January 28, 2019
LiDAR hillshade imagery highlights topographic evolution of the southern Appalachian Valley and Ridge
By Philip S. Prince, Virginia Division of Geology and Mineral Resources
Hillshade imagery brilliantly highlights the alluvial fans developed along the southeast slope of Brumley Mountain in the southwest Virginia Valley and Ridge. The fans represent an interesting stage in the topographic evolution of Brumley Mountain and the Valley and Ridge in general, whose namesake results from different rock types producing different types of topography. The Brumley fans are a good example of how rock strength controls topographic style, and how changes in surface rock outcrop patterns during Valley and Ridge exhumation can produce temporary erosional hot-spots in the landscape.
High-resolution surface imagery derived from LiDAR is as useful for looking at these features as it is for identifying landslides. Existing digital topography does show the fans due to their size, but details of their development and their association with the underlying sedimentary rock sequence is only visible with the 1-meter dataset. Without digital topography, the fans are much more difficult to delineate:
The fans have developed because stream erosion has worn through the extremely hard, erosion-resistant sandstone caprock that supports Brumley Mountain to expose extremely weak, weatherable, and erodible calcareous shale beneath the sandstone. In effect, this is an “erosional Cadbury Creme Egg” (no trademark infringement intended), in which the hard shell has been cracked open to expose the soft insides:
A close look at the headwaters of the funnel-shaped stream valley above shows the sandstone caprock horizons in good detail. The uppermost, extremely hard layer is Silurian-aged sandstone, the stratigraphic interval which dominates Valley and Ridge topography throughout the province, in Virginia and beyond. The hard rock layer beneath it is an upper Ordovician sandstone which is strong enough to produce a cliff line, but lacks the strength of the Silurian layers. Deeper in the Ordovician is very weak calcareous shale of the Martinsburg Formation (or Reedsville-Trenton, as some prefer this far south). In terms of Appalachian rock strength, the Silurian sandstones and the Martinsburg are at completely opposite ends of the spectrum.
The Cadbury analogy is okay for visualizing the general concept, but the weak rock doesn’t just “run out” once it is exposed; it is carried downhill in stream channels by moving water, with probable help from occasional debris flows. Locally amplified stream erosion is key to the formation of the fans. The small streams that formed the fans genuinely have moved more rock material than other streams of their size in the area because they are (and have been) steeper than they should be, given the weak rock that they now flow across.
Stream A, which has produced the largest fan lobe, crosses the same elevation interval as Stream B, which has not yet cut through the caprock and into the soft shale below. The inset shows comparative stream profiles of A and B, with exaggeration to better illustrate the point. Stream B roughly follows the tilt of the hard caprock, and remains steep as it gets longer and carries more water, giving the it good erosional capacity despite its small size. This geometry allows the hard sandstone to erode as quickly as softer rocks with more gently sloping streams and less topographic relief.
Stream A used to have a profile just like Stream B, which made sense when Stream A was flowing across the sandstone caprock. When Stream A wore through the caprock and exposed the softer shale, things changed. Stream A was sloped to erode hard sandstone but was now cutting into much softer rock, meaning it possessed much more erosional energy than it needed. Erosion ran wild and delivered a huge flux of sediment into the flat valley at the base of the mountain, forming the fan complex. The surge of erosion allowed Stream A to develop a more concave profile, where steep gradient is limited to headwaters, where the steam carries only a tiny amount of water. The dashed line shows what Stream A’s profile should ultimately look like. Fan development reflects erosional removal of rock to make stream channel slope more appropriate for the erosional resistance of newly exposed bedrock. It has happened in various parts of the Valley and Ridge throughout its exhumation, and will occur in several more locations on structures similar to Brumley in the region.
The localized nature of the fans is easy to see in a larger perspective. On the right of the image above, the caprock is not yet breached with the exception of a limited area near the top of the image. As a result, the streams are all very steep, but they do not transport tremendous volumes of sediment to be dumped in the valley. The valley thus looks quite different from the sediment-flooded valley below Brumley on the left, even though this is a small area that has experienced the same climatic and tectonic history.
Shown above is the mouth of the gorge of Brumley Creek, the largest stream draining the mountain and the source of a tremendous amount of fan debris. The Brumley Creek drainage is presently flowing at an elevation 30 meters higher than its neighbor. This does not necessarily mean that the fan complex is 30 meters (100 feet) thick here, but the deposits are certainly more than a thin soil horizon on the landscape. Brumley Creek is much larger than the drainages shown at the top of the post, but its continued erosion is now slowed by the accumulation of caprock boulders in its channel.
An interesting aspect of fan development is that Brumley Creek is now situated to be captured by its low elevation neighbor, which would offer the Brumley headwaters a faster, less sediment-choked path to the Holston River, but it would not present any large-scale increase in drainage network efficiency, as shown below.
Both fan-choked Brumley Creek and its northeast neighbor flow to the North Fork of the Holston River, which drains the area to the southwest. Both cross a ridge of Mississippian-aged sandstone to get there, so there is no advantage to the small stream at right carrying Brumley’s drainage to the Holston. If this re-routing did occur, the Brumely fans would end up just sitting around on the landscape and slowly weathering as the smaller drainages gradually moved the fan material to the Holston.
The hillshade imagery shows that channels are actively cutting into the fans and transporting fan debris to the larger Holston. Additionally, the forest cover and farming on the fans indicates that they have not been particularly active for some time. What incision into the fans actually means in terms of process is uncertain. It could mean that sediment over-supply has ended and the fans will no longer be built. It could also mean that Holocene climate conditions don’t favor fan development, but a long-term shift in precipitation patterns and freeze-thaw conditions could mobilize enough sediment to renew their formation. In either case, a lot of material has been moved off of Brumley Mountain, but it still has to make its way through the water gap above and into the Holston to continue its journey to the Gulf of Mexico.
This post was originally published on The Geo Models blog.