Category Archives: featured image

The Skippers Pluton

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NaturalSkippersOutcrop

A natural outcrop of the Neoproterozoic Skippers granite exposed in the eastern Piedmont, Greenville County, Virginia. The exposure, at the edge of a clearcut, is composed of more-erosionally resistant core stones that are elevated above the soil. William & Mary geologist Ace Yuan atop the largest core stone. Photo by B.E. Owens.

Finding bedrock outcrop in the Piedmont of Southside Virginia can be challenging.  Much of the terrain is covered by soil and saprolite, and geologists interested in this region must search high and low for outcrop.  The image above illustrates a ‘good’ natural outcrop in the Piedmont, likely the result of a relatively resistant rock that is only weakly fractured.  These are core stones of unweathered bedrock that are separated by …

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Simplified geologic map of the Skippers pluton near Emporia, Virginia. The pluton is covered, in part, by Cenozoic sediments of the Coastal Plain, and intrudes older metamorphic rocks

Simplified geologic map of the Skippers pluton near Emporia, Virginia. The pluton is covered, in part, by Cenozoic sediments of the Coastal Plain, and intrudes older metamorphic rocks.

Sea Level Change at Sewells Point

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SewellsMeanMonthlySeaLevel

Each blue square is the mean monthly sea level elevation. The red line is the linear mean sea level trend. Data from NOAA.

The level of the sea has been measured at Sewells Point, Virginia continuously since 1928.  In 1928 the mean level of the sea was ~45 cm ( ~18″) lower than it is in 2016.  Sea level rise and regional land subsidence is creating an ever-growing problem in Hampton Roads and most of coastal Virginia.

Chopawamsic Gneiss

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ChopGarbenSch

Complexly deformed gneiss in the Chopawamsic terrane near Cuckoo, Louisa County. Photo by C.M. Bailey

This exposure in a pasture near Cuckoo exposes a poly-deformed hornblende and plagioclase-rich gneiss. Foliation in the gneiss has been refolded.  Another interesting feature is the presence of garbenschiefer or “feather amphibolite.” This unusual texture is defined by stellate and featherlike hornblende clusters in the plane of foliation. Three amphibole species occur in this rock with hornblende and cummingtonite dominating the mafic layers and actinolite, as a late stage mineral overprinting the earlier fabric. The two-stage amphibole growth is evidence for two phases of metamorphism.

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Close up view of stellate hornblendes forming garbenschiefer.

The Appalachian Plateau’s rugged terrain

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In Virginia the Appalachian Plateau province is not particularly ‘plateau-like’, rather the region is characterized by rugged topography and steep slopes.  It is a well-dissected upland, and streams draining the region have a classic dendritic drainage pattern.  The featured image of Buchanan County (and the surrounding areas of Kentucky and West Virginia) is a shaded relief map that illustrates the aforementioned landscape features of the Appalachian Plateau. Green colors are elevations ~200 meters above sea-level (~650′), while orange colors are elevations ~700 meters above sea-level (~2,300′).

The Defenses of Yorktown

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Coastal defenses at Yorktown

Modern coastal defenses at Yorktown, Virginia. 2015 image from Google Earth.

Yorktown became a famous locale in 1781 after the surrender of the British troops which effectively ended the Revolutionary War.  British forces in and around Yorktown dug a number of defensive earthworks that were sieged by American and French troops over a three-week period prior to the surrender on October 19th, 1781.

In the early 21st century Yorktown acquired a new type of defense, coastal structures designed to protect the shoreline in Yorktown from erosion.  These structures include hard structures, such as breakwaters and groins composed of massive blocks (rip-rap) placed in the nearshore that are intended to break waves before they erode the shoreline and trap sand landward of the structure.  The sandy beach at Yorktown is not natural, rather it’s the result of beach replenishment– sand was brought in from elsewhere to create the modern strand.

Time will tell whether these defenses are up to the challenge of coastal erosion and sea-level change.

Breakwater at Yorktown composed of massive quarried blocks (rip-rap) to prevent coastal erosion. William & Mary geologists for scale.

Breakwater at Yorktown beach composed of massive quarried blocks (rip-rap) to prevent coastal erosion. William & Mary geologists for scale.

The Cumberland Gap

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The Cumberland Gap

Annotated photo of Cumberland Gap, a wind gap near the geographic triple point between Virginia, Tennessee, and Kentucky. View to the west from the Pinnacle Overlook. Photo by C.M. Bailey

Cumberland Gap, elevation 506 m (1,660′), in the far southwestern corner of Virginia is a wind gap near the geographic triple point* between Virginia, Tennessee, and Kentucky. The gap has long been an important crossing point through the Appalachian Mountains, and lies at the western edge of the Valley & Ridge province with the Appalachian Plateau province to the northwest.  Rocks that underlie the gap are tilted strata of middle to late Paleozoic age.

*Virginia has 4 geographic triple points- can you name them?

Twenty years of change at Cedar Island

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VABarrierIslands

Cedar Island is one of Virginia’s barrier islands that separates the Eastern Shore from the Atlantic Ocean. Barrier islands are dynamic environments that respond quickly to environmental changes. Over time barrier islands move, and their shorelines migrate. During the past two decades shoreline change at the south end of Cedar Island has been dramatic. Compare the 1994 aerial imagery to the 2014 aerial imagery (below). Cedar Island’s southern tip has eroded northward by more than a kilometer, and the shoreline has retreated ~400 meters to the west.  Wachapreague Inlet has gotten larger, and Parramore Island’s northern edge has been relatively stable.

1994 Wachapreague Inlet

Wachapreagure Inlet 2014

Geoscientists have long known that Cedar Island is a particularly mobile barrier island. Yet, Cedar Island is privately owned, and in 1990 dozens of houses were permitted (by the Virginia Marine Resource Commission) to be built on the barrier island.  Another example of scientific evidence being tossed aside by policy makers.  By 1997 27 homes had been constructed facing the Atlantic Ocean. In 2014 the last houses on Cedar Island had succumb to the shifting sand and moving island.  A legal quagmire continues as property owners are still taxed (albeit at a low rate) over lots that are now fully submerged beneath the Atlantic waves.

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Coastal erosion at Cedar Island, Virginia. U.S. Geological Survey photo