For the next few weeks I’m going to shift gears and focus on a subject that has long exerted a peculiar attraction for me. That subject is geology, an endlessly fascinating and notoriously complicated field of study to which I lay claim to no formal credentials beyond my having taken the introductory undergrad class decades ago. But as most of us understand, formal credentials aren’t everything: we’ve all known autodidacts who have become deeply conversant with their late-found passions. (With absolutely no arrogant comparison of stature intended, I point out that Johann Wolfgang von Goethe was not only the most eminent German-language playwright of his time: he was also a dedicated amateur botanist whose researches earned him the honorific “father of plant morphology.”)
As you might anticipate, my thinking about geology is deeply intertwined with my experience of childhood religious indoctrination, adolescent dysfunction, and the hard work of fighting my way out of that swamp of delusion and lies in which I was planted at birth. If you have read any of my previous posts, you already understand what I’m talking about. In addition to being an endlessly fascinating subject, geology is a corrective to many of the misconceptions I held earlier in my life.
I was reared in a small town in the Arkansas River Valley. From any number of vantages within the city limits, I could see the Ozark Mountains rising to the north and Mount Magazine – the vanguard of the Ouachitas – to the south. Those gentle topographies struck me as impressive during my childhood; since then, however, I have traveled and accrued some perspective. The Ozarks at their highest only rarely exceed 2,000 feet above mean sea level, and their local relief is no more than 500 meters. The same is true of the Ouachitas, with even Mount Magazine – the highest point in Arkansas – barely topping 2,700 feet. In fact, Mount Magazine (named for its profile) would, if it were located farther west and divested of its flora, be clearly seen for what it actually is: a long, low mesa. To a child growing up in an insular environment, however, these imposing prominences were demonstrations of the greatness of God and examples of his glorious handiwork.
I’m pretty sure that if someone had given me a gentle nudge in the right direction when I was young, I would probably have become a scientist of some sort. I like to think that the science of my choosing would have been geology. As it is, I grew up thinking that the Earth is about 6,000 years old, that the mountains that surrounded me were poofed into existence pretty much as we see them now in an instantaneous creative fiat, and that the fossils one finds in the Ozarks were more than likely washed up there during Noah’s Flood. As in so many other ways, childhood indoctrination by the ignorant and mendacious caused me to miss what I might later have reckoned to be my proper vocation. Instead, I ended up pursuing the only thing I could do halfway well, and I thus experience geology as an outsider looking in and wondering how he could have missed the invitation to such a splendid party.
Even though the Ozarks are called “mountains,” they are not proper mountains in the sense that geologists use the term. The area is actually a vast, dome-shaped plateau dissected by deep valleys carved by such rivers as the White, the Buffalo, the Kings, the Elk, the Current, and so forth. When one stands in the valleys and looks around, the erosional remnants that surround those valleys give the impression of mountains. When one looks at the strata exposed in roadcuts, stream banks and quarries, however, one sees (with very few local exceptions) layers of limestone, sandstone and shale that are as flat-lying as they were when they were deposited on the ocean floor over 300 million years ago. The topography of the Ozarks is a mere accident of erosion. Were it not for those rivers and their complicated history, plus the normal faulting that has caused great blocks of material to drop relative to their surroundings (the Arkansas River Valley being Exhibit A), the region would be as flat as the Texas Panhandle (which is considerably higher in elevation than the Ozarks).
In this post, I want to talk about the Ozarks primarily, but since I’ve made mention of them, I need first to spill a little more metaphorical ink over the Ouachitas.
The Ouachitas, low-lying as they are currently, are true mountains. They are in fact a small part of a large fold belt that includes the Appalachians and, in the region of Big Bend in southwest Texas, the Marathons. All of those mountains were raised some 300 million years ago in the continental collision that formed the supercontinent Pangaea. That collision also raised the Ozark Plateau (I’m now giving it its proper name), but in a much gentler fashion. A modern analog can be seen where the collision of the Indian subcontinent in its ongoing collision with Asia has raised (is raising, actually) the Himalayas, with the Tibetan Plateau lying to their north. In high contrast to the Ozarks, the Ouachitas are steeply folded and riddled with thrust faults. They are much more difficult to “read” than the Ozarks. They are also much more interesting mineralogically-speaking.
I will return to the Ouachitas and their mineral richness in a later essay, but for now I want to focus on the Ozarks. The Arkansas Geological Survey divides the Ozark Plateau into three sub-regions: the Boston Mountains, the Springfield Plateau, and the Salem Plateau. I have listed the three in order from highest to lowest. The Boston Mountains lie almost entirely within Arkansas’ boundaries, trailing off for a short distance into eastern Oklahoma. The Springfield Plateau extends well into Missouri and is named for the largest city in the southwestern part of that state. The Salem Plateau, named for a small town in northeast Arkansas, also extends into Missouri.
