Pictured above is an exposure of Prairie Grove Sandstone near Durham, Arkansas, southeast of Fayetteville. The ribbed, planar faces that are central in the photo resulted from a weathering phenomenon called zebra weathering.
Zebra weathering occurs in sandstones cemented with calcite – a soluble mineral. Calcite is common in marine sediment and, in the tidal environment where this rock was deposited, marine sediment mixed with insoluble sand from the continent. The ratio of marine sediment to sand changed continuously in that environment due to seasonal and climatic cycles. Today, the beds of sandstone weather at different rates depending on their calcite content. As the rock weathers, the sandier beds stand out in relief since they wear away more slowly than the soluble beds between them. Hence, the banded zebra pattern.
I was working near Lake Fort Smith State Park this last week when I came across a peculiar mineral deposit resembling Frankenstein’s Scars (Fig. 2). It was just in time for Halloween! The resemblance is uncanny. Despite the horror, there is a lot of geology illustrated in this rock.
The mineral that forms the “scars” seen in the photo is called limonite, and it was deposited within a cavity in a stylolite. A stylolite is a surface, typically a bedding plane, that has recrystallized due to pressure from the weight of overlying rock material. Stylolites can be recognized by their rough, jagged appearance (it’s difficult to see in this photo, but trust me – it’s there). The limonite “scars” formed in a pattern called boxwork and, surrounding the boxwork, limonite is also present in botryoidal form: a crystal shape resembling small round globs (the orange goosebumps around the scars).
At this time, rocks are not thought to celebrate Halloween, although more work needs to be done to verify that.
Above are two pictures of a trace fossil, Conostichus, from the Ozark Plateaus region of Arkansas. Like other trace fossils, Conostichus are structures found in sedimentary rock that represent the spot where an animal lived, fed, or travelled. Despite their abundance, especially in rocks of the Carboniferous period (299 to359 million years ago), it’s not certain what kind of animal made Conostichus, because the animal’s body wasn’t preserved.
The upper picture is the top of the Conostichus and shows the hole through which the animal entered or exited the structure. The lower picture is the same Conostichus with the top facing down. As you can see, they taper and come to a rounded point at the base, vaguely resembling a badminton birdie.
At present, the most widely accepted theory for their origin is that Conostichus are burrow traces left by Sea Anemone.
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.