By Philip S. Prince, Virginia Division of Geology and Mineral Resources

Karst landscapes are really cool to look at with good hillshade imagery. In the sedimentary Appalachian Valley and Ridge, karst systems aren’t terribly hard to find. Carbonate rock units susceptible to karst development are distributed throughout nearly all of the Appalachian sedimentary section, from Cambrian- to Mississippian-aged units. Stratigraphy does vary somewhat along strike; Mississippian carbonates that are prolific karst-formers in some portions of the central and southern Appalachian Valley and Ridge are absent from the Mississippian section in other areas, either for structural or stratigraphic reasons. Devonian and Silurian carbonate units are similarly variable along strike. The Middle Ordovician section, however, tends to be a prolific karst-former throughout the Valley and Ridge, particularly in southwest Virginia. The image below, from Scott County, Virginia, shows just how intense sinkhole development can be in parts of the Middle Ordovician section.

Just a few sinkholes here…The rugged hills at right are the core of a plunging syncline; the karst-forming unit with all the sinkholes wraps around the core. The average sinkhole diameter in this image is 30-40 meters, or 100-135 feet.

The portions of the landscape with large numbers of sinkholes have not been excluded from agricultural or residential use. This is the top image without hillshade overlay; the sinkhole pattern is not visible here.

Most of the sinkholes are not open pits or swallow holes. They are soil- or water-filled depressions, though water fill may be temporary and seasonal. Steeply-dipping limestone outcrops are visible at right center and upper left.

The pattern of sinkhole development seen here is interesting for two reasons.The first is structural; the sinkhole-forming zone curves around the rugged area because the area shown is a syncline in the footwall of a thrust fault. The rugged area is underlain by younger strata than the sinkhole-dense area, and is in the axis of the syncline. Second, both rock units are carbonates, but they clearly do not share the same susceptibility to dissolution and sinkhole development. Bedding can be seen in hillshade in the rugged area, and the beds that support enough relief to be visible in the hillshade are probably sandy or cherty, both possibilities in this area. Alignment of the sinkholes reflects the orientation of bedding as well, just in a different way.

Nearly flat-lying beds in the syncline core can just barely be seen. The sinkholes are hard to miss! They align along bedding on the steeply-dipping syncline limb.

Bedding on the southeast limb of the syncline was overturned to the northwest by thrust fault motion from right to left in this view.

The visible beds in the rugged hills aren’t particuarly dramatic. Good hillshade resolution!

45 kilometers to the west, on a different thrust sheet, the same age interval of rocks is behaving in the same way. The sinkhole-forming horizon is very obvious in the image below, where beds dip uniformly to the northwest (to the left of the image). The cherty limestone beds, with bedding slighlty visible, are to the left of the sinkholes, or up-section. The sinkholes are even the same average size as those shown above–about 30-40 meters in diameter. Other karst-forming units in the area may show equally numerous sinkholes, but their average size and distribution varies, even in moderately dipping, homoclinal sections like the one shown here.

The same karst-forming horizon is up to the same tricks here. View is to the northeast, with bedding dipping moderately towards the left of the image. Lower Ordovician through Devonian rocks are seen here; the prominent ridge at left is Silurian sandstone.

The dolomites in the foreground are also karst-prone, but they tend to form more isolated and very large sinkholes. Ridge in the distant is the Silurian ridge from the image above; Mississippian rocks are exposed on the steep slope cut off at the top of the image.

Interesting details of the karst landscape are apparent in the hillshade. Numerous sinking streams can be seen disappearing into swallow holes at the base of the very karst-prone units. This pattern can be seen in a normal topographic map, but is almost impossible to pick out from satellite imagery.

Needless to say, if you live in an area like this, have your well water tested!

Streams sink into swallow holes. These features have developed in the dolomite horizon that forms large, more isolated sinkholes, shown in the previous image.

This post was originally published on The Geo Models blog.