The Ouachita orogeny occurred ~300 million years ago when part of South American collided with the southern part of North America.
In this model Phil Prince takes us to the Ouachita Mountains of Arkansas and Oklahoma. Here you will see how the strength of different rock layers coupled with deep erosion can produced the characteristic shape of this range. Sinuous ridges formed from chert snake around valleys scooped out of shale abutting a broad sandstone plateau.
In this first part of a four part series, Dr. Phil Prince explains why we get the valleys and ridges that are the namesake of the Valley and Ridge province of Virginia.
Valleys and ridges can result from the erosion of anticlines and synclines. Knowing the ages of the rock layers can help determine, which you’re looking at when the topographic profiles have been worn away through time.
When rock strata are folded and produce an anticline, or “positive topography,” if then worn down from the top, older rocks will be exposed in the center. The opposite is true for a syncline.
Notice in the cartoon above that the original anticline and syncline are not expressed topographically. Instead two parallel ridges are present where the yellow, and presumably less-erodible strata intersect with the surface.
In this first video we get to see how geologic information overlaid in Google Earth can help illustrate this concept.
Strike-slip faults often seem like the least complex faults out of the bunch compared to thrust and normal faults, but it turns out that there’s a lot more to be said (and modeled)!
Strike-slip faults are where the crust is sliding past one another and can form linkages between areas experience convergence or divergence. Where strike-slip faults bend things get interesting as it creates zones of compression (called restraining bends) and tension (called releasing bends).
This video shows the development of a restraining and a releasing bend and then shows a cross-section of the model highlighting the different flower structures (for more information on flower structures see our previous post).
A real world example of where you can find these structures is the fault system responsible for the Mw 7 earthquake that affected Haiti on January 12th, 2012.
An exhibit called “The Poetry of Place” at the Greenville (South Carolina) Art Museum showcases a number of works portraying various settings in the southeastern United States, with particular emphasis on 19th century landscape painting. The majority of the pieces exhibited develop their “sense of place” through vegetation, overall appearance of topography, and, when applicable, representations of distinct architectural styles. Most of these details of setting are very directly stated and figure prominently within the paintings; Spanish moss, cypress trees, and standing water, for example, suggest coastal plain lowlands, while hills and rapidly flowing water immediately suggest uplands further inland. Two paintings, however, reached beyond these generalized indicators by portraying rock outcrops in accurate, identifiable detail consistent with the reality of the painting’s subject. As you might imagine, the painters were not Earth scientists (although one was connected to mining operations), and they would not have understood the process history of the outcrops they rendered in a world yet to understand plate tectonic theory. They were also painting at a time when the most realism-oriented landscape art was expected to carry a certain degree of exaggeration, with the purpose of creating a more awe-inspiring finished product. Even so, the two paintings I describe here preserve very interesting and truthful characteristics of their Appalachian settings through the representation of humble rock outcrops exposed, like any good Appalachian field site, along incised rivers.
William Charles Anthony Frerichs and William Louis Sonntag are not particularly well known artists (no offense, guys), and their respective renditions of a rapid in Linville Gorge and an unnamed river valley in West Virginia do not figure prominently within their body of work. The paintings themselves are large scale, dramatic landscapes in keeping with mid-19th century style, and the outcrops to which I will direct your attention are little more than details within the overall composition of the works. The rocks do, however, stand out to a geologist familiar with the subject areas as clearly having been produced by artists who were there, in person,and who obviously appreciated subtle natural features that helped to define the unique character and “feel” of their subject. The detail of these paintings is even more interesting in the geographic context of the painters’ professional lives, and I provide you with (very) abbreviated, internet-homogenized career summaries to supplement the images of what they painted and what I think they were trying to paint. These painters and their other works are certainly worth a Google Search, and their subjects–Linville Gorge, North Carolina and Kanawha Falls, West Virginia– deserve a visit on Google Earth and Google Maps Terrain.
The metamorphic Blue Ridge explorer who knew good quartzite when he saw it
William Charles Anthony Frerichs (1829-1905) spent the first 21 years of his life in Belgium and The Netherlands, where he trained as a landscape artist focused mainly on clouds and skies (a product of his Low Countries base of operation). He immigrated to New York in 1850 and began an association with the Hudson River School, delving into the representation of topographically dynamic settings and patterning his own work after the group’s distinct style of natural realism combined with a limited degree of Romantic exaggeration. After relocating to North Carolina to accept a position as professor of art and language at Greensboro Women’s College, Frerichs began to draw inspiration from the landscape of the southern Appalachian Blue Ridge west of Greensboro. His works began to show increasing influence of landforms in both the Blue Ridge and Piedmont, although his time in New York continued to manifest itself through an uncontrollable to desire to insert fictitious lakes into the pre-reservoir southern Appalachian landscape. This bit of license aside, it appears Frerichs sought to faithfully reproduce many aspects of what he saw, and I believe that his Hunters in Linville Gorge (ca. 1860) reflects a genuine interest in capturing the subtle, yet defining, details of the deeply incised Linville River in Burke County, North Carolina . The gray to tan outcrops at the right of the painting are produced with a caeful series of parallel brush strokes and, given the size of the hunters and trees, suggest a layered rock with well-defined partings spaced several inches to a foot or so. The outcrop shows gentle folding or warping and an orientation different from the outcrops across the river on the left of the painting. In my geologic opinion, these outcrops bear an uncanny resemblance to Cambrian quartzites exposed in the upper reaches of Linville Gorge, particularly at a significant rapid known to kayakers as Babel Tower Falls.
