Geology of Virginia

The geologic time scale is the “calendar” for events in Earth history. It subdivides all time into named units of abstract time called—in descending order of duration—eons, eras, periods, epochs, and ages. So on the geologic map, when there is a geologic unit named Cretaceous, that means it formed during the Cretaceous period about 145.5-65.5 million years ago during the Mesozoic era which is part of the Phanerozoic eon according to the scale on the right.

You can learn more about the geologic time scale and the history of our planet using the interactive geologic time scale to the right.

Geology 101

This story map is aimed at 8th grade Earth Science students and is going to assume that students are already familiar with the rock cycle and plate tectonics. As a brief refresher, here is a simplified diagram of the rock cycle. Things to remember:

1. Intrusive igneous rocks had more time to gradually cool, therefore, they may grow larger crystals (coarse grained).
2. Extrusive igneous rocks cool rapidly, preventing the growth of large crystals (fine grained).
3. Sedimentary rocks are the most abundant rocks on Earth, the product of weathering, erosion, deposition, compaction, and cementation.
4. Contact metamorphism takes place when existing rocks come into contact with extreme heat inside of the earth, but melting does NOT occur. Metamorphism is a solid-state process.
5. Regional metamorphism occurs due to the extreme pressure of overlaying rocks, often at convergent plate boundaries.

Theory of Plate Tectonics Review

Plate motion occurs due to convection in Earth’s mantle, including upwelling of material from the deep mantle in rift zones, the lateral movement of tectonic plates, and the pull of sinking dense, old plates at subduction zones.

The outermost layer of the Earth is composed of lithospheric plates.

The lithosphere is composed of the crust and upper portion of the mantle. There are two different types of lithospheres—oceanic and continental—that have very different physical and mineralogical characteristics. The oceanic lithosphere is relatively thin, young, and dense. The continental lithosphere is relatively thick, old, and less dense

At divergent plate boundaries plates move away from each other and magma rises to form lithosphere. Seafloor spreading occurs at these divergent plate boundaries.

At convergent plate boundaries plates move toward each other and one plate is subducted below the other. Lithosphere is destroyed. Most of the planet's major earthquakes occur at these types of boundaries.

At transform boundaries plates slide past one another and lithosphere is neither created nor destroyed.

Hotspots occur at divergent plate boundaries or within tectonic plates. Their position is fixed. Hotspots within plates may produce strings of volcanoes as lithosphere drifts over them (Hawaiian Islands). The volcanoes are active when they are over the hotspot, but die out as the lithospheric plate drifts away from the hotspot.

Geologic History of Virginia

The illustration to the right gives a brief, generalized overview of the geologic events that helped shape Virginia over the past billion years.

The Grenville Orogeny; A mountain-building event that took place approximately 1.2 billion years ago that is associated with the assembly of the supercontinent Rodinia. Grenville rocks can be found today in the Blue Ridge province as metamorphosed granite batholiths.

Iapetan Rifting; 600 million years ago Rodinia began rifting apart and the Iapetus (ee-ah-pet-us) Ocean began to form. Notice the rift sediments in gold depositing on the diagram to the right. The eastern continental edge at this time ran down the axis of the present day Blue Ridge province, and Virginia lay south of the equator in a tropical climate.

Passive Margin; A passive margin is the transition between oceanic and continental lithosphere that is not an active plate margin. 500 million years ago, as the Iapetus Ocean basin opened the new continental margin sank below sea level and an east thickening wedge of sediments was deposited. The climate was tropical and the sediments were mostly tidal flat limestones and dolomites. These rocks, depicted in blue in the diagram to the right, are now exposed throughout the Shenandoah valley.

Taconic Orogeny; About 450 million years ago a volcanic island arc terrane along with accretionary wedge sediment collided with present day southwest Virginia and southeast Pennsylvania creating a mountain range in the western Piedmont region. A deep basin formed in the Shenandoah valley area, and over time it filled with sediments from the eroding Taconic mountains. Desert conditions across all eastern North America also lead to salt deposition in the basin.

Acadian Orogeny; About 400 million years ago a second terrane (Avalon) collided with eastern North America, building a mountain range in the piedmont area containing Richmond. A large basin sinks in western Virginia and West Virginia and fills with a thick sequence of gray, green and red sediments now exposed in Shenandoah mountain at the western state line.

After the Acadian mountains are eroded a large inland sea spreads from eastern West Virginia across most of eastern North America and fills with limestone sediments containing abundant fossils.

