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.
“Turtle Rocks” are unique, mounded polygonal structures that resemble turtle shells. These features are found along the Arkansas River Valley in the Hartshorne Sandstone, a brown to light gray, massive, medium-grained sandstone deposited during the Pennsylvanian Period by ancient river systems. The processes that generate “turtle rocks” are not clearly understood. One explanation suggests that these features were created by a process known as spheroidal weathering, a form of chemical weathering that occurs when water percolates through the rock and between individual sand grains. These grains loosen and separate from the rock, especially along corners and edges where the most surface area is exposed, which widens the rock’s natural fractures creating a rounded, turtle-like shape. Additionally, iron is leached from the rock and precipitated at the surface creating a weathering rind known as case hardening. These two processes along with the polygonal joint pattern contribute to this weathering phenomenon.
Pedestals are a fairly common erosional feature in Arkansas in places where conditions are favorable. They typically form in massive sandstone units due to an increased rate of erosion along the joint set near a bluff line. Joints are vertical fractures within almost all rocks that formed in response to the tectonic stresses they have undergone in the distant past. Joints are most often expressed as sets oriented in rhombohedral patterns. Water can more easily penetrate the rocks along these joints, eventually opening a gap. When this happens along joints parallel to a bluff face, the gap essentially cuts off the incipient pedestal from the influence of groundwater, isolating it from most of the processes of chemical weathering. Once that happens the majority of weathering of the newly formed block of sandstone is done by wind and rain. Because the corners and edges of a rhombohedron have more surface area, weathering is concentrated there, eventually rounding it off to form the typical pedestal shape. In many places, a capstone of more resistant sandstone is present which contributes to the top-heavy pedestal or mushroom shape. Also, the pedestal-forming unit is commonly underlain by shale or silty-shale on which the fully intact pedestal can slowly creep downslope. Some of them end up quite a distance from the bluff where they started. If you would like to view several fine examples of this erosional phenomenon, consider a visit to Pedestal Rocks Natural Area in the Ozark National Forest.
This type of weathering produces pits of varying size on a rock surface. The pits in the picture above are small approximately less than 1 inch up to 2 inches in diameter. This type of weathering is also known as tafoni, a Sicilian word for window. The dominant process that forms these features is probably chemical weathering. Sandstones are made up of sand size grains of quartz and other minerals. The grains are held together with a cement or glue such as quartz, calcite, or iron. In most instances, there are patches where the cement is no longer present allowing the grains to fall apart. Honeycomb weathering is abundant in the sandstones in the northern part of the state. This picture was taken of the Wedington Sandstone Member of the Fayetteville Shale in northwestern Arkansas.