Why do rocks have beds? Are rock beds where geologists sleep? Sometimes, but that’s not the point of this article. The picture above, taken on the Goat Trail at Big Bluff, overlooking the Buffalo National River, is a great example of a sedimentary rock composed of many individual beds (layers). The reason that rocks are bedded is due to either gaps in deposition or abrupt changes in the grain size of sediment being deposited in an environment.
Here’s an example; when a storm causes a river to flood its valley, the water deposits sediment as the flood recedes. Typically, there’s a period of non-deposition before the next flood event deposits a new layer of sediment over that one. This time between floods allows weathering to alter the character of the first flood deposit. That weathered surface will eventually differentiate the flood deposits into distinct beds of rock.
Bedding can also form as a result of flowing water gaining or losing velocity. The size of sediment that water carries (and eventually deposits) is directly related to flow rate. A sudden change in flow rate creates bedding distinguished by differences in grain size.
Everyone in the photo above was eventually air-lifted to safety… Just kidding! They’re still up there
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.
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
Pictured above is a bluff of St. Peter Sandstone exhibiting some spectacular black staining. The bluff is exposed near the confluence of Sylamore Creek and the White River north of Mountain View, Arkansas. Bluffs with this staining are referred to as “painted” because it looks like paint has been poured over the face of the rock.
The stains, which are manganese oxide, were deposited by groundwater as it seeped from the sandstone. The St. Peter Sandstone contains a minute amount of manganese that gets picked up by water as it flows through the rock. When the groundwater flows out of the sandstone, some of it evaporates leaving the manganese behind. Over time, a coating of manganese builds up on the bluff face.
The St. Peter Sandstone is also found along certain reaches of the Buffalo National River. The “Painted Bluff” – as it is known locally to river folk – is another great example of manganese staining.
This is a picture of a paleokarst feature from the Upper Buffalo River in Newton County, Arkansas. Paleokarst features, like this one, are ancient caves or sinkholes that have been preserved in the rock record.
In this case, a sinkhole formed when bedrock was exposed above sea level and acidic rainwater dissolved a vertical pit in the bedrock. When sea level rose and covered the area again, more sediment was washed in and the sinkhole was filled with sand. Eventually the sand became sandstone and a cast of the sinkhole is preserved today (center of photo).
All of this happened about 450 million years ago. Paleokarst features are one more clue geologists use to decipher earth’s history. If you didn’t know better, you might float right by and never give it a second thought.