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.
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.
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Pyrite, also known as Iron Pyrite (FeS2), is the most common sulfide mineral. Its most frequent crystal structure is cubic, as seen in the picture above. It also forms octahedral (8 sided) and dodecahedral (12 sided) structures. Its brassy-yellow color and metallic luster can sometimes cause it to be mistaken for gold, hence the nickname “fool’s gold”. While it may look like gold, it is much lighter and harder. Typically pyrite cannot be scratched with a knife.
Pyrite is found in many counties in Arkansas. It is used in the production of sulfuric acid, although its use is declining. The primary value of this mineral currently is as a collectible specimen. Individual crystals are commonly found up to 1 inch in diameter.
Any rockhound worth their salt knows that the best place to hunt for interesting minerals is in the void spaces in rock. Void spaces come in two types; vugs and veins. Vugs are usually found in igneous rock and result from trapped gas bubbles. Veins, on the other hand, can be found in any type of bedrock.
Veins are fractures, that have been plugged with minerals, typically by precipitation from circulating water. The above picture was taken in the Ron Coleman quartz mine, near Hot Springs, Arkansas. The near-parallel white streaks that riddle the sandstone are quartz-filled veins. The fractures resulted from the intense deformation of the Ouachita Mountains, by plate tectonic forces, around 300 million years ago. That deformation opened up space for quartz to grow in, and the tremendous heat and pressure from the mountain-building generated the mineral-rich fluid that deposited the crystals.
Pictured above is sandstone displaying classic herringbone cross-beds. Cross-bedding results from either sediment transport by flowing water, such as in this example, or by wind flow, as in the case of dunes.
Cross-beds form by the migration of sediment, and tilt in the direction of flow. As sediment grains are carried by the current, they migrate up the gentle ramp of previously deposited cross-beds. When they reach the end, they tumble down the steeper face there and are deposited to become part of the next cross-bed. In this way the sediment migrates in the downstream direction.
Each group of similarly tilted cross-beds is known as a set. In herringbone cross-bedding, the sets are oriented contrarily, which gives the outcrop a fishbone appearance. These differently oriented cross-bed sets indicate changing flow directions.
A tempestite, like the one pictured, is a rock composed of debris deposited by a storm. It’s mostly a sandstone but also contains various fossils, pebbles, and other clasts that were picked up and tossed about by the waves.
Waves are generated as wind energy is transferred to water. Naturally, during a storm, waves are bigger and more energetic. This increased energy allows the waves to pick up, and in some cases rip up, various relatively large clasts and fossils and transport them. The large elongate fossil above is an extinct squid-like creature known as a conical nautiloid. Other marine fossils in this sample include gastropods, and crinoids. It also contains plant material.
The presence of tempestites in a rock outcrop indicate the area was a shallow marine environment when those rocks were being deposited. This sample was collected in Northwest Arkansas from the Pennsylvanian Prairie Grove Member of the Hale Formation.