Geology

What does the geology of Harpers Ferry National Historical Park (NHP) have to do with natural communities? More than you might think. The underlying bedrock, in combination with the sculpting power of the Potomac and Shenandoah rivers, largely determines the shape of the land. Bedrock also provides ingredients for soil, as do loose sediments on the surface. Landscape and soil both play major roles in creating habitats for plants—and natural communities. 

Map tip: To explore geology on the interactive map of Harpers Ferry NHP, switch the main data layer from “Natural Communities” to “Bedrock Geology” or “Surficial Geology.” To see how natural communities relate to the geology, turn on “Natural Communities (Outlines).” 

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​A Meeting of Two Rivers

The Shenandoah River joins the Potomac River in Harpers Ferry NHP. Together they carved gaps through high ridges in the Appalachian mountains, creating a natural travel corridor in the rugged landscape. American Indians used this corridor as did European explorers and settlers.

In 1761, Robert Harper set up a ferry across the Potomac just upstream of the where the Shenandoah joins it. Two years later, the town of Harpers Ferry was established as “Shenandoah Falls at Mr. Harper’s Ferry.”1,2 It is located in the triangle of land between the two rivers at the point where they converge.

In 1794, George Washington chose Harpers Ferry as the site for a U.S. Armory. Mills, railroads, and factories soon followed.3

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A Pair of Water Gaps

Just after the Shenandoah joins it, the Potomac River carves a gap through a ridge, creating a water gap between Maryland Heights on the north and Loudoun Heights on the south. The ridge is made of a very erosion-resistant rock called quartzite, so it stands taller than the surrounding areas, which are underlain by softer rock.

The river flows through a mile or two of less resistant rock, then cuts another water gap through a second ridge of quartzite called Short Hill Mountain.

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View from Jefferson Rock

You can get a good view of the double water gaps from Jefferson Rock on the banks of the Shenandoah. Thomas Jefferson took in the view here in 1783 and called it a view worth a trip across the Atlantic.4

It’s unusual that the river cuts through these ridges. Typically rivers follow the path of least resistance, running along a resistant ridge rather than through. It’s a clue that the Potomac may be older than the ridges: the river would have been already flowing on its current path when a period of regional uplift slowly pushed the bedrock up. As the rock moved up, the Potomac cut down.

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The Rivers Today

While cutting through quartzite ridges took ages, the Shenandoah and Potomac make smaller but still visible changes on a daily basis. They deposit and erode sediments along their banks and, during floods, on the floodplains. The banks and floodplains host a variety of natural communities that take advantage of the nutrients brought in by new sediments. The plants in these natural communities can handle saturated soil and the physical stresses of scouring floods.

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Two Ecoregions

Harpers Ferry NHP is in the Appalachian Mountains where the states of Maryland, Virginia, and West Virginia come together. The park lies in two different ecoregions, which are regions defined by topographic and ecological features.

The main part of Harpers Ferry NHP, including the town of Harpers Ferry, is in the Blue Ridge ecoregion. (The very easternmost tip of the park sits near a zone of transition between the Blue Ridge ecoregion and the Northern Piedmont.) Blue Ridge bedrock is metamorphic. Variation in the erosion-resistance among these metamorphic rocks creates high ridges, deep valleys, and steep slopes.

The western part of the park, in the vicinity of Bolivar and Millville, is in the Ridge and Valley ecoregion. Here, in the park units called Schoolhouse Ridge North and Schoolhouse Ridge South, bedrock consists of sedimentary rocks called dolomite and limestone, which were formed at the bottom of an ancient sea. These calcium-rich rocks weather into fertile soils. They can also dissolve in places over time to form caves.

Map tip: To see where these ecoregions occur on the interactive map of Harpers Ferry NHP, switch the main data layer from “Natural Communities” to “Ecoregions.” Be sure to look at the Legend.

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The Blue Ridge Ecoregion

The Blue Ridge ecoregion extends from southern Pennsylvania to northern Georgia. The rocks of the Blue Ridge ecoregion have been through a lot of stress and strain. Heat and pressure generated by continental collisions have transformed them into metamorphic rocks. By looking at minerals in the rocks, as well as structures such as folds and faults, geologists can begin to piece together ancient events. 

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Geologic History of the Blue Ridge

During the Paleozoic, a series of landmasses collided with ancestral North America, creating the supercontinent Pangea, and pushing up the Appalachian Mountains. The compression of the collisions caused large folds in the rock, like the folds that form in a rug if you push on one side. In some places, folding was so extreme that the rock layers were completely overturned, with older rocks ending up on top of younger layers. In other places, fault lines developed and rock layers were pushed many miles along these faults.

