February 4, 2019
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.
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.
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.
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!
This post was originally published on The Geo Models blog.