandstone is a member of the Mississippian Stanley Formation.
(Click picture to see large high-resolution version)
At top is a scan of a hand-drawn map of downtown Hot Springs Arkansas ca. 1859. It was drawn By Dr. David Dale Owen, the first State Geologist of Arkansas. It shows Bathhouse Row, the area renowned for its hot mineral-water springs (a photo of the area depicted on the left side of the map is included for comparison). Bathhouse Row remains a popular attraction today, though a lot has changed since 1859.
Hot Spring Creek, which displays across the bottom of the map north to south (note that north is to the left here), now flows underneath Central Avenue in downtown Hot Springs. Central Avenue is the street at the bottom of the photograph (see photo). In 1860, there was no Central Avenue and people crossed Hot Spring Creek on wooden bridges (see map). The bluff east of the creek from which the hot springs flow is now Hot Springs National Park.
This map was included in the second of two geological reconnaissance reports published by Dr. David Dale Owen concerning Arkansas geology. During the field work for that publication in 1859, Dr. Owen, only fifty three years old, contracted malaria. He died a short time later. In the introduction to the final volume of that publication, Dr. Owen’s brother writes that David was dictating the report, from bed, until 3 days before his death.
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.
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.
Igneous rocks are rare in Arkansas, making up about 0.1% of the surface area of the state. Nonetheless, we find some interesting and unusual rocks and minerals within our igneous rocks. The above picture is a pseudohexagonal, zoned, green biotite crystal collected from Magnet Cove, Arkansas just last week. That’s a mouthful!
Magnet cove is a 100 million year old igneous intrusion, now exposed at the surface 12 miles east of Hot Springs, Arkansas. In that little area, over 100 mineral species have been identified, including some that were first discovered there. Students, researchers, and mineral enthusiasts come from all over to visit Magnet Cove, collect samples, and learn about this geologically fascinating place.
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.