Recently, we posted a blog explaining that the Ozark Mountains are actually incised plateaus and that the hills are remnants standing between the incised river valleys. If you missed that one you can see it here. Now, we will talk about how a river is able to erode solid rock.
The picture above is of the Buffalo National River in its valley. As you can see, an impressive volume of rock has been excavated by this little river. A common misconception is that the water is carving the rock. Water is soft and softer things generally do not abrade harder things. Slightly acidic water can dissolve rock very slowly, particularly carbonate rock like limestone, However, the majority of the erosion in a river is due to the sediment suspended in the flowing water. As the sediment – which can range from tiny grains of silt to boulders– is carried downstream by the current, it skips along the channel, colliding with the bedrock. The repeated collisions break down the sediment, chipping off edges and rounding it. By the same process, new sediment is ground away from the bedrock and the valley is slowly enlarged.
The same thing is true of wind erosion such as in a desert setting. The wind itself really can’t erode the rock. The erosion is due to strong winds lifting loose sand and blasting it against the solid rock, slowly wearing it away.
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.
If you live in Arkansas, chances are you’ve heard of the Ozark Mountains. Actually, the correct geologic term is Ozark Plateaus. Unlike typical mountains in which the bedrock has been squashed and folded, the Ozarks are one broad dome-like structure made up of flat-lying sedimentary bedrock. The hills and valleys of the Ozark topography are the result of rivers carving into this dome, rather than compression or deformation.
The picture above was taken overlooking the Buffalo River. The various hills, from the foreground to the distance, are roughly the same height. Of course they are! If not for this and other rivers, the landscape pictured here would be one solid flat surface, as tall as the highest peaks in the picture, stretching to the horizon.
Skolithos is a common type of trace fossil that has been found in rocks as old as 541 million years. Trace fossils are not the fossilized remains of organisms but rather the burrows, footprints, and other structures that resulted from the animal’s activities.
In the case of skolithos, it’s widely believed that a vermiform (resembling a worm) animal created the straight, vertical, tube structures. These worm-like critters probably lived by filtering plankton from the turbulent water of a shallow marine environment. The vertical tubes may have been a dwelling place to retreat to, though their specific purpose is not known.
In the above picture, captured in north central Arkansas, a sandstone has weathered to reveal skolithos traces permeating the approximately 460 million year old rock. This example is from an exposure of the St. Peter Formation, Buffalo National River Park, Marion County, Arkansas.
To see more views of skolithos traces from Arkansas click here
Pyrite, also known as Iron Pyrite (FeS2), is the most common sulfide mineral. Its most frequent crystal structure is cubic, as seen in the picture above. It also forms octahedral (8 sided) and dodecahedral (12 sided) structures. Its brassy-yellow color and metallic luster can sometimes cause it to be mistaken for gold, hence the nickname “fool’s gold”. While it may look like gold, it is much lighter and harder. Typically pyrite cannot be scratched with a knife.
Pyrite is found in many counties in Arkansas. It is used in the production of sulfuric acid, although its use is declining. The primary value of this mineral currently is as a collectible specimen. Individual crystals are commonly found up to 1 inch in diameter.
Any rockhound worth their salt knows that the best place to hunt for interesting minerals is in the void spaces in rock. Void spaces come in two types; vugs and veins. Vugs are usually found in igneous rock and result from trapped gas bubbles. Veins, on the other hand, can be found in any type of bedrock.
Veins are fractures, that have been plugged with minerals, typically by precipitation from circulating water. The above picture was taken in the Ron Coleman quartz mine, near Hot Springs, Arkansas. The near-parallel white streaks that riddle the sandstone are quartz-filled veins. The fractures resulted from the intense deformation of the Ouachita Mountains, by plate tectonic forces, around 300 million years ago. That deformation opened up space for quartz to grow in, and the tremendous heat and pressure from the mountain-building generated the mineral-rich fluid that deposited the crystals.