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.

Fans are highlighted in transparent red. The main fan complex at center is 3.5 km (1.5 miles) across. The broad summit plateau of Brumley Mountain is about 640 meters (2100 feet) above the top of the fans. Brumley Mountain is about 1,300 meters above sea level at it highest point (a bit over 4,200 feet)

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:

Presence of the fans is implied in the satellite imagery, but the hillshade shows them in great detail. Land use shows that the fan surfaces are highly agricultural, but the stony character of the fan surfaces can be seen from Google Earth. The fans likely provide good shallow groundwater with a pristine recharge area on the mountain slopes.

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:

The Silurian sandstone caprock (hard chocolate shell) is arguably the hardest rock in the Appalachians, and the Martinsburg calcareous shale below it (soft filling) is one of the weakest, eroding rapidly on the steep slopes to produce the fans in the valley. The formation of these fans is a process controlled by the appropriate arrangement of contrasting rock layers, and does not occur on all steep Silurian slopes in the Valley and Ridge.

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 top image shows the caprock layers. which are strong enough to support bold cliffs. Logging roads cut on the mountain do not ascend past the lower hard rock layer, where topography becomes too steep. In the bottom image, the caprock layers are visible along the mountain crest. The fan covers a large area, but is quite thin compared to the depth of the stream valley from which the sediment was sourced. It’s interesting to visualize taking the fan sediment and using it to fill in the scoop-stream valley from which it was sourced. The summit of the mountain plateau is 640 meters (2100 feet) above the top of the fans.

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.

The steadily steep profile of Stream B gives it the necessary energy to erode hard sandstone, particularly in the reach about halfway between headwaters and mouth. Stream A used to look like B, and when it cut through the sandstone to expose the soft shale, it had far more erosional energy than it needed due to its combined length and gradient. Erosion went crazy, forming the fans as Stream A cut its channel into a more energetically “comfortable” shape for the shale as shown above.

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 caprock layer that has breached on Brumley Mountain is slightly less folded to the northeast, and the steep streams draining it will not breach it to expose the soft shale beneath. As a result, the valley system at the right of the image does not have a huge fan complex, even thought it is fed by streams of comparable size and steepness to those on Brumley. Weather, uplift rate, and all other variables have been the same in the small area shown here (about 15 km or 9 miles across), so the fan formation definitely reflects bedrock contrasts.

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.

The Brumley Creek braidplain now sits 30 meters (100 feet) above the fan-free valley system on the right. At least some of this elevation difference represents the Brumley Creek valley building itself up on the huge amount of sediment produced by steep exposures of weak Martinsburg upstream. The low, 535 meter valley is now poised to capture Brumley Creek and offer it a less choked path to the Holston River.

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.

Brumley Creek has cut deeply into the soft Ordovician rocks, and debris flows from the caprock-supported gorge walls have introduced large sandstone boulders to its channel. One flow is highlighted in transparent red, but all ravines in this image have carried debris flows. Despite its larger size, Brumley Creek remains steep due to the addition of these boulders and the fact that its headwaters flow for a few kilometers across the caprock before descending to the valley. The boulder fill from the debris flows results in a steadily sloping whitewater stream instead of a set of large waterfalls at the edge of the caprock following by a rapidly flattening shale- and limestone-floored stream.

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.

The fan-choked Brumley Creek drainage at left and the fan-free drainge at right both end up in the N. Fork Holston River at comparable elevation after crossing the same ridge-forming layers of Mississippian-aged Price sandstone. This image also shows rock type-specific topography. At lowermost right are Cambrian shales carbonates of an overlying thrust sheet. The extremely rugged, but not high, area in the foreground has developed on Mississippian ages limestones and clastic interbeds. The ridge at center is Mississippian-aged Price sandstone, and the valley filled by the fans is underlain by Millboro shale. Brumley’s unbreached slopes are Silurian sandstone, and breaches expose Ordovician shale and deeper carbonate in the Brumley Creek gorge.

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 fan material will ultimately need to make its way through the gap in the foreground and into the Holston River. Don’t hold your breath. The summit of Brumley is 6 kilometers (3.7 miles) away in this image.

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.

The channel at right is cutting into the fan and slowly transporting the sediment towards the Holston. Detail in the hillshade shows boulders at the gully mouth–they look like a slightly rougher land surface texture. Debris this size deposits quickly after exiting the steeply sloped gully and did not move very far across the fan.

This post was originally published on The Geo Models blog.