Final Assembly of Pangea; About 280 million years ago Africa converged with North America closing shut the Iapetus Ocean and creating the supercontinent Pangaea. A Himalaya size mountain range existed throughout much of Virginia. During this orogeny, most older rocks in Virginia were folded and thrust faulted toward the west. Coal swamps formed in southwest Virginia.

Atlantic Rifting; About 200 million years ago the supercontinent Pangaea rifted apart along a zone now located off the Virginia coast. As Africa moved away, the present Atlantic ocean began to open and continues to do so to this day.

Following the rifting of Pangea, the new continental margin sank below sea level and accumulated an eastward thickening wedge of sediments. Virginia eroded completely down to sea level, and the sea may at times have covered most or all of Virginia.

Gentle uplift of the eroded roots of the Alleghenian mountains started erosion again, leading to the formation of the present mountains with their long ridges and water gaps.

Modern Passive Margin; Virginia presently sits on a passive margin.

Physiographic Provinces of Virginia

A physiographic province is a region (or area) with similar landforms that are distinctly different from the landforms in adjacent areas. Virginia is divided into five physiographic provinces as seen on the map to the right.

The graphic below displays how key geologic events helped to shape Virginia's provinces. You may click on the image to enlarge it.

You can use the slider to the right to observe how the geology of Virginia is the basis for the boundaries of the physiologic provinces of the state.

The cross-section to the right is very generalized, but it helps us to visualize the topography and underlying geology of Virginia across the physiographic provinces.

Touring the Physiographic Provinces

Stop 1: Appalachian Plateau

The Appalachian Plateau contains rugged, irregular topography that is underlain by flat-lying sedimentary rocks. The region is actually a series of plateaus separated by faults and erosional down-cut valleys. The upper layers of the plateau are covered with forests and are rich in coal, natural gas, and petroleum formed 250–540 million years ago.

Agriculture and Soil in the Appalachian Plateau

Some of the land in the Appalachian Plateau is used for pasture, and farmers raise beef cattle and sheep here. Cooler temperatures in the region of southwest Virginia promote the growing of burley tobacco, some livestock, and Christmas trees.

In Buchanan County, the largest Virginian county in this region, the most economically important agricultural enterprise is timber production. Tobacco farming is scattered throughout the county in small plots, especially in the southern part of the county. Beef cattle farming is carried out to a lesser degree. The steep mountainsides and hillsides throughout the county are too steep and stony for the use of conventional farm machinery. The most level areas of the county are restricted to narrow flood plains and terraces along the drainageways. These areas are commonly too small for major farming enterprises. Many of these areas have been utilized for housing, road construction, and industrial development.

Stop 2: Ridge & Valley

The Ridge and Valley province is an area with long parallel ridges (3,000-4,000 feet high) and valleys underlain by ancient folded and faulted sedimentary rocks. The folding and faulting of the sedimentary rocks took place during the late Paleozoic (~480 Ma) when Africa and North America collided. The Appalachian Mountains are a result of this collision. The oldest sedimentary layers in the Ridge and Valley are Cambrian (~520 Ma) and the youngest are Carboniferous (~300 Ma). Limestones and shales are more susceptible to erosion and make up much of the valleys, whereas more resistant sandstones and conglomerates form the ridges. This process is called differential weathering.

Karst processes in the limestone beds have produced beautiful caves, well decorated with stalagmites and stalactites. These include the Skyline, Luray, Endless, and Shenandoah Caverns. Natural Bridge, northeast of Roanoke, Virginia is large enough to support a major highway, spanning 30 m (90 ft) and rising 65 m (215 ft) above the Cedar Creek. Learn more about Virginia's karst features here: https://www.dcr.virginia.gov/natural-heritage/karsthome

Agriculture and Soil in the Ridge and Valley

Agriculture is the number one industry in this region of Virginia. In the southern part of this region, the land is very hilly and rocky, similar to the Appalachian Plateau. This marginal land is used primarily for pasture and farmers raise beef cattle, horses, dairy cows, and sheep. As you move further north, the land becomes flatter and more productive.

The valleys contain incredibly productive soils due to their limestone parent material. The valley soils are deep, fertile, and well drained, which makes them great for growing grass for livestock, hay, timber, apples, produce, grapes, and much more. Rockingham County in the Shenandoah Valley is the number one agricultural county (total agricultural products sold) in Virginia, and one of the top 100 counties in the U.S. for agricultural production. Famous for its poultry output, Rockingham County is called the “Turkey Capital of the World”, but dairy cattle are also important to this county. 

