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.

It ain’t the Grand Canyon, but it still looks cool and the Pizza Hut in Big Stone Gap is a short drive away. The steep “banded” zone shows about 300 meters (1,000 feet) of Mississippian stratigraphy; the base of the Pennsylvanian section is almost visible in the upper right corner. Valley to mountaintop is ~500 meters (1,650 feet). View is to the northeast; the perspective is from above US 23 north of Duffield, Virginia. The valley at the bottom of the image is underlain by Devonian shale. The “drippy” landslides and debris flows (I think they look like melted wax) have deposited on the lowermost Mississippian shale slopes, which lack the steepness to keep the flows moving. They are well-defined here, but vegetation patterns indicate they are not recently active.

As usual, not a lot to see here due to vegetation. In this leaf-off imagery, the Mississippian bedding is subtly visible. The dark, conifer-covered band of the mountain face is the darkest, steepest package of layers in the top image.

The northeast end of the Powell Valley Anticline (PVA) is the curving shape at the center of this Google Maps image. Where its Mississippian/Pennsylvanian sandstone crest is intact, it is covered by National Forest and is colored green. More populated valleys are Devonian shale (northeast of red box) or Ordovician/Cambrian carbonate (southwest of red box). The red box shows the Big Stone Gap 1:24,000-scale quadrangle, referenced below. Cliff Mountain and its cool exposures are at the bottom center of the box, just above the US 23 marker.

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.

Rock mass strength dictates steepness on this slope. The dark band in the middle, which hosts the steepest slopes, is the Greenbrier Limestone. US 23 is the highway in the valley; 500 meters (1,650 feet) of relief separate it from the top of Cliff Mountain. Less steeply sloped units above and below are sandstone-shale interbeds. The sandstones are quite resistant, but their interbedding with weak shale reduces the overall rock mass strength and limits hillslope steepness. The Greenbrier is softer in a “Mohs hardness” sense, but its thick and uniform beds, which may be cherty, contribute to an overall unit strength. Beds don’t dip very steeply in this part of the anticline’s crest, but this perspective shows a gentle east-northeast dip (towards the right).

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 sandstone caprock at the top of Cliff Mountain would produce even more impressive topography, but it is too thin. The Google Earth elevation model matches the overlay imagery very nicely.

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.

The dark Greenbrier band can be traced along the backlimb of the PVA until it disappears below the overriding thrust sheet at far right center. The flat uplands at top right center are underlain by strong Pennsylvanian sandstones.

The same image as above, and not much to see here…in this instance, the hillshade overlay really pulls its weight.

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 dark, layered Greenbrier band at left stops suddenly when it is buried by a complex of colluvium and landslide debris, the “blob” at the center of the image between the Greenbrier labels. A tiny exposure of Greenbrier is visible to the right of the “blob.”

A similar topographic expression is seen on the PVA’s northwest-dipping forelimb. The Greenbrier still stands out, and hillslope deposits still intermittently bury visible bedding or outcrop. A small “dribbly” (another excellent descriptor) debris flow can be seen atop the fan/debris cone near the center of the image.

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.

The light blue Mississippian and Pennsylvanian sections can be seen on the limbs of the PVA; pink Ordovician units are exposed in the structure’s core in the Big Stone Gap quadrangle. Cambrian units are exposed in the core just west of the quad. Complex faulting in the Ordovician units makes the map pattern locally very complex.

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.