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Highland County Geologic Map

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HighlandGeoMap

This image illustrates part of the geological map of Highland County, Virginia in the Valley & Ridge province.  Note the linear belts Paleozoic sedimentary rocks that are repeated by folding.  From oldest to youngest these include O1 = lower Ordovician strata, O2 = upper Ordovician strata, S1 = Silurian strata, SD = Silurian-Devonian strata, D1 = lower Devonian strata, and D2 = middle and upper Devonian strata.  The red blobs are igneous intrusions (primarily basalt and andesite) that formed 35 to 55 Ma, and are Virginia’s youngest igneous rocks.

Micrograph of Deformed Quartz Sandstone

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This micrograph colorfully illustrates deformed quartz grains in a well-cemented quartz sandstone from the Cambrian Antietam Formation, a prominent geologic unit in the Blue Ridge geologic province.  Many of the individual sand grains are elongated (~horizontal orientation), but not recrystallized.  Photo taken in cross polarized light with the gypsum plate inserted.

Hylas Zone Mylonite

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HylasZoneMylonite

This micrograph illustrates a mylonite from the Hylas Zone in the eastern Piedmont ~5 km west of Doswell, Virginia.  The rock is characterized by a well-developed foliation with elongate quartz ribbons (transparent grains) and feldspar porphyroclasts.  The rock originally was a granodiorite that was strongly deformed and sheared, under ductile conditions, into a mylonite. Checkout this blog post for more information on the Hylas Zone. Plain light micrograph.

Cliffs of Westmoreland

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Westmoreland Cliff

The Cliffs of Westmoreland rising above the Potomac River on Virginia’s Northern Neck. photo by C. M. Bailey

The Cliffs of Westmoreland rise 30 to 45 meters (~100 to 150′) above the waters of the Potomac River.  These bluffs expose Miocene to Pliocene sedimentary units, some which are quite fossiliferous.  For more information on the geology of Westmoreland checkout the 2005 Virginia Geological Field Conference guidebook by Buck Ward and others. Note the erosion-control structures along the shore (lower right).

Nottoway Falls

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Nottoway Falls in the Piedmont of south-central Virginia

Nottoway Falls in the Piedmont of south-central Virginia. photo by C. M. Bailey

Nottoway Falls is an impressive set of cascades along the Nottoway River in the south-central Piedmont.  Here the Nottoway River forms an impressive knick zone, dropping ~30 meters (~100′) in less than 1 kilometer (0.6 mi.).  A strongly lineated granitic gneiss is well-exposed during low water conditions.  The origin of this knick zone is poorly understood.

Virginia’s State Fossil

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Chesapectean Fossil

Chesapecten jeffersonius

This distinctive scallop is the state fossil of Virginia and is the first fossil described from North America in 1687. Chesapecten sp. are commonly found in strata exposed along Coastal Plain cliffs along major rivers in southeastern Virginia and eastern North Carolina. Chesapecten jeffersonius is the index fossil for the Lower Yorktown Formation, and is distinguished by the number of ribs (9 to 12), and a rather rounded shell edge. Immature scallops attach themselves to the sea floor, whereas adult individuals are free-swimming.

Chesapecten is a lineage of scallops that flourished in the shallow seas along the Mid-Atlantic during the Miocene to Pliocene (~8 to 3 million years ago). Different species dominated during different intervals of time:  middlesexensis during the Miocene (Eastover Formation);  jeffersonius during Early Pliocene (Lower Yorktown Formation, about 4.5 to 4.3 million years ago); and  madisonius during Late Pliocene time (Upper Yorktown Formation, about 4 to 3 million years ago). Other scallops lived at the same time, but Chesapectens were the most abundant.

Old Rag Mountain

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The NW face of Old Rag Mountain, which is situated in the Shenandoah National Park. This mountain is famous for the rock scramble that is at the top.

The northwest face of Old Rag Mountain. photo by C. M. Bailey

Old Rag Mountain is nestled in the Blue Ridge Mountains in Shenandoah National Park. The trail leading to the top is one of the most popular hikes in Virginia. This is due to the fantastic views, and the scrambling nature of the hike required to reach the summit.  The large granitic boulders provide a unique rock climbing experience, and bouldering is be possible along many points in the trail. The summit tops out at 3,284 ft (1,001 m), and affords wonderful views of the Piedmont and Blue Ridge.

The mountain itself is underlain by the Old Rag Granite, a coarse-grained alkali feldspar granite, that crystallized ~1,060 Ma during the Grenville Orogeny.  Massive volumes of granitic rock intruded the Earth’s crust during the long-lived Grenville Orogeny, which ultimately formed the supercontinent of Rodinia.

For more information on the geology of Old Rag Mountain consult Paul Hackley’s U.S. Geological Survey Guidebook to the mountain.

 

Mount Rogers

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Mt Rogers

A view of Mount Rogers, Virginia’s highest peak. Photo by C. M. Bailey

Mount Rogers is the highest peak in Virginia, with its summit 1,746 meters above sea level (5,729 ft). The peak is located in the Blue Ridge province in southwestern Virginia on the border between Grayson and Smyth counties.  Mount Rogers is underlain by Neoproterozoic rhyolite.  The upper reaches of the mountain are cloaked in alpine forests of red spruce (Picea rubens) and Fraser fir (Abies Fraseri).  The peak is named for William Barton Rogers, who was educated at William & Mary, and became Virginia’s first State Geologist in the 1840s.

Fault Breccia in the Blue Ridge

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Photomicrograph of fault breccia in the Antietam Formation, Blue Ridge province. Breccias form when rocks are extensively fractured in fault zones and are cemented together when minerals precipitate in the cracks and fractures. Note the angular fragments (fr) of quartz sandstone in a matrix of fine-grained iron oxide cement (ic). Field of View 4 x 2.7 mm, Cross Polarized Light.

Photomicrograph of fault breccia in the Cambrian Antietam Formation, Blue Ridge province. Breccias form when rocks are extensively fractured in fault zones and are cemented together when minerals precipitate in the cracks and fractures. Note the angular fragments (fr) of quartz sandstone in a matrix of fine-grained iron oxide cement (ic). Field of view 4 x 2.7 mm, cross polarized light.