Stop 3: Blue Ridge

The Blue Ridge is a high ridge separating the Piedmont from the Valley and Ridge Province. The billion-year-old igneous and metamorphic rocks of the Blue Ridge are the oldest in the state. The Blue Ridge province contains the most rugged and highest land in Virginia. In the southwestern Virginia Blue Ridge, Mount Rogers forms Virginia’s highest peak with a summit elevation of 5,729 feet.

North of Roanoke, this province is narrow and forms a distinct ridge of resistant rock. South of Roanoke, it forms a broad area of high ground ( or "upland"), with a steep slope (escarpment) on the east side.

The Skyline Drive and Blue Ridge Parkway north of Roanoke in Shenandoah National Park follows the crest of the Blue Ridge. These roadways boast spectacular views both to the east and west. South of Roanoke, the Parkway follows the edge of the escarpment, with great views mostly toward the east.

Agriculture and Soil in the Blue Ridge

The Blue Ridge province is mostly mountainous with poorly developed soils along the ridges, however, the province does contain flatter, less rocky areas in its eastern region. This is an instance where the physiography and the geology do not exactly correspond. The geologic province is defined primarily by the rocks underlying it, (coarse grained igneous and metamorphic Grenville basement rocks) than its topography (the eastern part of the geologic province blends in topographically with the Piedmont in many places, and appears distinct from the Blue Ridge mountains).

Apple orchards are common in this region, as are Christmas tree farms. Similar to the Appalachian Plateau, many farmers raise beef cattle, horses, dairy cows, and sheep because the land is too steep for crop production.

Stop 4: Piedmont

“Piedmont” is a French word meaning “foot of the mountains.” The Piedmont province is Virginia's largest region. It is characterized as an area of rolling hills that gradually rises to 1,000 feet, then doubles in height at the foothills of the Blue Ridge Mountains where the terrain becomes more rugged. The Piedmont is geologically complex and is divided by geologists into several "terranes" or subprovinces.

In general, the Piedmont is underlain by mostly ancient igneous and metamorphic rocks. The igneous rocks are the roots of volcanoes formed during ancient episodes of subduction that occurred before and during the formation of the Appalachian Mountains. Other belts contain metamorphosed sedimentary and igneous intrusive rocks. Large faults separate the terranes from each other.

The region also includes Mesozoic Basins (formerly called Triassic Basins), early Mesozoic (~225 to 190 Ma) sedimentary rocks, diabase dikes, and basalt flows that formed during the early stages of rifting associated with the opening of the Atlantic Ocean. Some Mesozoic basins preserve dinosaur, fish, and insect fossils. Coal was also deposited in swampy lowlands in some of these basins. The Mesozoic Basins consist of Triassic to Jurassic age sedimentary and igenous rocks. These rocks are distinctively younger than the surrounding Piedmont and Blue Ridge.

Because they are younger than the formation of the Ancestral Appalachians, they are not strongly metamorphosed or deformed. They formed during the break-up of Pangaea and opening of the Atlantic Ocean.

Many of the sedimentary rocks are easily eroded, causing the Mesozoic Basins areas to form regions of lower and flatter landscapes than the surrounding Piedmont or Blue Ridge.

Agriculture and Soil in the Piedmont

The parent material for Piedmont soils consists mostly of gneiss, schist and granite rocks at a typical depth of between 2 and 10 feet. Soils developed from these rocks and minerals form acid, infertile soils. While the natural fertility of Piedmont soils is low, these soils do respond well to liming and fertilization. Historically, much of the region was cleared of trees and farmed intensively, which led to extreme erosion. Agricultural production eventually diminished and most farms reverted back to forests. Today, over two thirds of this region is wooded.

In the southern reaches of the Piedmont, where the soils are composed of heavy clays, tobacco is a common cash-crop. The soils of the northern Piedmont, by contrast, are good for growing grass. Turfgrass and crops like winter wheat thrive. As the name implies, winter wheat is planted in the fall and lives through the winter at a height of a few inches. In the spring, it grows rapidly and, come summer, is harvested. The northern Piedmont is also home to dairy cows, horses, corn, and peach orchards

The Fall Line

The Fall Line, or Fall Zone, is located at the boundary between the Coastal Plain and the Piedmont where the elevation sharply drops 50-100 feet and the geology transitions from hard rock to soft sediments. Along this transect, rivers draining the Piedmont drop steeply to sea level and in the process form rapids and waterfalls. These rapids prevent travel upstream. In Richmond, where the James River flows, the Fall Zone is seven miles wide with half of the elevation loss in the final half-mile.

