Dynamic underground and surface forces, although occurring slowly over many millions of years, created the Shenandoah Valley and its surrounding hills and ridges. The processes continue today as the Valley region gradually weathers, erodes, levels, and develops rich soil.
It all began with moving rock. Earth’s rocky crust, or outer shell, averages about 30 miles thick and is subdivided into at least fourteen huge blocks called “tectonic plates.” These plates move and shift on Earth’s surface, riding slowly on hot, semiliquid magma below the crust.
Nearly 500 million years ago, the rocks that now make up the Shenandoah Valley solidified from ancient tidal flat sediments into vast, horizontal layers of limestone, shale, sandstone and the mineral dolomite. After approximately 100 million years, the North American Plate met the oncoming African Plate. The two plates began a series of long, grinding, mountain-building collisions called orogenies. These events caused the underlying limestone, shale, sandstone, and volcanic rock to slowly fold upward into the ridges and mountains now called the Appalachians in North America and the Atlas Mountains in Africa. Collisions continued for another 100 million years or so until the Appalachians grew to elevations possibly as high as 20,000 feet above sea level. The two plates eventually separated and have moved further apart ever since. Although no longer near plate edges, or boundaries, Virginia contains enough active fault lines (areas where rock was once broken over several miles) that seismic equipment, and people, in the Shenandoah Valley occasionally detect small earthquakes.
Exposed to millions of years of weathering and erosion from rain, wind, ice, and sun, weaker overlying rocks on the ridges and mountain tops broke down into smaller particles and fragments called sediments. The sediments washed downhill into the valleys between the ridges. In what is now Virginia, the Blue Ridge Mountains became the eastern boundary of a wide valley called the Shenandoah Valley, with the western boundary being another series of hills and ridges called the Alleghenies. Not a single, long ridge, but rather an interconnected group of hills and ridges, much of the Blue Ridge’s overlying sedimentary rock such as limestone, shale, and sandstone eroded away to leave the core rocks of very hard igneous granite, and metamorphic gneiss, slate, and quartzite. The Alleghenies still retain most of their overlying sedimentary rock.
Slowly evolving over many years, the Shenandoah Valley is now approximately 150 miles long and averages about 30 miles wide. Bisecting the Valley is Massanutten Mountain, a long ridge of approximately fifty miles that also still has much of its overlying sedimentary rock. Sediments of limestone, shale, and other rocks and minerals collected in layers (strata) on the floor of the Shenandoah Valley, giving the Valley its rich, limey soil and agricultural fertility. Erosion of the highlands has been so effective over the past 300 million years that most Appalachian hills and ridges are now under 5,000 feet above sea level.
Limestone and Karst Terrain
The Valley’s limestone is evidence that the area was once under ocean water, long before the tectonic plates collided. Limestone forms in warm, shallow seas when millions of tiny sea creatures die and accumulate on the ocean floor. The process produces large formations of limestone which is still abundant in the Valley. The many caves and caverns in the region resulted from dripping water slowly dissolving limestone, creating holes in the rock. Also present is red clay, an oceanic sediment containing iron oxide, along with countless fossils of sea creatures including shell-like brachiopods and extinct swimmers called trilobites.
Because of its limestone base, the Shenandoah Valley is called a “karst” area, which means that slightly acidic water has worn away limestone and dolomite bedrock over many millions of years, producing caves, caverns, sinkholes, streams, and springs. Streams disappear and reappear, as do springs over time. Thus, karst is a terrain type with distinctive landforms and water resources including underground reservoirs known as aquifers. Highly productive but extremely vulnerable to contamination, aquifers provide approximately 40% of the groundwater used in the United States.
Aquifers are underground rock layers saturated (filled) with groundwater. The aquifer can be porous and permeable (allowing water to filter through) and include fractured limestone, silt, gravel, and sand. An aquifer is not an underground river, but a layer of rocks with water occupying the spaces between the rocks. Aquifers vary in depth and the ones closer to the surface, which are mostly used for irrigation and water supplies, are re-charged by rainwater.
