The STATEMAP geologic mapping team came across an unusual outcrop in the Atoka Formation while working in northwest Arkansas earlier this year. Luckily they took lots of photos to document the odd features that formed in this bluff during weathering.
We refer to the process that created these features as differential weathering, meaning the rock does not weather uniformly. In sedimentary rock it is typically caused by variations in the degree or type of cementation binding the grains of the rock together.
One example of differential weathering is the gray masses of rock that seem to cling to the bluff shelter in several of the photos. For reasons not entirely clear, those masses are cemented with calcite. The surrounding rock has been mostly leached of cement. The picture at top right shows sub-concentric bands of calcite that precipitated in the rock at some point in its history. Perhaps that phenomenon explains the clinging calcite-cemented rocks. Where this secondary calcite cement has formed the rock is harder. The softer rock surrounding it weathers away faster and those calcite-cemented areas progressively stand in relief.
At any rate, it’s a visually stunning outcrop and well worth a look. Enjoy the pictures, courtesy of the STATEMAP crew!
The above picture is of a syringodendron, the fossilized trunk of a giant tree-like fern that lived in Arkansas in the Pennsylvanian Period (318 to 299 Million years ago). The Pennsylvanian Period is a sub-division of the Carboniferous Period of geologic history which was so named due to the preponderance of lush vegetation that existed at the time. Because there was so much vegetation , the geologic record for the Pennsylvanian period is coal-rich and also preserved a lot of plant fossils. This syringodendron was rescued from an abandoned coal mine in Scott County Arkansas by the land’s owner prior to the land being reclaimed.
The syringodendron is a somewhat rare as a fossil. It is actually from the same plant that produces another more common plant fossil called a Sigillaria. Here is an artists representation of what that plant may have looked like:
Syringodendron is a Sigillaria trunk that has lost its bark prior to fossilization. The double impressions that run vertically at regular intervals are called Parichnos scars, or “hare’s trails” colloquially. Between the vertical impressions are slight linear protrusions called ribs. If you examine the top edge of the fossil, the ribs give a wavy or undulating shape to the margin of the trunk.
Ooids are tiny grains that are typically composed of CaCO3 either as calcite or aragonite. They precipitate from seawater in concentric bands around a nucleus (for instance a fragment of rock or fossil) in turbulent shallow conditions.
Once ooids form, they can accumulate and be cemented to form a sedimentary type of limestone called oolite. The above picture is a magnified and tumbled piece of oolitic chert collected fromgravel on Crowley’s Ridge in northeast Arkansas. The difference between this and typical oolite is that it came into contact with silica(SiO2)-rich ground water after it formed. The SiO2 then replaced the CaCO3 the rock was initially composed of. The polished surface provides an ideal view of the internal structure of the spherical ooids.
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.
(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.