Recently, I’ve had the opportunity to eat lunch down by the Arkansas River several times a week, and, as I am always curious about the world around me, I’ve taken to collecting various interesting pebbles lying around on the riverbank. Mostly, I’m picking up well-rounded, fine- to very-coarse-sized quartzitic pebbles (≈1/4” to 2.5”). Some are dark gray or dark red in color, some have abundant muscovite mica inclusions, and some even appear to have gneissic banding—none of which is commonly found in Arkansas, if at all. I began to wonder from how far upstream these rocks could have come.
As it happened, last week the sun was high overhead when I noticed a bright flash spread over the entire surface of an otherwise ordinary-looking pebble lying there among countless others. Now, many if not most of the pebbles there are highly polished and/or fractured chert which commonly glints in the sunlight, but this one flashed very brightly and seemingly only when viewed from a particular angle. A flash like that usually means you are seeing the cleavage faces of a mineral crystal. Due to its molecular structure, certain crystals will cleave, or split, in predictable ways along zones of weakness in its crystal lattice. Calcite is the most common example as it will typically break along its cleavage planes to form a rhombohedron. But seeing rhombohedral calcite in a terrace deposit is very unlikely since it readily dissolves in the weathering environment. Seeing a crystal with perfect cleavage like this on a riverbank in Arkansas is a rare occurrence, and, after picking it up, I noticed something else: scrawled across the surface of the pinkish cleavage faces was what appeared to be lines of an odd script written in a dark-colored mineral inclusion. This was really getting strange, but something about that inscription pattern jogged a memory, and, after a quick check in the Manual of Mineralogy, I was sure this pebble was composed of what is known as graphic granite.
Graphic granite, also known as runic granite, runite, Hebraic pegmatite, and schriftgranit, is an igneous rock named for its most characteristic feature: the presence of what at first glance appears to be lines of an ancient text, such as runes, cuneiform, or Hebrew, written on its surface. This “text” is actually composed of quartz crystals intergrown in a “tablet” of feldspar crystals. It was the cleavage in the potassium (K) feldspar, also known as orthoclase, that produced the flash in the sunlight.
Finding an igneous rock of any kind is rare in Arkansas as less than 1% of the surface area of the state is composed of igneous material. That being said, I had actually seen several other igneous rocks there on the river bank—most notably mafic lamprophyre, possibly ouachitite, with abundant biotite mica inclusions. I had previously seen rocks of this type at localities along the Arkansas River below Morrilton where they were weathering out of the riverbank from sills intruded into black shale of the Atoka Formation. That’s a distance of about 30 river miles, and, being so close, these rocks are usually rather large and angular. In fact, it’s a bit surprising they made it this far as these lamprophyres typically weather to clay in outcrop. In contrast, most of the pebbles I had collected were very well rounded, some almost polished, indicating they may have traveled a great distance in the bedload of the river. Could this unusual pebble be traced to a source in the headwaters of the Arkansas River? I thought it might be a distinct possibility.
During college, I had been on several field trips to the Cañon City, Colorado area and had seen granite pegmatite cropping out near the Royal Gorge. Large crystals of feldspar, quartz, beryl, and muscovite comprise the bulk of the rock there, with inclusions of garnet, tourmaline (shorl), and other minerals. The word pegmatite literally means “to bind together” and was first used in reference to the interlocking feldspar and quartz in the graphic granite commonly associated with it. Later, the meaning of the term was expanded to include any igneous rock with an exceptionally coarse-grained texture. Typically, it forms by the slow cooling of a water-rich, silicate melt during the later stages of crystallization in pockets and veins along the margin of an igneous batholith deep below the surface. These conditions favor the growth of the unusually large crystals characteristic of these rocks. After a quick search on the internet, I found several pegmatite localities along the Arkansas River west of Cañon City, some near the Royal Gorge and others even further upstream, where graphic granite was mined historically. In fact, Mica Lode, located on the rim of the Gorge, was the largest mine in the Eight Mile Park District and the largest producer of feldspar in Colorado at one time. As a final check, I looked up the published state geology for the intervening states and found that the Arkansas River basin doesn’t cross Precambrian granite anywhere else. I began to think the most likely explanation for how the pebble had come to be there on the riverbank was that it had traveled more than eleven hundred miles from central Colorado!
