RVF’s Geology Presentations with Chuck Bailey and Callan Bentley
A Brief Geologic History This Region
Where we are standing, about 355 million years ago, billion year old granites and granite-like rocks were crushed, as if in a vise, in a one or two mile wide zone running several tens of miles from northeast to southwest, roughly from Charlottesville to Lynchburg. That area of compression and resulting shortening became the “Rockfish Valley high-strain zone”. When compressive stress is applied to a body of rock deep in the crust, shortening may occur without folding and faulting of the rocks. That compression was supplied by pressure from the southeast as Africa and other land masses gathered together creating, ultimately, Pangaea. Let us look at that story.
More than one billion years ago a mountain chain, called the Grenville Mountains, was formed about where the Appalachians stand today. This occurred as several brittle tectonic plates joined together to form a supercontinent called Rodinia. As the edge of proto-North America was squeezed by a continental plate (now postulated to have been a piece of present day South America) molten rock from the mantle rose into the crust, cooled slowly, and formed what would become the coarse-grained granites and gneisses of the basement rocks, those rocks that underlie all others.
Later, some 600 million years ago, after erosion had exposed the granites at the surface, this supercontinent would rift or break apart. As it did so molten rock from the mantle once again rose, this time to the surface, through cracks in the stretched and fractured basement rock. Thick flows of black basalt flooded the region from what would become Lynchburg up into southern Pennsylvania. With time and subsequent mountain building events this basalt would be changed or metamorphosed to greenstone now found up on the slopes of the Blue Ridge just to the west of here.
The ocean that was created in the widening basin as Rodinia rifted apart opened, then closed. 300 million years ago a new supercontinent, Pangaea, was formed as continental plates once again joined together. This time it was Africa that collided with the eastern edge of proto North America. Again, a great mountain chain was formed where the two continental plates collided. The Appalachians, which include the Blue Ridge, were born. As that ocean closed collisions by land masses accreting to the eastern edge of North America created the stress that ultimately formed the Rockfish Valley high-strain zone. Later, 200 million years ago, Pangaea rifted apart as had Rodinia long before. The Atlantic Ocean filled the gap as Africa drifted away from North America. Slowly the majestic Appalachian Mountains were eroded down to a gently rolling topography. As they were later resurrected by uplift, tough resistant rock formed mountains and softer and broken rock, valleys.
Where the rock was weakened by faulting and fragmentation from crushing pressure, streams would often flow. The Rockfish River runs in one of these fault zones. The wide flood plain of the river speaks of large volumes of water carrying the weathered products of the mountains to the James River and thence to the Atlantic. As the river seeks equilibrium it wanders back and forth in the floodplain cutting down new streambeds and filling old channels, leaving terraces behind.
The weathering of these mountains may be gradual, grain by grain, as a result of physical and chemical processes. Often it can be catastrophic as was the case in 1969 when Hurricane Camille struck. It has been estimated that 50% of denudation or wearing down of the mountains may occur in these frightening events that may occur every two or three thousand years at any one location. Hundreds of debris flows or landslides occurred in this part of Nelson County. Everything from the underlying rock upwards, soil, water, rocks, vegetation, slid down the mountainsides at speeds of up to 40 miles per hour destroying everything in its path. More than one hundred people lost their lives in the tragic event.
Greater detail of these events is available at the kiosk at the trailhead. Knowledge of that story helps to better appreciate how this beautiful valley came into being.
Compiled and written by Chip Morgan with help of resources from VA Division of Mines and Minerals.
