The above pictures are of sandstone beds from a quarry in North Arkansas developed in a rock formation called the Batesville Sandstone. Though it formed in a marine setting, the Batesville is typically composed of fairly homogeneous, flat-bedded rock with little evidence of inhabitation. This spot is an exception. The pictures clearly indicate the depositional environment was teaming with sea life at the time the sediment was emplaced.
The abundant trace fossils, which preserve the activity of organisms rather than their physical form, show a variety of behaviors common to marine invertebrate animals that lived in Arkansas more than 330 million years ago. Remnants of grazing traces of various snail and worm-like critters (A), resting traces (starfish; B), Dwelling burrows (sea-anemone or bivalve?; C), and locomotion trails (D) are indicators of the conditions present in North Arkansas near the end of Mississippian time.
The photo above shows a vertical dark rock in the center of flat-lying white rock. The dark rock is a sandstone deposit, probably Mississippian-aged, and the white rock is Silurian-aged limestone. If one were to follow the sandstone dike upward, it would lead to a sandstone bed sitting on top of the limestone. Since the limestone was deposited first, we can infer that it was exposed to weathering. The limestone was solutioned and deep fractures or cracks formed. Afterwards, sand was deposited in the area, filled the fractures in the limestone, and eventually lithified into sandstone. There are several of these sandstone-filled fractures present along the Buffalo National River in Silurian-aged limestone. The one pictured above is located at Shine-Eye.
This rather handsome outcrop of the Wilcox group consists of alternating layers of sand and clay of the Eocene Epoch which lasted from about 56-34 million years ago. The Wilcox Group is a non-marine unit mostly composed of sand with lesser clay, silt, gravel, and lignite (low-grade coal).
This geologic unit is part of a larger sequence of loosely-consolidated sedimentary rocks exposed in south central Arkansas, south of Pulaski county. These rocks are the northern extent of the West Gulf Coastal Plain, a physiographic province that stretches from central Arkansas, south, to the Gulf of Mexico.
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
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.
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.