Linville Gorge quartzites are exposed within the Grandfather Mountain Window, an extensive feature produced by erosional removal of the overlying gneiss- and schist-dominated thrust sheets. Rocks within the window are part of a large but localized structural horse block carried within the Blue Ridge Megathrust system. The horse, consisting of thoroughly metamorphosed quartz-rich sediments and a bit of underlying basement, locally arched the overlying thrust sheets, causing them to be completely eroded away in the arched zone while still persisting in neighboring structural lows. A bizarre but potentially illustrative analogy can be constructed with a rug, a football, and a riding lawnmower (don’t stop reading). Imagine placing the football on the ground and dragging the rug, representing the Blue Ridge-Piedmont megathrust sheet, across the football, which represents the horse block. The football produces a bump in the rug, locally arching a portion of the rug above its lower surroundings. Now drive the riding lawnmower (an erosive instrument) across the rug. The first place that the rug will be completely cut away is atop the football, where it is arched highest. You will then be able to see the football through the “window” in the rug. The Grandfather Mountain Window exposes the football , so to speak, while the rug (schists and gneisses of the Blue Ridge thrust sheets) persists all around it. Just like the football is clearly different from the rug, the rock in Linville Gorge looks very different from exposed gneissic and schistose rock in surrounding areas (particularly just to the northwest, outside of the window), and visitors and regulars alike quickly take note of the unique landform style supported by the extremely erosionally-resistant quartzite. Were 19th century visitors struck by the unique character of Linville features as visitors are today? Perhaps Frerichs was, leading him to apply what I think is a very specific and intentional style in the painting of the outcrops by the falls.
Now let’s step back for a moment…Do I have any direct evidence that Frerichs was observing, or at least thinking about, the characteristic quartzite outcrops when he painted Hunters in Linville Gorge? Indeed I do not, and it would be reasonable to ask if I am letting my own geologist’s perspective lead me down a path of over-interpretation and assumption. This is possible, but an equally worthwhile question is whether Frerichs painted all the rock in his North Carolina paintings in the same way as the outcrops in Hunters. I could deflect this question by telling you that, regrettably, many of his works were destroyed by fire in 1863…but I don’t have to. Fortunately, enough of his paintings survive to definitively say that he does not paint rock outcrops in a single style. Interestingly, his paintings show a variety of approaches to outcrop painting, with a blockier, joint-intersection style that does not emphasize foliation-related features being fairly common in his other southern Appalachian works. Some paintings do clearly portray foliated rock, but in a style distinct from Hunters in Linville Gorge. Frerichs’ apparent appreciation of outcrop character was not limited to metamorphic southern Appalachia. His Little Falls on the Passaic River (New Jersey) shows a completely different type of rock, as well it should, given its Triassic rift basin setting. In the context of these surviving works, it is indeed interesting to consider that Frerichs was seeing, appreciating, and recreating the “sense of place” developed by bedrock exposures in the locations he painted. This idea is supported by the fact that Frerichs worked as an artist and inspector of mines with the Confederate Corps of Engineers. In this capacity, he would surely have been familiar with defining characteristics of many different rock types, enabling him to appreciate and recreate subtle details key to developing the “feel” of a regionally unique setting like Linville.
The Appalachian Plateau native familiar with jointed sandstone (we should worry if he wasn’t)
Another notable work presently displayed in Greenville is View in West Virginia, by William Louis Sonntag (1822-1900). Sonntag certainly had extensive experience in the Appalachian Plateau setting of the painting, having been born in Pittsburgh and trained as a landscape artist in Cinncinnati, Ohio. He was another member of the Hudson River School of landscape painters, and aspects of his style reflect the same interest in a significant degree of realism “enhanced” by careful exaggeration of impressive natural features. Sonntag spent some time painting in Italy, but he moved permanently to New York in 1856 and established himself a leading American landscape artist of the period. He produced numerous region-specific landscapes of east coast subjects, including the James and Shenandoah Rivers of Virignia and the upper Hudson River of the Adirondacks. West Virginia was also a popular subject for him, and would have been relatively easy to access by major rivers from his early base in Cincinnati.
