M.A.G.I.C. Outcrops
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The Rocky Islands are exposures of Proterozoic-age metagraywacke of the Mather Gorge Formation. Graywacke is a "dirty" sandstone deposited by turbidity currents in submarine fans (a.k.a. "abyssal fans"), which are essentially big piles of sediment in deep ocean basins. This is likely the original enviro...
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Looking at an anticline in the Ordovician-aged Edinburg Formation from Frederick County, Virginia, on the western shore of Opequon Creek, gazing across into Clarke County. Strata are Ordovician; deformation is Alleghanian (late Paleozoic). Another set of folds is just upstream: gigapan.org/gigapans/120300 ...
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Viewed from Frederick County (Opequon Creek is the dividing line), you can see strata of the Ordovician-aged Edinburg Formation limestones and shales. These anticlines and synclines formed during Alleghanian mountain-building in the late Paleozoic. Another set of folds is just upstream: gigapan.org/gigapans...
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There is one rock exposed along the Billy Goat Trail which is a bit more enigmatic. It is amphibolite, a metamorphic rock very rich in the mineral amphibole (a.k.a. hornblende), which shows up as black in this picture. The white portions of the rock are plagioclase feldspar. This is a very mafic metamorphic rock, with ...
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The Rocky Islands are exposures of Proterozoic-age metagraywacke of the Mather Gorge Formation. Graywacke is a "dirty" sandstone deposited by turbidity currents in submarine fans (a.k.a. "abyssal fans"), which are essentially big piles of sediment in deep ocean basins. This is likely the original environment where t...
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This image shows three generations of Potomac riverbed, called straths. How does a river deepen its channel? One way is through nickpoint propagation. A nickpoint develops when the river's base level drops. (Base level dropping could be caused by sea level dropping, or the land uplifting, or other reasons.) On th...
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The Acadian Orogeny took place mainly to the northeast of here. (As the name implies, Acadian-aged rocks are well-exposed in Acadia National Park in coastal Maine.) Along the Billy Goat Trail, they left one set of important structures: a series of four parallel dikes filled with the igneous rock called lamprophyre. ("L...
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- Lamprophyre Dikes on the Virginia Side, Visible From the Billy Goat Trail by Robin Rohrback-Schiavone
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The Acadian Orogeny took place mainly to the northeast of here. (As the name implies, Acadian-aged rocks are well-exposed in Acadia National Park in coastal Maine.) Along the Billy Goat Trail, they left one set of important structures: a series of four parallel dikes filled with the igneous rock called lamprophyre. ("L...
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As North America collided with the Chopawamsic Terrane during the Taconian orogeny, temperatures within the crust started to rise. As things got warmer, partial melting occured. Minerals present in the graywacke that had low melting temperatures, like quartz, potassium feldspar, and muscovite mica (felsic minerals) all...
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Devil's Racecourse in Ft. Ritchie, MD is a 700-meter-long blockfield created when the area experienced a periglacial climate during the Pleistocene. It is thought that boulders of Weverton quartzite from South Mountain on the west and Catoctin metabasalt from the east weathered out of their respective mountains an...
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