Even as mountains were being pushed up, erosion was already tearing them down through the work of rain, ice, and wind. Rivers carried away ancient mountains bit by bit, depositing the sediment in valleys, floodplains, and eventually, the sea.

Later, during the Mesozoic, Pangea broke apart. Large basins and cracks opened in the continent, just as cracks open up in clay as you pull it apart. One of the cracks got wider and wider until it became the Atlantic Ocean.

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Steep Slopes in Harpers Ferry NHP

The landscape in the part of the park that’s in the Blue Ridge ecoregion is noticeably steep, which is a result of different rates of erosion.

Quartzite, made mostly of the very hard mineral quartz, tends to resist erosion and therefore create ridges. Maryland Heights, Loudoun Heights, and Short Hill Mountain are all made of quartzite.

The oldest rocks in the park, which are among the oldest in the Appalachians, are metamorphosed granite that erodes more easily. The valley between Short Hill Mountain and the Maryland Heights - Loudoun Heights ridge is underlain by this rock.

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Metamorphic Rocks of the Blue Ridge in Harpers Ferry NHP

There are many different bedrock formations in the Blue Ridge ecoregion of Harpers Ferry NHP. Below are descriptions of some of the most widespread.

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Weverton Formation

The Weverton Formation is widespread in Harpers Ferry NHP, forming the ridges of Maryland Heights - Loudoun Heights and Short Hill Mountain. The Weverton Formation is primarily composed of metamorphosed sandstone, called quartzite. As its name implies, quartzite contains a lot of quartz, which is very resistant to erosion. For that reason, quartzite often creates topographic high points.

The sand that became Weverton quartzite was originally deposited on an ancient beach in more or less horizontal layers.5 The layers have been folded and faulted many times over the years, as continents collided. The result is that the Weverton quartzite crops out at the surface in Harpers Ferry NHP in two separate ridges.

The Weverton Formation weathers into soil that is acidic and not very fertile.

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Harpers Formation

The Harpers Formation is widespread in Harpers Ferry NHP and underlies the town itself. If you go to Jefferson Rock, you’ll be climbing up steps cut directly into the Harpers Formation.6

It consists of layers of metamorphosed sedimentary rock that originated as siltstone, shale, and sandstone. The sediments were deposited in an ocean long ago, and contain fossils of burrow holes made by a marine worm called Skolithos.7 During the continental collision that created the Appalachians, the sedimentary rock layers were metamorphosed into metasiltstone, phyllite, and quartzite. The Harpers Formation weathers into soil that is acidic and not very fertile.

Harpers Formation was used as building stone in some Harpers Ferry buildings, including Robert Harper’s house.8

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Garnetiferous leucocratic metagranite 

These rocks are older than the Harpers and Weverton formations and are thought to be among the oldest in the Appalachian Mountains. They are made of granite that was metamorphosed during an ancient continental collision. They erode relatively easily, which is why this bedrock is found in the valley between the two ridges in the park—Maryland Heights - Loudoun Heights and Short Hill Mountain.

These rocks weather into soil that is acidic and not very fertile. Surprisingly, though, the interactive map of Harpers Ferry NHP shows a fairly lush natural community growing on a slope with this kind of bedrock in the Short Hill park region. How can this be? Look closely at the bedrock geology on this map, and you will see narrow bands of a very different rock—metadiabase dikes—intruding this hillside.

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Metadiabase dikes

Dikes are intrusions of magma that cut across layers of pre-existing rock. These dikes are made of diabase, a medium-grained igneous rock. The prefix “meta” indicates the rock has undergone metamorphism, which occurred during continental collisions.

In Harpers Ferry NHP, metadiabase dikes can be found in relatively narrow bands within older metamorphosed granite rocks that occur between the ridges that form Maryland Heights - Loudoun Heights and Short Hill Mountain. Diabase is rich in minerals such as pyroxene and calcium feldspar that act as fertilizer for plants. Even if they are not big enough to be mapped, bands of metadiabase embedded in other kinds of bedrock can radically improve the quality of soil for natural communities!

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The Ridge and Valley Ecoregion

The western part of Harpers Ferry NHP is in the Ridge and Valley ecoregion, which stretches from New York to Alabama.9 The bedrock here is very different than in the Blue Ridge ecoregion. It consists of sedimentary rocks called limestone and dolomite, with minor amounts of other sedimentary rocks, such as sandstone.