Painted around the same time as Hunters, View in West Virginia captures the same Hudson River School inspirations as Frerichs’ work, and also shows an impressive appreciation for minor details of setting. Sonntag took great care to reproduce the nearly right-angle corners at the edge of the broad, flat outcrop in the foreground of the painting, and this outcrop style is quickly recognizable to anyone who has spent much time in rivers of the Appalachian Plateau in West Virginia.
The Appalachian Plateau province represents the undeformed, and now uplifted and dissected, Appalachian foreland basin. While not deformed by orogenic stresses in the same way as units further east, Plateau rocks do show the effects of stress field shifts over time through the presence of several sets of fractures, known as joints, along which no displacement has occurred (a fracture with displacement in the fracture plane is a fault). Joints reveal the orientation of past stress fields, and the presence of several joint orientations within a region may hold valuable clues to tectonic history in the absence of displacement-related tectonic structures such as folds or faults. In the Appalachian Plateau, two main joint sets have been extensively studied (Engelder et al., 2009 provides an excellent overview). One set is northeast-southwest trending, and is inferred to relate to fluid overpressure caused by hydrocarbon generation during burial of the units to appropriate depth. A second, younger joint set, trending generally northwest-southeast, is believed to reflect tectonic stresses generated by encroachment of the growing Appalachian tectonic wedge on its foreland basin sediments. These two joint sets intersect at angles approaching 90 degrees, producing conspicuously shaped corners and recesses, particularly in strong, thickly-bedded sandstones. Other still younger joint sets may reflect the effects of Mesozoic extension, flexure due to loading of the crust by ice sheets to the northwest, or more recent tectonically-induced stresses. Regardless of origin, erosional result of these many intersecting joints in flat-lying sedimentary rock is a distinct, “geometric” outcrop pattern that is readily apparent at scales up to many 10’s of meters. The cleanly-broken planes and oddly geometric patterns are certainly as eye-catching to today’s Plateau visitor as they were to Sonntag, and probably caused him to wonder what sort of natural process could produce such neat surfaces and sharp corners.
Beyond the classically Appalachian Plateau outcrops, the painting offers few clues to place Sonntag in a specific West Virginia setting. If the body of water in the painting is indeed intended to be a river, its size, the depth of its incision into the landscape (which, admittedly, may be somewhat exaggerated), and the thickness of the widely-jointed beds suggest to me that Sonntag was influenced by the New (Kanawha) River at the northwest (downstream) end of the New River Gorge. The joint spacing and bed thickness of the strata suggest the rocks are probably Pennsylvanian sandstones of the Kanawha Formation, which are very capable of supporting bold, cliff-style outcrops. A similar widely jointed, thick-bedded outcrop style is visible today mid-river at Kanawha Falls, but any similar outcrops along the banks of the river, if they did exist, have been blasted for railroad construction. The appearance of the rock is not consistent with the upper reaches of the New in southeastern West Virginia, where more thinly bedded Mississippian units fail along closer spacing of the same joint sets. These units produce more “ragged” outcrops that cannot support a few meters of clean, vertical joint surface as depicted in the painting. These Mississippian outcrops also tend to show minor, low-amplitude folding due to their position at the very leading edge of the Appalachian orogenic wedge. The stress field that produced the gentle folding increased overall damage to the rock, further limiting its potential to support large, clean joint surfaces after block failure.
The New (or Kanawha, in its lower reaches) River is certainly not the only option for Sonntag’s inspiration; the lower Gauley, Elk, Little Kanawha, Cheat, and Tygart might also be viable candidates, and he represented the Tygart in Scene near Grafton, West Virginia. The sheer size of the channel rendered, however, along with its depth of incision, is not consistent with these smaller streams. I would also suggest that geography favors Sonntag encountering the New/Kanawha during the early part of his career. Sonntag completed much of his training as an artist in Cincinnati, Ohio, and a trip up the Ohio River towards West Virginia would have reached the mouth of the large Kanawha River first, providing an easy route into the interior of the Appalachian Plateau and encounters with plenty of Pennsylvanian sandstone outcrops.
The deliberately vague title of the painting might also suggest that it does not depict a specific river at all, instead showing a composite of observations made throughout the Ohio River basin in West Virginia. A review of Sonntag’s landscapes shows remarkable consistency in their layout, with high bluffs to the right, water at center or left, and light originating from the left. Sonntag appears to have enhanced this basic composition with discernibly “local’ details (rock structure, vegetation, rapids or flatwater, etc.) to produce generally appropriate representations of a variety of settings. Ultimately, however, the particulars of location are not what makes this painting interesting to an Earth scientist. Accurate depiction of the outcrops in the painting clearly indicate that Sonntag had actually “been there” prior to completing the work, and that he, like Frerichs, most definitely tailored his representation of rock to specific settings. View in West Virginia, however, shows exceptional attention being directed towards rock, perhaps because jointed sandstone was such a familiar and region-defining feature to Sonntag.