Stop 5: Coastal Plain

The Coastal Plain extends from the Fall Zone eastward to the Atlantic Ocean. It is a flat area consisting of young, unconsolidated sediments underlain by older crystalline basement rocks. Two-thirds of this sedimentary wedge is composed of late Jurassic and Cretaceous clay, sand, and gravel, materials that were eroded off the Appalachian highlands, carried eastward by rivers and deposited in deltas at the margin of newly formed Atlantic Ocean basin. Fossiliferous marine sands of Tertiary age overlie the older strata. These sediments were deposited in warm, shallow seas during repeated marine transgressions across the Coastal Plain.

The Coastal Plain is a terraced landscape that essentially "stair-steps" down towards the coast and to the major rivers. The ‘steps’ are topographic scarps that formed as ancient shorelines, and the ‘treads’ are emergent bay and river bottoms. This landscape was formed over the last few million years as sea-level rose and fell in response to the repeated melting and growth of large continental glaciers and as the Coastal Plain slowly uplifted. During the last ice age much of the continental shelf was emergent, and the Susquehanna River flowed through the Chesapeake lowland and out across the exposed shelf reaching the Atlantic Ocean over 60 miles to the east of the present-day shoreline. The Chesapeake Bay was created about 5,000 to 6,000 years ago when the lower course of the Susquehanna River was flooded by rising sea level.

Agriculture and Soil in the Coastal Plain

Coastal Plain soils promote the growth of evergreens and pine forests, as well as cotton, corn, wheat, and soybeans. Peanuts also grow well in the sandy soils of the this region. The peanut farms in Southeast Virginia are famous for producing large peanuts of excellent quality. You-pick strawberry and pumpkin farms are also popular in this region. Since the soils in the Coastal Plain are fertile and well-suited for crops, pastureland is not as common.

Farmers on the Eastern Shore of Virginia raise over 60 kinds of vegetables and fruits. Potatoes, cucumbers, squash, and green beans are vegetables common to the area. Fruits raised include tomatoes, apples, peaches, and strawberries. Poultry farms are also abundant on the Shore.

Aquaculture is another agricultural industry along the coast of Virginia. Clam and oyster farming is a booming, multi-million dollar industry in Virginia. These shellfish provide important economic and environmental benefits. In fact, a single adult oyster can purge 50 gallons of water a day! And shellfish gardening and farming reduce harvest pressure on wild stocks, while increasing the overall number of shellfish that help clean the water and serve as habitat for fish and crabs.

Virginia is 1st in the U.S. for hard clam production and 1st on the East Coast of the U.S. for Eastern oyster production!

Agriculture is Virginia's #1 Industry

Agriculture is Virginia’s largest private industry by far. Agriculture has an economic impact of $70 billion annually and provides more than 334,000 jobs in the Commonwealth.

Virginia’s agricultural production is one of the most diverse in the nation. Many Virginia commodities and products rank in the top 10 among all U.S. states. These include leaf tobacco, 3rd; apples, 6th; grapes, 8th; peanuts, 9th; fresh market tomatoes, 10th. Livestock rankings based on number of head include turkeys, 6th in the nation, and broilers, 10th.

Based on the 2017 Agriculture Census:

• Virginia has 43,225 farms
• The typical Virginia farmer is 58.5 years old
• The average farm size is 181 acres
• Farms cover 7.8 million acres
• Approximately 36% of Virginia’s primary farm operators are female
• Less than 15 cents of every consumer dollar spent on food actually goes to the farmer

Soils in Virginia

If agriculture is Virginia's #1 industry, then Virginia's soils must be an important natural resource! Soil Scientists have surveyed and mapped all of the soils in Virginia, and there are about 500 named soil series in the state.

The geology of Virginia serves as the parent material of the soils, so we can see that the soils in our state are subdivided similar to our physiologic provinces.

The U.S. Department of Agriculture maintains all of the nation's soil data in an online database called  Web Soil Survey . Anyone can access Web Soil Survey and use it to create soil maps that display a great variety of soil data.

Virginia even recognizes a state soil. Just like the Northern Cardinal is the state bird, Pamunkey is the state soil! You can learn more about Virginia's state soil  here . Pamunkey was selected as the state soil because it is formed from sediments which originated in every physiographic province in Virginia and therefore represents the whole state better than most other soils. It's also a very fertile soil that is important to Virginia's agriculture, so most of this soil is farmed.

