February 12, 2019
By Philip S. Prince, Virginia Division of Geology and Mineral Resources
The Mississippian-aged sedimentary section in the northeastern portion of Virginia’s Powell Valley Anticline (PVA) offers up stunning hillshade imagery on the flanks of the aptly-named Cliff Mountain. The face of Cliff Mountain shown below exposes well over 300 meters (1,000 feet) of section on the back limb of the PVA, from Devonian shales in the valley to uppermost Mississippian sandstones at the top of the steep slope. Debris flows and landslides from the caprock layers at the mountaintop have traveled down gulleys through the steep portion of the slope, slowing and coming to rest on the shales that form the valley topography.
The boundaries between the different sedimentary units look particularly crisp in the hillshade because each unit displays its own distinct slope steepness, a result of the differing mechanical strengths of the layer packages. The strongest layers support the steepest slopes, while weaker layers produce gentler slopes. The crisp breaks between slope domains couple with the orientation of Cliff Mountain relative to the hillshade light source to produce a beautiful, well-defined image. Rotating the perspective in Google Earth illustrates the unit-specific slope steepness clearly.
Interestingly, the steepest zone on Cliff Mountain is developed on the Greenbrier Limestone. In the Appalachians, limestones typically weather quickly to produce valleys and karst landscapes. In some instances, however, limestone outcrops evolve under detachment- or failure-type erosional processes that are limited by the physical strength of the rock mass, not its susceptibility to chemical weathering. In these cases, limestones are actually very good cliff-formers (or rapid/waterfall-formers) and can be topographically distinct. The Greenbrier produces the dark band along the face of Cliff Mountain, which is the steepest portion of the exposed section.
The dark band of Greenbrier beds makes a good marker horizon to look for along the limbs of the PVA. On the back limb, it can be followed as a dark band (due to its topographic steepness) until it disappears beneath an overlying thrust sheet.
In some places, bedrock outcrop (or at least bedding visible in hillshade) is buried beneath landslide or colluvium deposits that blanket the hillslopes. Prominent formations like the Greenbrier show this pattern very clearly, and it can be a very frustrating issue for the field geologist. Frequently, areas that seem like a sure bet for measurable outcrop turn out to be blanketed by surficial deposits like the ones seen here. Topographic maps often lack the detail to show this effect, and it’s easy to waste hours of work seeking outcrop where there is none. Hillshade imagery finds a useful application in helping the field geologist plan worthwhile transects.
The northeastern portion of the PVA produces an interesting map and topographic pattern because it involves nearly the entire Appalachian sedimentary section (Cambrian-Pennsylvanian) and is an upright ramp anticline, allowing both of its limbs to stand out in comparable topographic prominence. The Big Stone Gap 1:24,000 scale topographic map (from the 1960s) is shown here draped over topography. Everyone’s favorite Mississippian limestone is also marked to define the limbs of the anticline.
Draping this map from the 1960s over LiDAR-based imagery raises an interesting question: How does a map of this vintage stand up against 1-meter hillshade imagery? I admit I was a bit surprised by the answer, and it will receive its own post in a few days.
In addition to providing a great perspective on ramp anticlines and Paleozoic stratigraphy, the PVA and its rivers and topography played an interesting role in travel and conflict along the colonial frontier in the 18th century. Because the area is a bit off the beaten path today, it doesn’t receive much attention, but it’s a good story to tell and will also get its own post down the line.
This post was originally published on The Geo Models blog.