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Geologic History of the Ridge and Valley

Limestone and dolomite bedrock form on the floor of the ocean over time, as tiny marine organisms die and accumulate. The limestones and dolomites of the Ridge and Valley were formed on the floor of an ancient ocean that existed before the Atlantic Ocean. During the continental collisions that pushed up the Appalachian Mountains, the ancient ocean closed up. The layers of limestone and dolomite were caught between enormous landmasses, and were compressed until they buckled into folds and faults.

Limestone is made of calcium carbonate, while dolomite has a similar composition but includes magnesium as well. These rocks erode very easily because calcium dissolves in rainwater and groundwater, both of which are naturally slightly acidic.

After the continental collisions, layers of folded bedrock containing more sand tended to resist erosion more than the limestone and dolomite layers. This led to the development of long ridges (sandstone) and valleys (limestone and dolomite), all trending northeast-southwest.

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Limestone and Dolomite Bedrock = Rich Soils

As limestone and dolomite weather, they add to the fertility of the soil. Limestone and dolomite are both rich in calcium, and dolomite also contains magnesium. These minerals are important nutrients for plants.

When rain or groundwater percolates through these rocks, it dissolves some of the calcium and magnesium, which may later be absorbed by plant roots or deposited in the soil.

Natural communities in the Ridge and Valley often contain plants that thrive in rich soils, such as bitternut hickory, white ash, American basswood, chinquapin oak, and eastern red-cedar. In Harpers Ferry NHP, areas in the Ridge and Valley often have a history of agriculture because of the rich soils.

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Ridge and Valley Bedrock in Harpers Ferry NHP

Below are descriptions of the most common types of bedrock in the Ridge and Valley section of the park.

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Tomstown Formation

The Tomstown Formation underlies much of Bolivar Heights and parts of Schoolhouse Ridge, in the western part of Harpers Ferry NHP. It’s made of limestone and dolomite. The Tomstown Formation contains abundant calcium, which promotes plant growth.

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Waynesboro Formation

The Waynesboro Formation underlies much of Schoolhouse Ridge in the far western part of Harpers Ferry NHP, in the Ridge and Valley ecoregion. It is composed of limestone, dolomite, and sandstone.

The Cavetown Member of the Waynesboro Formation is primarily limestone and dolomite, both of which are high in calcium. Soils atop the Cavetown Member are fertile and were often used in this area for agriculture.

The Red Run Member of the Waynesboro Formation is mostly dolomite and sandstone; it underlies parts of Schoolhouse Ridge. It weathers into soils that can be infertile (over areas of sandstone) to somewhat fertile (over areas of dolomite).

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Surficial Geology: Sediments on the Move

The term “surficial geology” refers to loose rock or sediment on the surface. The sediment can range in size from tiny (silt) to huge (boulders). The type of rock that makes up the sediment can be very different from the bedrock beneath it. That’s because the sediment may have originated somewhere else—upstream or uphill.

Map tip: To explore surficial geology on the interactive map of Harpers Ferry NHP, switch the main data layer from “Natural Communities” to “Surficial Geology.” Be sure to look at the Legend. To see how natural communities relate to the geology, turn on “Natural Communities (Outlines)” and zoom in as necessary.

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Alluvium

Sediment deposited by rivers is called alluvium. It’s often composed of a mixture of different rocks and minerals, because the sediments were eroded from various places upstream.

In Harpers Ferry NHP, the shores of the Potomac and Shenandoah rivers are lined with alluvium. Where there is a thick layer of alluvium, the make-up of the sediment plays more of a role in the fertility of soil than does the bedrock far below.

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Colluvium

Not all sediments that have been transported were moved by rivers. On steep slopes, rock fragments move downhill over time as a result of gravity, rain runoff, and frost action. Sediment deposited this way is called colluvium. It often accumulates on the bottom of slopes—toeslopes—where the shape of the land is slightly concave or almost flat.

Consider Maryland Heights, which is a ridge underlain by quartzite bedrock. Quartzite boulders and rock fragments gradually move downhill from the crest of the ridge, accumulating on the toeslopes. On the western side of the ridge, the bedrock is Harpers Formation, but quartzite colluvium has accumulated on the surface. Likewise, on the eastern side, quartzite colluvium lies atop bedrock of ancient metamorphosed granite.

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