With the exception of the St. Francois Mountains in east-central Missouri, which would have appeared as a mostly granitic island during the Paleozoic Era, the entire Ozarks region was a great continental shelf. As is true of continental shelves generally, the region was sometimes inundated, sometimes exposed. Those periodic inundations and exposures can be attributed to a number of factors: crustal flexing due to isostatic adjustment, falling sea levels during periods of extensive glaciation, and so forth. That up-and-down situation prevailed for at least 300 million years prior to that uplift that occurred some 300 million years ago, which raised the area finally and forever (at least so far) beyond the reach of Poseidon. Thus we have a marine fossil record in the Ozarks that encompasses most of the Paleozoic (only the Permian Period is completely missing) but riddled with unconformities – places where portions of the planet’s history are missing from the local depositional record. This makes the Ozarks both a challenge to read, and a useful place to study the evolution of benthic organisms over a vast period of time, yielding its fossil information in discrete chunks. In the Ozarks, one is more likely to note sharp contrasts in the fossil record than smooth transitions.
The lowest elevations in the region under examination are those of the Salem Plateau. These are also the oldest rocks exposed at the surface in the Ozarks, consisting mostly of Ordovician-age dolomite. This dolomite (more properly called “dolostone” to distinguish it from the carbonate mineral known as dolomite) is a diagenetic product in which the calcium of limestone has been gradually replaced by magnesium. In that process, fossils tend to disappear. Fortunately, there is enough unaltered limestone in the region to give us a fairly good idea of the marine fauna of the time. Moreover, these rocks are closely correlated with rocks lying much farther to the east, so it’s pretty easy to flesh out the picture (the Cincinnati Arch, celebrated for its fossil richness, is also of Ordovician age and represents a similar depositional environment).
In the Springfield Plateau, which consists mostly of Mississippian-Period limestone, the fossil record is much more complete. Many of the limestones of northern Arkansas consist almost entirely of the fossilized fragments of animals that lived from 360 to 320 million years ago (they are mostly clastic limestones rather than chemical limestones), and the depositional record, despite periodic interruptions when sea level was low, is more complete than in the lower-lying Salem. The Mississippian Period is sometimes referred to as “the age of crinoids,” as those stemmed echinoderms dominated the ocean floor as at no time before or since. Walking along an exposure of crinoidal limestone in northern Arkansas is like looking at a snapshot of an ancient ocean floor.
The highest area of the Ozarks – the Boston Mountains – is so because of a hard, dense Pennsylvanian-Period sandstone that caps the region. That sandstone, comprising the Atoka, McAlester, Hartshorne and Savannah Formations (all named for type localities in Oklahoma) is the caprock of both the Ozarks and Ouachitas (mostly missing from the latter, as I will explain in a future essay). It is the Boston Mountains that I viewed with admiration as a child, and visited every chance I got, and that eventually raised troubling questions in my thoroughly-washed brain.
In contrast to both limestone and shale, sandstone usually represents a near-shore deposit. Depending on the amount of abuse it’s taken in the surf, sand can range from quite coarse to almost microscopically fine. In the case of the latter, many of the grains are actually the ground-up shells and other hard parts of sea-dwellers. The sand may be cemented by various agents, also. In the case of the sandstones of the Boston Mountains, their extremely tiny clasts are well-cemented by silicate cement enriched by hematite. The result is some of the hardest, densest sedimentary rock imaginable, and in those areas where that rock has remained unbreached by faulting, its resistance to mechanical weathering and erosion has kept the region high. That, in a nutshell, is the story of the Boston Mountains. The lower reaches of the Ozarks – those of the Springfield and Salem Plateaus – represent areas where the sandstone caprock was never as hard and dense as it is in the Boston Mountains, or where it was breached by fractures that enabled ground water to eat its way through to the underlying limestone – a process that finally obliterated the sandstone cap and yielded the karst topography of the Springfield and Salem Plateaus. (The splendid limestone caves of northern Arkansas and southern Missouri are a renowned tourist destination.)
The sandstone I’ve described isn’t a very good medium for fossilization, and since my hometown sits on sandstone of the McAlester Formation, I didn’t really grow up with fossils. (Had they been readily available, the forbidden questions might have arisen in my mind earlier than they did.) Most of the local fossils I grew up seeing were not those of marine animals, but those of plants that grew in the estuaries of receding oceans. It is in that great swampy stretch of the ancient North American continent that the extensive coal beds of the Carboniferous Period (comprising the Mississippian and Pennsylvanian) were laid down. During my childhood, I was also surrounded by the many strip mines by which the Philpot Coal Company desecrated the landscape, and in the waste piles of those mines one could often pick up the fossilized roots of large tree-like ferns (Stigmaria). These were fascinating fossils, but not overly troubling: I could imagine their having been caused by Noah’s Flood inundating forests not unlike those I knew as a child.
I’ll close this account with a sidebar: in a no doubt laudable attempt to restore the region’s natural beauty, most of the strip mines have been filled in by shoving the tailings into the voids whence they came, and planted over to erase all traces of such invasive activity. This no doubt represents an aesthetic gain of some kind. Cattle now graze where fish once swam, amphibians laid their glistening clusters of eggs and migratory waterfowl found temporary respite from their exertions: artificial wetlands have been obliterated (why? so we can feel a little better about our planet-eating selves?). All of those fossils have disappeared into the chasms from which they were dug. I’m not convinced that we’re actually better off for the “reclamation.” The real damage has been done and is irreversible: that ancient, safely-sequestered carbon has been burned, its oxides released into a rapidly warming atmosphere that is going to give us and our progeny no end of grief in the decades ahead. We humans are awfully good at cosmetic fixes, papering over, sweeping under. It probably shouldn’t escape our notice that cosmetology – in high contrast to cosmology – is the science of denial.
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