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.
In the picture above, large black rectangular aegerine crystals are prominent in a rock type known as a pegmatite. Pegmatites are igneous rocks characterized by extremely large crystals. Sometimes they also contain unusual mineral species. This sample was collected from Magnet Cove, Arkansas. Magnet Cove, which is approximately 10 miles east of Hot Springs, is one of the few places in Arkansas where igneous rock is exposed at the surface.
Between 84 and 100 million years ago, magma was injected into the earth’s crust under central Arkansas where it slowly cooled and crystallized into igneous rock. Millions of years of erosion eventually unearthed that rock. Despite only being exposed over approximately 5 square miles, the rocks of Magnet Cove have yielded more than 100 different minerals. Rare minerals have been discovered there including a new variety of zirconium-rich garnet called Kimzeyite.
Phantom quartz refers to quartz crystals that show the outline of smaller quartz crystals inside of them. They are thought to form when there are at least two periods of growth. The phantom crystal is visible either because it differs slightly in composition or because it was coated with some material prior to the second round of crystal growth.
In the crystals above, all of which were collected near Mt. Ida, Arkansas, partial dissolution of the phantom crystals prior to the second round of growth left the phantoms with a ragged appearance.
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.