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.
Freshly exposed igneous rock at 3M quarry near Little Rock, Arkansas. About 100 million years ago, molten rock from the earth’s hot interior was forcibly injected into the bedrock in this area. The dark-colored rock you see at the center of the picture is what geologists call a xenolith, or accidental inclusion. It’s basically a hunk of the bedrock that broke off and got trapped in the magma before it cooled. The bedrock was originally shale, but the heat and fluids from the igneous rock cooked and altered the shale so it’s now a metamorphic rock called hornfels.
Pictured above is clay and sand of the Nacatoch Formation of southern Arkansas. The clay beds, which stand slightly in relief, have been churned, and the sand has numerous cylindrical structures of various sizes. These are hallmarks of bioturbation, or reworking of sediment by living things. The cylindrical structures are the preserved casts of roots. The disrupted clay beds are evidence of the burrowing of mud-loving critters.
Bioturbation structures are commonly preserved in rock and offer glimpses into the environment where sediment was deposited. Specifically, it tells us that this sediment was near the surface in a relatively calm near-shore marine environment long enough after deposition for living organisms to move in and set up shop. Clues like these are one of many tools geologists use to piece together the early history of the earth from the rock record.
Pictured above are tracks of an Acrocanthosauras Atokensis: a bi-pedal predatory dinosaur. The tracks were discovered in 2011 in a gypsum mine north of Nashville Arkansas by workers at the mine. It is one of two such “dinosaur trackways” – as they are called – that have been discovered in this mine; The first one was unearthed in 1983. Dinosaur tracks are not common in Arkansas as most of the rocks here, which are very old, were deposited long before the dinosaurs existed.
The rocks where the tracks are preserved were deposited in the early Cretaceous Period sometime between 145 and 100 million years ago. At that time, the area south of the Ouachita Mountains was a broad coastal plain and the Gulf of Mexico waters reached all the way to southern Arkansas. A variety of dinosaur species tracks, both herbivore and carnivore, have been discovered in these trackways, indicating that the coastal area at that time was quite the dinosaur stomping ground.