Author Archives: mkdonohue

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.

Virginia’s State Rock: Nelsonite

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Nelsonite-scalebar

On July 1, 2016, Governor Terry McCauliffe signed a bill into law that made nelsonite the first official state rock of Virginia. The initiative for this project was led by students from Piedmont Virginia Community College.  Michelle Stanislaus and her classmates from their Historical Geology class and Government class ran the petition for this law starting in the fall of 2015. Why pick Nelsonite?  Nelsonite has a unique historic role for Virginia, as it served as a key economic resource in the early 1900s after it was first discovered near Roseland in central Virginia.  Nelsonite’s type locality is Nelson County, Virginia and is one of the few rocks that is named after a state county.  Nelsonite is a distinctive igneous rock composed primarily of the minerals ilmenite and apatite, and as such it’s rich in both titanium and calcium phosphate.  Titanium is used in paint pigments and steel alloys, whereas the calcium phosphate was used as agricultural fertilizer and even as a filler for artificial teeth. Although Nelsonite is no longer mined in Virginia, mining is still active in parts of China for its rare and highly useful minerals.

Nelsonite occurs as both dikes and segregation layers in the 1.0 to 1.2 billion year old granitic gneisses of the Blue Ridge province.  The rock is medium-grained and comprised of ilmenite (FeTiO3) which is a black metallic and slightly magnetic material, and apatite (Ca3(F,Cl)(PO4)3) a light-colored, non-metallic mineral with small amounts of rutile scattered throughout.  Nelsonites are magmatic in origin. The iron-titanium-phosphorous-oxide magmas from which they crystallize result from liquid immiscibility within a granitic or anorthositic magma, this is followed by intense fractionation of the parent magma to produce the nelsonite.

At the Pluton’s Edge

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When large quantities of magma intrude and solidify in the Earth’s crust they form bodies of intrusive igneous rock known as plutons. The featured image nicely illustrates the edge (geologic contact) of a granitic pluton in the Blue Ridge Mountains of central Virginia.

PWCcontact

Exposed geologic contact between ~1 billion year old granitic gneiss of the Blue Ridge basement complex (left) and the 706 ± 4 million year old granite of the Polly Wright Cove pluton (right).

The granite is part of the 706 ± 4 million year old Polly Wright Cove pluton that intruded into older rocks, ~1.0 to 1.1 billion year old granitoid gneisses, of the Blue Ridge basement complex. The Polly Wright Cove pluton occupies an area of ~9 km2 in Nelson County, about 5 km north of Lovingston.

NEWPollyWrightCoveMap

Geologic map of the Polly Wright Cove pluton in Nelson County, Virginia (from Bailey and Tollo, 1998)

Prior to the intrusion of the Polly Wright Cove pluton, the space into which the pluton would intrude was occupied with older rocks (known as the country rock). What processes worked to “create the space” into which the Polly Wright Cove magma would intrude and crystallize?

Did the magma wedge apart the country rock? Or perhaps the hot buoyant magma rose upward while the cooler country rock sank deeper into the crust, by a process similar to what happens in lava lamps?

The Polly Wright Cove pluton consists of two varieties of granite. The margins of the pluton are composed of fine- to medium-grained biotite granite that is variably foliated with aligned biotite and plagioclase phenocrysts. The interior of the pluton consists a of younger massive (unfoliated) leucogranite

.PWCxenolith

Detailed geologic mapping of the Polly Wright Cove pluton demonstrates that it contains inclusions (xenoliths) of the older country rock completely surrounded by granite. Some of the inclusions are hundreds of meters in length (discernible on the geologic map), whereas others are 10s of cm in length. Nearly all of the xenoliths are elongate in two-dimensions (with aspect ratios from 3:1 to 25:1). The long axes of xenoliths are always oriented northeast-southwest, and parallel to the overall shape of the Polly Wright Cove pluton.

Sheet-like projections of granite intrude the country rock at the both northeastern and southeastern ends of the Polly Wright Cove pluton. Extensional ductile faults zones (high-strain zones) are localized in these regions as well.

Bailey and Tollo (1998) interpreted the Polly Wright Cove pluton to have formed as a series of sheeted dikes intruded into the Blue Ridge country rocks during a period of crustal extension. As dikes coalesced the pluton grew. Elongate blocks of country rock were caught between dikes and completely surrounded by the magma. Extensional ductile fault zones created dilational voids which further enhanced magma emplacement.  The younger and massive leucogranite at the center of the pluton did not form as dikes and was perhaps emplaced by different mechanisms such as stoping or ballooning.

For Further Reading:

Bailey, C.M. and Tollo, R.P., 1998, Late Neoproterozoic extension-related magma emplacement in the Central Appalachians: an example from the Polly Wright Cove Pluton: Journal of Geology, v. 106, p. 347-359.

http://www.journals.uchicago.edu/doi/abs/10.1086/516027

field photos of plutons

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.