If you were expecting or even – God forbid – hoping for another rant, I’m sorry to be the bearer of bad news: I think I need to give it a rest for a while. I’ll say only this before taking my leave of that somewhat continuous, reiterative and baleful project: because of the way fundamentalist religious dogma with all its patriarchal connotations warped the members of my family for generations, I’m genuinely sorry I was born into that family, and somewhat resentful as well. That’s a pretty heavy thing to say, ain’t it? I have always tried to treat my son in such a way that he won’t feel about his father the way I feel about mine. Some of you who read this know full well what I mean because that’s the way you feel – and chances are, religious dogma played a role in it. Those of you who can’t imagine what it must be like to feel that way, also don’t know how lucky you are to have dodged such a bullet by a fortuitous accident of birth.
Now, on to brighter things. On Friday, I met my friend Nicole King (whose beautiful, thought-provoking and touching essays you’ve probably encountered on this blog) for a long-overdue visit to the Wichita Mountains in southwest Oklahoma. The Wichitas are a fascinating igneous province whose history is quite unlike that of any other mountain range known to me. It’s a series of granite outcrops that trend roughly east-west for some sixty miles from near Lawton to a bit beyond the appropriately-named town of Granite. They aren’t large as mountains go: the maximum topographical relief is probably no more than 1700 feet or so. On approaching them, one is immediately struck by the fact that there are no foothills: the massifs simply rise directly out of the surrounding plains. This is, to say the least, unusual, and there is of course a good reason for it, which I’ll get to eventually. (One will not discover that reason by reading the Holy Bible.)
After lunch at the celebrated restaurant in Meers – a charming establishment that occupies a ramshackle collage of old mining structures and serves up wonderful food and delicious locally-brewed beer in 22-ounce bottles – we headed up into the mountains to enjoy that great proliferation of wildflowers that has followed in the wake of unprecedented flooding in this geologically-fascinating region. The three hours we spent kicking around up there afforded a golden opportunity to revisit some of the unusual features of one of my favorite places on Earth.
The granite of the Wichitas has been dated to early in the Cambrian Period, about 524 MYA give or take 1.2 million either direction. That’s a very good date, established and corroborated by a number of radiometric “clocks” – various minerals (especially zircons) contained within the granite that incorporated radioactive isotopes into their structure at the time the magma chamber that produced that granite was slowly cooling under miles of overburden.
I should say a few words about why that granite came to be emplaced in the first place. A little over half a billion years ago, the part of the North American continent where I now live was subjected to extensional forces, as the crust rifted and attempted to break into two continents. This is a familiar process that can be seen all over the world. Notable examples include the separation of the Arabian Peninsula from Africa, the East African Rift Valley which proceeds in a south-southwesterly direction from the elbow formed by the Red Sea and the Gulf of Aden, and the jigsaw-puzzle match between the eastern edge of South America and the western shore of Africa. At times, these rifts “fail,” and become permanent features of a continent that remains intact. Such failed rifts are called aulocogens, and familiar examples include the Mississippi Embayment that runs from New Madrid, Missouri all the way to the Gulf of Mexico, and the Rio Grande Rift in New Mexico. The latter is the site of a great deal of volcanic activity, touched off by the thinning and fracturing of a crust that was until recently being pulled apart.
The same sort of thing happened to produce the granite of the Wichita Mountains. The Central Oklahoma Aulocogen runs from near Lawton to a little north of Amarillo, and the Wichitas’ granite was emplaced in the thinning crust as a large batholith that cooled slowly enough that the distinctive grainy texture had plenty of time to develop.
The overburden that I mentioned earlier was gradually weathered and eroded away, as the crust of continents is continually doing, and the granite was eventually exposed during the Carboniferous Period. At that time, the usual forces (rain, wind, running water, ice, freeze-and-thaw cycles and so forth) sculpted the mountain range that we see today.
The thing is, what I just described took place well over 300 million years ago. Granite mountains don’t last 300 million years: the always slightly acidic rain water that periodically bathes them reduces the granite’s feldspar to clay minerals which readily wash away, leaving sharply faceted grains of quartz sand – the small quartz crystals that were originally interlocked with the feldspar to give granite the “grainy” texture from which the rock takes its name – lying around in the wake of the process. Most of the quartz sand found on beaches worldwide is the weathering product of continental granite.
So why do we see those granite massifs today, when the weathering rate of granite guarantees that we will see none? The answer is fascinating: the Wichitas are a fossil mountain range.
