February 26, 2019

Some examples of the geology of “gaps” and travel on the early American frontier

Posted by larryohanlon

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

(Note: I don’t use any footnotes or attributions here; this material is easily found online at various sources that have borrowed from one another so many times it’s hard to tell who said what first. I try to offer a general synthesis of some major concepts and events, and the reader is encouraged to search for these terms and check out the various presentations of transcribed primary documents.)

The topographic features of the Powell Valley Anticline (PVA) played a significant role in the lives of both indigenous and Euro-American peoples on the American frontier in the late 18th century. Because of its location, length, and impact on the layout of Appalachian river systems, the PVA facilitated both western travel for Euro-Americans and strategic movement of indigenous peoples seeking to stop this westward expansion. Here I focus on two subjects inextricably connected to PVA topography: The Wilderness Road and Robert Benge, also known as Chief Benge, Captain Benge, Bob Benge, or simply “The Bench.” Benge and Wilderness Road users had two very opposite goals, leading to numerous clashes and Benge’s ultimate demise in the mountains of the PVA.

For all of this to make sense, it’s necessary to look at the physical extent of the PVA (transparent red shape below) in southwest Virginia and Tennessee along with the boundaries of the Pine Mountain Thrust Sheet (red dotted lines), of which the PVA is part. Dots are modern day cities: (lower left to upper right) Chattanooga, TN; Knoxville, TN; Asheville, NC; Roanoke, VA.

The Powell Valley Anticline (PVA), covered in the red shape, is part of the Pine Mountain Thrust Sheet, outlined in dashed red. Because these structures are quite large and involve very erosion-resistant Pennsylvanian-aged sandstones, they strongly influence topography and the layout of river systems.

Now look at how river systems are laid out with respect to the PVA and Pine Mountain Thrust Sheet. The PVA is still marked in transparent red below; the outline of the Pine Mountain Thrust Sheet is omitted to reduce crowding of lines.

Rivers radiate away from the Pine Mountain Thrust Sheet and the limbs of the PVA. Clockwise from bottom, the dark blue rivers are the uppermost Tennessee River system; the Powell River flows through the PVA. Brown: Cumberland River. Red (north-flowing): Kentucky River. Light yellow: Russell Fork of Big Sandy. Yellow: Tug Fork of Big Sandy. Pink: New River system.

Several river systems, all of which take very different courses to their respective mouths, fan out from the edges of the PVA and Pine Mountain Thrust Sheet. In early America, river valleys were frequently used as routes of travel, making the PVA a large-scale “intersection” where travelers using the topography of one river system could cross dividing ridges and access another system heading in an entirely different direction. The Cumberland Gap on the Wilderness Road is a prime example, as it allowed travelers to pass from the Powell River (dark blue river in the heart of the PVA) valley into the Cumberland River (brown) system and then, if desired, into the north-flowing Kentucky River system.

The “intersection” concept becomes important when the groups of people using various travel paths are considered. Westward-moving Euro-American settlers used the purple routes shown below, while southern Native American groups had long used a variety of routes following the upper Tennessee River prongs, shown in red, for war and trade. Not surprisingly, things got interesting where the two routes overlapped within and immediately southeast of the PVA. Conflict was especially common in the 1790s when a group of Cherokees from south and east of present-day Chattanooga, Tennessee undertook a guerilla campaign against westward expansion of Euro-American interests.

Lower Cherokee travel and raiding (red) overlapped with Euro-American expansion (purple) around the PVA. The southern prong of the purple travel route starts in Morganton, NC, follows the Yadkin River to its Blue Ridge headwaters, then follows the Watauga River to the Holston at Kingsport, Tennessee, where it meets the northern prong of purple. The northern purle prong begins at Fort Chiswell, Virginia, and passes from the New River basin into the Holston. The three main prongs of the red travel route are (southeast to northwest) Holston River, Clinch River, and Powell River.

