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
A sinkhole is an area of ground that has no external surface drainage. Water that enters a sinkhole exits by draining into the subsurface. Many people are leery of sinkholes because of the damage they sometimes cause. Every now and then, a catastrophic sinkhole-collapse makes headlines, typically by swallowing someone’s house, or even draining an entire lake.
Not every kind of sinkhole is the dangerous kind though. The picture above shows a solution sinkhole. Unlike the feared collapse sinkhole, the solution sinkhole forms by chemical weathering of rock at the ground surface resulting in gradual lowering of the surface to form a depression. Solution sinkholes form in areas where fractures and joints in the bedrock create pathways through which rainwater can infiltrate the ground.
In, Arkansas, sinkholes are common in the northern part of the Ozark Plateaus where much of the bedrock is limestone or dolostone. These types of rocks are notorious for sinkhole development because they are soluble in weakly acidic rain water.
Like the White Cliffs of Dover, England, the “White Cliffs of Arkansas” (pictured above) are composed of chalk. Chalk is a marine sedimentary rock that forms of calcite-rich mud that accumulates in semi-deep marine environments. The mud is composed of the accumulated skeletal remains of algal microorganisms called coccolithophores. These algae grow and shed skeletal parts called coccoliths which they arrange around them, in life, in a structure called a coccosphere. Below is a scanning electron microscopic image of some coccospheres (borrowed from news.algaeworld.org).
Chalk in Arkansas is found in the Annona Formation, which formed in the late Cretaceous Period, and crops out in southwest Arkansas as well as parts of Texas. In addition to being mined to make blackboard chalk, this resource is also used in brick, and cement manufacture.
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 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.