During the Permian Period, that entire region of the continent subsided enough for the ocean to come in, and for well over a hundred million years, sediments poured into one of the deepest continental depositional basins on Earth from the newly-raised Ouachitas to the east and the ancestral Rockies to the west. The Wichita Mountains were buried under at least a mile of sediment, and thus preserved.
The exhumation that makes it possible to admire those mountains began during the late Cretaceous Period while the region was once again being uplifted tectonically, and continues to this day. The ancient profiles of a mountain range that had not been seen for a good slice of an aeon were gradually exposed, yielding some distinctive features – a couple of which I would like to attempt to describe.
Near the charming little town of Medicine Park, a small river runs directly through a granite outcrop. How is this possible? There is only one explanation: that river is an ancient one that used to snake all across the plains that surround the Wichitas. It was a part of that great drainage system that gradually carted away the sedimentary overburden under which the Wichitas had lain unseen for a very long time.
If a region that already contains a river is further uplifted, steepening the stream gradient and making the flow more energetic, the river will cease meandering in its floodplain and start downcutting. This is the origin of the great incised meanders one sees along the Colorado River in the upper reaches of the Grand Canyon, and also along the breathtakingly scenic Buffalo River in the Ozarks of Arkansas.
I don’t know how to describe this next step without introducing a bit of anthropomorphism, so here goes: a river that is now downcutting, and thus “trapped” in its own bed, doesn’t know that there’s solid granite beneath it. When it reaches that granite, it has no choice but to continue downcutting. This eventually results in a river that runs through solid granite, such as one sees at Medicine Park.
Another feature of the Wichitas that I’ve always enjoyed is a series of ancient stream channels also cut through solid granite. If you’ll look carefully at the accompanying photograph, you’ll realize that there are no headwaters: the exposed granite face is immediately cut by streams. This is of course impossible. The only explanation is that those channels were once part of a much larger, taller mountain face over which streams were running and carving out their steep-sided, V-shaped valleys on their way down the mountain. What’s left is only a remnant of that former system, the ancient channels now being re-used. (They’ve seen a lot re-use over the past three weeks!)
One of the most appealing features of the Wichitas is the ubiquitous rounded granite boulders that range in size from that of grapefruit to that of small houses. Such rounding is the result of subterranean weathering. As a granitic area is uplifted and the overburden removed, the granite tends to “relax” and crack into great joint-sets into which meteoric water – always slightly acidic, as I mentioned earlier – can seep and begin to weather the feldspar. It is where those joints intersect at right angles that the greatest surface area is exposed, so most of the weathering takes place at the “corners.” The result is striking: many of the smaller boulders that lie all over the surface are almost perfectly spherical, and in nearby Medicine Park a great many of the houses have been constructed by pouring formed concrete walls and rotating such granite spheres into the still-wet cement. The architectural results are certainly distinctive: a Google image search for “Medicine Park” will show you what I mean.
Another thing that granite does when exposed at the surface is called exfoliation: the granite gradually sloughs off its surface. In the Wichitas, this often yields a notable result, as lichen-covered surfaces abut pristine areas that have recently been “renewed” by exfoliation.
I want to close this account by relating a life-changing insight vouchsafed to me by the Holy Spirit about five years ago, as I was collecting calcite geodes and petrified wood in an alluvial plain near Granite (whose main economic engine appears to be a large maximum-security prison). All around me were the familiar granite massifs of the Wichitas, but the area where I was collecting was mostly flat-lying. I parked my car and hiked out into the plain along the stream channels for maybe a quarter of a mile and encountered a fairly large pile of granite boulders, whose top came in at roughly eye-level. My first thought was that someone, for some unaccountable reason, had dumped a truckload of granite boulders there. My second thought, following immediately on the first, was that my first thought was preposterous. There was no road leading to the site, and no reason whatsoever for dumping a load of granite boulders there.
A flash of instinct led me back to my car to retrieve my geologist’s hammer, which I then used to pull some of the boulders off the pile (one really doesn’t want to do that with bare hands in rattlesnake country!). The further I went into the pile, the more obvious it became that this was no “pile” – this was the top of a granite mountain whose base lay a good thousand meters beneath my feet! That’s how much Permian-Period sediment remains – roughly a kilometer’s worth – and that’s the reason one sees no foothills as one approaches the Wichitas.
It was one of life’s great moments, humbling and ennobling simultaneously: humbling, because it made the vast expanse of “Deep Time” real to me, and ennobling because it is only during an era of scientific enquiry that one can both know such things and care about them.