Some of the most intriguing geological features in the Shenandoah Valley are caves and caverns. These are natural underground openings large enough for humans to enter. They are primarily formed by volcanic activity or the eroding effects of water and wind. The caves and caverns of the Shenandoah Valley are of the latter variety, the result of the dissolution of limestone and other soluble rock throughout the area’s mountainous regions.
Although similar and born from the same erosional processes, caves and caverns are somewhat different. Caves are normally one large room with a single opening, while caverns make up a network system that can extend for several miles underground with many rooms, interconnecting chambers, and multiple openings. In some caves and caverns, the continuous chemical reactions caused by dripping water on limestone create the classic calcium carbonate features of stalactites, stalagmites, and columns. Stalactites resemble large icicles hanging down from a cave’s ceiling while stalagmites grow upward from the floor. Columns develop when stalactites and stalagmites connect.
Besides limestone, the Shenandoah Valley region contains many other rocks and minerals. Rocks are mixtures of earth materials while minerals are solid chemicals. Notable igneous (volcanic) rocks are granite, gabbro, and basalt. Sedimentary rocks (those often solidified from particles) include limestone, sandstone, shale, and coal. Metamorphic rocks (rocks hardened from many years under extreme heat and pressure) include slate, gneiss, and quartzite. Relatively common Shenandoah Valley minerals are manganese, various iron ores, zinc, lead, sulfur, gypsum, and pyrite (fool’s gold). Small amounts of oil and natural gas are also extracted.
The entire Shenandoah River drainage system covers hundreds of square miles and includes many feeder streams, or tributaries, that send water to the Shenandoah. With its sources, or headwaters, high in the Blue Ridge and Alleghenies, the Shenandoah River consists of two major branches, the North Fork and the South Fork. Both forks generally flow northeast until merging just north of Front Royal, Virginia, to create the main stem of the Shenandoah. The river then continues northeast to empty into the Potomac River at Harpers Ferry, West Virginia. Since the Shenandoah River system flows downhill to the northeast, the northern end of the Valley is considered the “lower” Valley; the southern end, upriver, is called the “upper” Valley. All this freshwater eventually flows into the Chesapeake Bay and the Atlantic Ocean, part of a massive river system called the Chesapeake Bay Watershed. What this means is that nearly every drop of precipitation that falls on the Shenandoah Valley region, unless it evaporates first, ultimately travels downstream to the Atlantic Ocean to mix with saltwater.
Over millions of years, the Shenandoah River and its many tributaries carved out the Shenandoah Valley, picking up rock particles and other earth materials, then dropping them along the way to create wide, fertile floodplains. Because streams constantly change, the
Shenandoah also developed snake-like, U-shaped, looping bends called meanders. Many of the meanders are in areas with underlying shale, a relatively soft sedimentary rock consisting primarily of hardened clay, silt, and mud.
While the processes of weathering and erosion have continually broken the hills and ridges down into particles of sediment, the Shenandoah River drainage system carried the particles downstream and deposited them throughout the Valley to create rich, fertile soils. Soils are complex mixtures of rock particles, mineral particles, water, air, and decaying organic matter, called humus, from plants and animals. The particle content of the soils in the Valley is primarily derived from sedimentary rocks like limestone, shale, siltstone, and sandstone. However, the reason the soil is so productive is because most of it is born from the fertilizer-like limestone and mixed with organic humus. These soils are deep, fertile, and well-drained, great for growing grains, hay, timber, apples, peaches, and grass for livestock. Because of the productive soil and more-than-adequate rainfall, agriculture is still the top industry in the Shenandoah Valley.
Driving the entire process of continuous geological surface change in the Shenandoah Valley area is precipitation, water from the atmosphere. Rainfall averages over 38 inches per year with 60 inches of total precipitation including rain, snow, sleet, and hail, while the highlands receive 55 inches of rain per year and about 100 inches of total precipitation. Thus, the highlands continue to erode lower, the rivers carry water and sediment downstream, the caves and caverns grow, and the soil produces crops. The cycle of life and replenishment in the Shenandoah Valley appears stable.
Last updated: July 23, 2020