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Patterson Creek anticline and syncline along Corridor H, West Virginia. Folds like these are the result of ductile deformation of rock. When rock is at great depth and is under directional pressure (compression, in this case), it will bend. Why? It's quite hot down there - and so while the rock surpasses the "elastic limit" (it doesn't bounce back like a rubber band), it deforms in a "plastic" manner. It retains the shape. These folds were formed in the same way the valley anticline and Massanutten Syncline were formed - when Pangaea was formed! The mountains around us are remnants of these folds.
The formation of Pangaea 240 million years ago formed our mountains. Continents came together and formed a very long, wide, and tall mountain range. It was very similar to today's Himalaya. This is our area's third and final pulse of official mountain building. After this time, no longer would a marine basin cover Virginia. Some new sedimentary deposition would begin in parts of the Virginia Piedmont during the breakup phase of Pangaea and, during the late Cenozoic Era, the coastal plain would be the source of new sedimentary basin deposition. But, here in our Shenandoah Valley, our stratigraphic record was complete. The Alleghenies would erode to a plain by the end of the Cretaceous Period. But, the story wasn't over... Whence came our mountains?
The North Fork of the Shenandoah River from Hawkinstown to just south of Edinburg, near Red Banks. Here, the river meanders, but not in an iconic seven bends fashion. The bedrock is limestone, which pretty evenly weathers. The area us underlain by significant karst/cave deposits. Based upon the physical attributes of the river: Sinuosity: 1.35 Gradient: 0.001 Entrenchment Ratio ~ 3 Stream Type: C3-C4 it is behaving in a predictable manner that is consistent with other streams of this type in similar valleys.
The North Fork of the Shenandoah River from Seven Bends State Park to just north of Woodstock. Here, the river meanders in its iconic fashion, which is very different from the reach near Mount Jackson and other points further south. It is, however, the same behavior that occurs on the other side of Massanutten Mountain, in northwestern Page County. In both areas, the river traverses over the same rock formation, the Martinsburg Formation. Consisting of limestone that grades upward into shales, siltstones, and sandstones, the Martinsburg has beds that are very resistant to weathering and others that are not. This is called differential weathering. Coupled with the deformation of the rock that occurred during the formation of Pangaea 260-248 million years ago, the resulting weathering patterns and additional fractures in the rock provide a geological control on the formation of meander patterns. Here are the river stats for this area: Sinuosity: 3.63 (High) Gradient: 0.001 (Same as near Mount Jackson) Entrenchment Ratio ~ 3.4 (A bit higher than at Mount Jackson) Stream Type: E3-C4 The river is more entrenched here, meaning that it has essentially downcut more deeply with steeper valley walls than normal. This is expected in a situation where the bedrock geology controls the meanders. The sinuosity is really high, which is more unusual for a stream in this type of valley with the gradient/change in elevation that is present.Slide1
A classic rendition of the history of the Appalachian Mountains - our portion of the Alleghenian Orogeny. A) Pangaea begins to rift apart, forming in Virginia rift basins like the Culpeper Basin. Ultimately, our side of the Atlantic Ocean takes shape; B) By Cretaceous times, about 120 Ma later, these mountains are eroded to a plain; C) During the Cenozoic Era, ~50 Ma, tectonic upwarping from below raises our area, rejuvenating the land; D) Since that time, changes in Cenozoic climate have weathered and eroded the mountains to what we see today, including the mountain on which this image was taken!
There have been two primary hypotheses posited for the remains of the Appalachian Mountains still existing: 1) The Appalachian Mountains have lasted longer than normal because they have eroded more slowly than would be normal. They are more resistant. 2) Upwarping of mantle material below the passive eastern margin of North America caused rejuvenation of the land. In the image above, taken from Mazza et al. (2014), provides a cross-sectional model of what this might have looked like. Eocene Epoch volcanics in the Shenandoah Valley attest to some kind of igneous activity having taken place in our region. Using computer modeling, Liu (2014) demonstrated that tectonic processes occurring under the western half of North America at the time could have provided the means for this. During the Eocene (56.5-35.4 Ma) - Mantle activity causes arrays of igneous intrusions, including dikes and some volcanoes, to emerge in the Shenandoah Valley; During the Miocene Epoch (23.3-5.2 Ma) - Uplift and rejuvenation of land in our area raised up the Appalachian Mountains as we see them around us today (Gallen et al., 2013); During the Pliocene and Pleistocene Epochs (5.2 Ma to 11,700 ka) - Intense erosion, likely due to a very wet and much colder environment, causes today's relief to come into existence.
By 300 Ma, The Taconian Mountains were long gone and our area was in the midst of a new mountain-building event, the Acadian Orogeny. By this time, Virginia had migrated a bit more northward and we were sitting along a north-facing coastline of a large, shallow marine basin.Lidar
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