HIGHWAY 65: MARSHALL ROADCUT

Highway 65 Roadcut

This is the highway 65 roadcut a couple of miles south of Marshall. The cutaway section faces south, so the sun is always blazing on it, lighting it up like a big layered cheesecake, especially in the winter months. It comes into view suddenly as you top the hill, and it's so big and so structured that it just looks important. I've driven past this site hundreds of times, and each time I've promised myself I'd look up the meaning of this thing. This month I did.

As you might expect, the Arkansas Geological Commission publishes a guidebook (publication number GB-79-1, price $4) that explains in fairly technical terms exactly what is going on here. And not just here. There are other cutaways along this road. You can match up the layers from one to the next and construct a profile of over 150 vertical feet of sedimentary rock.

First we learn the names of everything. These rocks were made in the Mississippian epoch of the Carboniferous period. The thick layers on top are Pitkin Limestone. The numerous thinner layers below are some of that Fayetteville Shale that has been in the news in the business section lately. The vertical marks you see are drill holes that were packed with explosives when this hillside was excavated.

These names, like Fayetteville Shale and Pitkin Limestone, come from places where these layers break the surface and are easily found and technically identified. For example, Pitkin Corner can be found aside highway 540 between West Fork and Winslow. This layer of limestone makes some prominent outcrops there.

The first thing you notice about these layers is that they are all very nearly horizontal, just as you'll find throughout the Ozarks. Visit the Ouachita Mountains on the south side of the Arkansas River and you'll see a different arrangement. On the left is a picture from an old quarry at Pinnacle Mountain State Park, which is right on the eastern end of the Ouachita Mountains. These layers are also from the Carboniferous, but notice how steeply angled they are. And here on the right is a snapshot from a trailhead at Daisy State Park about a hundred miles to the southwest in that same Ouachita range. These are Mississippian rocks, same age as those in Marshall, but notice these layers are almost vertical.

So the Ozarks have flat, horizontal layers and the Ouachitas are made of layers that are tilted. The reason is that the Ouachita acted as a kind of crumple zone as the South American tectonic plate pressed into the North American tectonic plate.

Enough of the Ouachitas for now. Back to the Marshall roadcut.

The guidebooks say that the Pitkin Limestone was formed at the bottom of a shallow sea. You don't have to climb or dig to verify this. You can walk along the base of the cliff and pick up chunks of limestone that have fallen down over the years. Here's archimedes, a typical carboniferous fossil, a soft-bodied marine animal living in a spiral mineral shell. These three are cross sections. At first I thought they were fish bones, but archimedes is corkscrew-shaped.

The archimedes fossils are imbedded in rock that is chock full of other fossils, mainly crinoids, which are relatives of starfish and sand dollars. Up close, a lot of this limestone looks like fragments of crinoids cemented together in a mass. Here's a closeup. The crinoid fossils look like stacks of poker chips. In life, those little poker chips were strung together and topped with a rosette structure, kind of like a flower on a puka shell stem.

These creatures and others like them made shells of calcium carbonate, which is what limestone is made of. So what you're looking at when you see the Pitkin layer is millions of years worth of old seashells and crinoids and other stuff piling up and getting compacted and fused together with minerals from the seawater.

When the seas are deeper that doesn't happen the same way. Where the sunlight doesn't penetrate to the bottom of the sea, such life is rarer. Vast quantities of calcium carbonate shells do not accumulate and limestone is not formed. The deeper the sea when the sediments are accumulated, the less limestone will be formed; so geologists can make some general guesses about the depth of the sea at any given point on this wall based on the amount of calcium carbonate in the rock and the types of fossils imbedded there.

However, some stuff does find its way to the bottom of the deep oceans, mostly fine silt and dust, and when these sediments are pressed together for millions of years they become shale, like the Fayetteville shale on the bottom of this cutaway.

As you walk ten thousand years at a step down the north side of the hill along the roadcut, new layers of shale emerge from the ground. Looking up, you see that the Pitkin limestone you could have reached up and touched with a little help is now forty feet over your head. These crumbly layers in varying shades of gray are the Fayetteville shale. Fossils from these layers are few and far between. Even so, I did turn up a couple. Here's one.

Since I found this bivalve in the rubble at the base of the roadcut I can't say for sure which layer it came from, but the rock is not packed with the crinoids like the higher levels and the color is gray and the texture is fine like the shales, not coarse and grainy like the limestones. The literature mentioned "lime mudstone" layers within the Fayetteville strata, and I'm guessing that's what I've got here. A layman's guess.

Don't expect dinosaur bones. These rocks were made long before dinosaurs, even before reptiles. There were no birds, turtles or even flowers. We're talking 365 to 325 million years ago, give or take a few million years. We can't be exact about rock layers the way we can about tree rings. One of those thin shale layers might represent five million years or a half million. A ten-foot-thick slab of limestone might have accumulated over ten thousand years or a hundred thousand. Geologists seem to agree that limestone generally accumulates faster than shale.

The Mississippian was about the time when the first four-legged vertebrates started moving onto the land. Paleontologists call them tetrapods, and they looked something like salamanders about the size of a house cat. The insects, on the other hand, were huge. There were creatures like centipedes six feet long, giant scorpions. That kind of thing. I didn't find any fossils that looked like that, but then this was not their environment.

There's a rest stop with picnic tables opposite this site, and there's a scenic overlook offering a view of the countryside around Marshall. Also a rock shop.

RTJ--11/1/2006


SOURCES:

Arknasas Geological Commission Website

Handford, C. Robertson and Manger, Walter L., A Field Guyide Prepared for the Midcontinent Section of SEPM 1990 Field Conference October 19-21, 1990.

MacFarland, John David, Stratigraphic Summary of Arkansas, Information Circular 36.

A Guidebook to the Odovician-Missippian Rocks of North-Central Arkansas, Arkansas Geological Commission publication GB-79-1

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