Royal Gorge, also known as the Grand Canyon of the Arkansas, was formed by the persistent downcutting of the Arkansas River through the granitic rocks of the Precambrian Migmatitic Gneiss as it was uplifted during the Pliocene Epoch about 3 million years ago. The gneiss is composed of the same minerals as granite, but has undergone alteration by heat and pressure. The rate of incision is estimated to be rather leisurely—about a foot every 2500 years. Of course that’s only a blink in the pebble’s history as the Migmatitic Gneiss from which it may have originated is estimated to be between 1600 and 2500 million years old. The Gorge, though ancient, formed relatively recently by comparison. To use the old analogy of the clock to represent all of geologic time, this rock formed between 8:30 am and 1:00 pm, and the uplift occurred during the last minute before midnight. If you’ve never been there, it’s quite spectacular and certainly worthy of its acclaim as a preeminent tourist attraction in central Colorado. Along its seven mile reach, there are places where it is only 300 feet wide and 1250 feet deep making it one of the narrowest and deepest gorges in the world. Carving this vast canyon would have required the Arkansas River to erode an enormous amount of material and transport it across the Great Plains. Presumably, some of that material would have been carried from there to the Gulf of Mexico and all points in between. During periods of glaciation, large quantities of detritus was abruptly added to the river by the breakup of ice dams formed by glaciers that were upstream of the Gorge. One of them was approximately 670 feet high near what is now Granite, Colorado. It formed a lake that extended upstream about 14 miles and was 500 feet deep. When the ice dam broke, catastrophic flooding deposited a sheet of boulders as much as 60 feet thick 10 miles downstream. The flow was strong enough to deposit four-foot boulders as high as 160 feet up the canyon wall. How far out onto the Great Plains would a wall of water over 160 feet high take a pebble? I’m not sure, but that would be some boost!
Imagine the pebble’s journey from the Rocky Mountains to Little Rock. Aside from catastrophic floods, it would have literally taken ages for it to be pushed along the bottom of the Arkansas River across eastern Colorado, Kansas, Oklahoma, and western Arkansas to finally reach the bank at Cook’s Landing below Murray Lock & Dam No. 7. Starting as a much larger piece of rock, it was tumbled and rounded by the gravel in the bedload of the river, perhaps only transported from place to place during periods of high flow. It was deposited and buried in countless point bars as the channel moved toward the cut bank, only to be exposed again and moved downstream as the river meandered back. By the time it reached Little Rock, this little rock would have dropped over 5000 feet in elevation. Feldspar is typically unstable at the surface and normally weathers to clay rather quickly, but perhaps because it’s intergrown with resistant quartz in the pebble the weathering process was slowed enough to allow it to make its long trip in relatively good shape.
As it leaves the Front Range of the Rocky Mountains near Cañon City, flow on the Arkansas is seasonal because it’s mostly derived from snowmelt. Due to recently constructed controls and pumping for irrigation, flow has become minimal on the western Great Plains, sometimes dropping to zero in places. The McClellan-Kerr Arkansas River Navigation System, which came online in 1971, now controls flow on the river by utilizing a series of stair-stepped reservoirs between Tulsa and its mouth on the Mississippi River.
At the head of these navigation pools, flow is diminished which tends to cause the heavier gravel in the bedload to settle there as it can no longer be supported by bed turbulence. Also, the extensive use of revetments, dikes, and levees along the river have almost completely eliminated its ability to meander freely. And to maintain each navigation pool at a precise elevation, the Corps’ design directs most of the river’s flow through Tainter gates at the top of each dam creating further impediments to the heavier clasts. Due to these factors, transport of the larger sizes of sediment has effectively stopped on the Arkansas River between Keystone Dam near Tulsa and the Mississippi River, possibly only occurring during major flood events. Suspended sediment composed of clay-, silt-, and sand-sized particles is most of what the river carries these days, meaning my pebble has probably not moved much since the dams were built. It’s even possible that it was deposited below Murray Lock & Dam more than 50 years ago and has waited down there ever since for me to pick up!
Since I found that first pebble and as my eye has become better trained, I have found more pebbles of graphic granite along with pure K-feldspar, fine- to coarse–grained granite, and other granitic or possibly volcanic rocks that are definitely not from around here. In the end, I guess that rock I found didn’t really say much, but I think I was still able to figure out what it was trying to tell me. You never know what you’ll discover when you take a look around outside and try to make sense of the world. That is the work of a field geologist and if you’re doing that, then you’re in the field wherever you go. Some might consider looking for pretty, rounded pebbles after lunch a mundane or, at best, mildly interesting pastime. But finding a connection via our state’s namesake river to a mountain range three states away can make it a whole new adventure!
Until next time, I’ll see you in the field!
Richard Hutto
Office of the State Geologist Blog 