Every state also has Soil Scientists that work for the U.S. Department of Agriculture. If you ever need information on the soils in your area or want to know more about career paths as a soil scientist, you can find contact for Virginia's soil scientists online  here .

Virginia's Geologic Economic Resources

The information on this page was obtained from the Virginia Department of Mines, Minerals and Energy and contains active links to their website.

Virginia’s diverse geology provides a wealth of opportunities in the form of mineral and energy resources.

 Energy resources  include conventional fossil fuels such as coal, natural gas, and oil that are produced from mines and well fields located mainly in the Appalachian Plateaus province in southwestern Virginia. Other non-conventional energy resources that have a geologic origin, but currently remain undeveloped include geothermal and uranium-enriched rocks.

Non-fuel  mineral resources  include crushed stone, sand and gravel aggregate, clays and shale, and a diverse range of industrial minerals such as kyanite, vermiculite, dimension stone, titanium and zirconium sands, among others.

Watch the video below to learn more about fossil fuel resources.

Rock and Mineral Resources in Virginia

In Virginia, major rock and mineral resources include coal and natural gas for energy, gravel and crushed stone for road and building construction, silica for electronics, zirconium and titanium for advanced metallurgy, and limestone for making concrete.

The map to the right is provided by the Virginia Department of Energy and displays the non-fuel mineral resources that are presently mined in Virginia. What mined mineral resources do you find where you live?

The Virginia Department of Energy  website  provides further information. They also have their own Story Map about  Aggregates in Virginia  which describes why aggregates are important to us, supply and demand by county, location and production of aggregate mine sites, how geology can contribute to aggregate mining and land use planning, and the general geotechnical characteristics of aggregates in Virginia. This Story Map also provides two activities for K-12 education.

Tools for Topography

In your Earth Science class you're not only learning about the geology of Virginia, you're also learning about the topography of the state. Topographic maps are historically the most common tool used to visualize the 3-D surface of a landscape on paper. However, in flat landscapes such as the Coastal Plain, topographic maps don't always provide a clear visualization of the lay of the land.

New technologies have led to improved methods of visualizing 3-dimensional landscapes on a 2-dimensional platform. LiDAR, which stands for Light Detection and Ranging, is one such technology. LiDAR data is collected when aircraft or drone-mounted laser systems measure 3-D coordinates of features from reflection on Earth's surface.

The data can then be processed to create a Digital Elevation Model (DEM) that can be utilized in a Geographic Information System (GIS). A digital elevation model is a 3D computer graphics representation of elevation data to represent terrain on the surface of the Earth and is the basis for most relief maps. A geographic information system is a framework for gathering, managing, and analyzing geographic data. Learn more about GIS  here .

The slider on the right side of the screen contains one layer with USGS Topographic Maps for Chesapeake, Virginia and a LiDAR DEM for the same area. The physical geographic features of the the city do not particularly stand out on the topographic map, but on the LiDAR DEM you can clearly see the changes in elevation and various geographic features.

Tools for Topography Continued

Let's do another comparison of a topographic map and a LiDAR DEM at a smaller scale. To the right is a comparison of the two using the Virginia Wesleyan University campus in Virginia Beach, Virginia. On the topographic map we can see a few labeled contour lines (10, 15, 20). On the LiDAR DEM the color gradient helps us to visualize the changes in elevation on the campus.

The gradient legend on the bottom right corner of the LiDAR map tells us the scale of the gradient in feet.

The highest elevations are indicated in white and are about 28 feet above sea level. The lowest elevations are in light blue and are about 3 feet above sea level. These areas are low-lying streams and natural drainageways. What other features can you see on the LiDAR DEM map?

You can learn more about LiDAR from the USGS  here . More advanced learners may be interested in this  LiDAR online training module  from NOAA.

The Chesapeake Bay Impact Structure

You can click through the presentation to the right to learn about the Chesapeake Bay Impact Structure, which is believed to have led to the formation of the present day Chesapeake Bay. According to the National Park Service, About 35.5 million years ago an exploding meteor collided with Earth and formed a massive crater. Because rivers flow along the path of least resistance, the depression created by the crater caused river valleys to converge, setting the stage for the formation of the Chesapeake Bay.

Millions of years later, the prehistoric Susquehanna River carved a 400-foot deep canyon as it flowed from the Appalachian Mountains toward the Atlantic Ocean. Rising sea levels at the end of the last ice age, about 10,000 years ago, flooded the Susquehanna River valley. Sediment has since filled much of the channel, forming the shallow Bay.