This is a picture of Pinnacle Mountain – one of several steep-sided hills up to about 1000 feet tall, located at Pinnacle Mountain State Park, northwest of Little Rock, Arkansas. Though its appearance may be misleading and its origin is debatable, Pinnacle Mountain is not a volcano!
What’s intriguing about Pinnacle and the smaller nearby hills is that they’re sandstone lenses surrounded by shale. That, from a geologic perspective, is difficult to explain since sedimentary rocks are suppose to form in layers, not lenses or blocks. This has led geologists to a variety of interpretations for their origin ranging from giant undersea landslides, to sand that got trapped in the empty gouges left by large undersea landslides, to beds of sandstone caught up and scrambled with shale beds along huge thrust faults.
Whatever their origin, because they are sedimentary rock and contrary to some satirical publications I’ve seen circulating online, they are not volcanoes and they are not going to blow up!
Satin spar is a variety of the mineral gypsum and, aside from it’s attractive fibrous appearance, it’s used for many practical purposes, including for making plaster, chalk, and drywall. Some ideally suited varieties are carved by sculptors. This piece was collected from a gypsum mine in Howard County, Arkansas, near the town of Nashville, where it’s mined and processed to make drywall.
Around 100 million years ago, the water of the Gulf of Mexico reached all the way to southern Arkansas, forming a huge marine bay. Because that water was somewhat isolated from the ocean’s circulation, evaporation concentrated dissolved minerals there, to the point that the water became oversaturated and minerals, such as gypsum, began to crystallize out of it. It’s the same process by which most of the world’s salt deposits formed. In fact, gypsum is often found associated with salt.
According to the American Geosciences institute, a castle, in the geologic sense, is a natural rock formation bearing a fancied resemblance to a castle – sophisticated science, I know! The limestone boulder pictured above, which is from north central Arkansas, is one such castle. Rocks like this one owe their appearance to their solubility in weak acid.
Most rain water is actually slightly acidic, due to the CO2 it absorbs from the atmosphere and soil it passes through. Over time, this acidic water is capable of dissolving limestone bedrock into features such as caves, sinkholes, and, in this case, castles. The boulder pictured here has been flipped over by the creek’s current; they typically form with the castle side down.
below is an example of a castle that is still forming. The base of the rock dissolves faster than the upper part, because it is under the water more often. This differential weathering is what gives the boulder its characteristic castle shape.
Photos by Richard Hutto
Pictured above is a Loess doll, collected from Crowley’s Ridge in the Mississippi River Valley, eastern Arkansas. A loess doll is a type of concretion, or chemically precipitated mineral mass. They grow in deposits of wind-blown sediment, known as loess, by a process of leaching and concentration of minerals within a soil. They typically form by nucleating around a particle in the loess, such as a fragment of a fossil.
They were given the name loess doll due to the whimsical shapes these concretions take, which sometimes resemble people or animals; the one above looks like a lamb to me. Aside from their charm, they are important to geologists because they’re associated with buried soils, or fossilized soils, which are useful markers for correlating land surfaces across large areas.
To see more loess dolls click here
In this photo we are looking at rock beds, tilted till they are nearly vertical, and exposed in three levels in a quarry near Kirby, Arkansas, Ouachita Mountains. Like a humongous 3-step staircase, each ascending level of the outcrop provides a deeper view into the rock formation. An outcrop like this one illuminates a couple of basic but important concepts of geology: key beds, and strike and dip.
The key beds or beds that can be traced across the outcrop, such as the one marked with red dots above, appear to shift to the right as your eyes ascend the steps. These are not faults! It’s an optical illusion. If our view were aligned parallel with the sides of the beds, they would appear aligned, but our view is actually diagonal to the bedding. To illustrate this, here is the same picture with a drawing of the key bed as if it were jutting out of the outcrop.
This is why geologists measure the orientation of rock beds – known as the bed’s strike and dip. Knowing how a bed is oriented in one place can help you to predict where it will be in another, perhaps inaccessible, place such as deep in the subsurface. If that bed is full of oil, gas, or other precious commodity, predicting where it is becomes very important.
Pictured above is the contact between two very different rocks: the Brownstown gravel above, and the Jackfork Sandstone below. This is what geologists call an angular unconformity. The contact represents a vast period of time.
The lower sandstone was deposited in the Pennsylvanian period, around 300 million years ago, when Arkansas was at the bottom of an abyss. It was deposited in horizontal layers, but later, when the South American continent collided with the North American continent, it was deformed into mountains and raised above sea level. For about the next 200 million years it eroded and subsided, until eventually, a shallow sea covered it again. Rivers washed gravel into that sea, which became the Brownstown gravel.
This picture was taken near De Gray Lake State Park in south central Arkansas.