The complicated topography of the PVA provided a good place for Cherokee forces (and sometimes Shawnee) to stage attacks and escape through the connected valley networks and numerous gaps in the mountain ridges. Cherokee war captain Robert Benge was particularly fond of using the PVA to access and escape after raids, and the gaps marked on the image below were all well-known to him and those he attacked in the 1780s and 1790s. They are called by name in first-hand accounts of his raids and abductions, and the old map below also takes care to note most of these features. Despite being produced in 1859, well after the area was thoroughly explored and settled, the old map still struggled to capture the layout of complicated ridge patterns within the PVA. This complex topography was used by Benge to great advantage until 1794, when it was ironically used against him and led to his demise…

The PVA looks like a finger about to tap someone on the shoulder. The black dashed line is the Wilderness Road, connecting Fort Chiswell, Virginia and Kingsport, Tennessee to Cumberland Gap and Kentucky.


This 1859 map captures the general shape of PVA topography, but the intricately curved ridges are not perfectly reproduced. Place names used here date from the 1700s. A link to this map is found at the end of the post.

Benge was only half Cherokee. His father was a trader of Irish descent, but Benge lived in the world of his Cherokee mother, who was of a prominent lineage. A simple Google search of Benge’s name is highly recommended, as it will provide a wealth of information about his life, and the reader will find that most accounts borrow heavily from each other. I won’t go deeper into his biography, but suffice it to say that he was obviously capable and rowdy enough to still be talked about in 2019. Despite being based out of the general area of Chattanooga, he and his band were exceedingly well-traveled, not hesitating to go as far as the Ohio River. He raided heavily along the Holston River in Virginia southeast of the PVA, but was well-known throughout the entire region. He engaged in plenty of mischief in and around the PVA, which provided a useful avenue of travel back towards the Tennessee-Georgia border area. I wish to present three of his PVA-related exploits below, all of which involve the use of topographic features that are expressions of PVA structural geology.

  1. In 1785, a Cherokee raid on Wallen Creek, on the southeast limb of the PVA, resulted in the abduction of Mrs. Frances Scott. She reported hearing Benge’s name repeatedly used during the raid, but it is not definitively known if Benge himself was present. She was taken up the Powell River through Big Stone Gap, across Black Mountain into the Cumberland River headwaters, and then into the uppermost Big Sandy River headwaters. She eventually escaped when most of her captors left to steal horses on the Clinch River (they left her with one older man), followed the Russell Fork of Big Sandy through the Breaks Gorge to its headwaters, and then crossed the dividing ridge to reach New Garden on the upper Clinch River. The curving gray path marked below shows her approximate route.

    Mrs. Scott’s journey upstream along the Russell fork (light yellow river) would have been very challenging due to topography in the Breaks Gorge.


    Mrs. Scott was taken out of the Powell Valley through Big Stone Gap, seen here in LiDAR hillshade. This gap offers an obviously preferable route to ascending the adjacent mountain ridges and cliffs. View is to the north; the Powell River flows towards the observer from modern-day Appalachia, Virginia.

  2. In 1794, Benge and a war party ambushed well-known frontier ranger Moses Cockrell and two associates on the Wilderness Road in Kane Gap on Powell Mountain. Cockrell’s associates were killed, and Benge chased Cockrell for two miles on foot to Scott’s Station on Wallen Creek. Cockrell barely escaped into Scott’s Station, and his survival was considered impressive due to Benge’s reputation as an athlete. The slopes traversed on this run tend to be covered with variable amounts of sharp sandstone talus, and I would be very curious to know what kind of pace the men were able to maintain. The fact that this event is remembered today even though Cockrell was not killed or captured suggests that the activities of both of these men were of great interest to the locals in their day. Also noteworthy is that Cockrell was travelling from Rye Cove when he was ambushed–Rye Cove is the sinkhole-riddled plain shown in the karst blog.

    Scott’s Station is 2 miles (3.2 km) and several hundred feet below Kane Gap


    LiDAR hillshade shows one obvious road grade passing through Kane Gap, and a smaller, more faded grade just to its left near the gap. One of these grades is supposedly the original Wilderness Road, according to online materials provided by Scott County, Virginia.

  3. Benge’s final raid ended with his death on the forelimb of the PVA near present day Norton, Virginia. After raiding the Livingston farm on the Holston River, Benge, his party, and his captives headed northeast, crossed Clinch Mountain, and headed northwest towards their secret PVA escape route (bright red arrow at right, below). Their captives and the rough topography slowed their progress, and a rider who heard of the raid left the Holston Valley on horseback, presumably following the Wilderness Road (black dashed line) to Yokum’s Station in the Powell Valley. From Yokum’s Station, a group of rangers had an easy trip up the Powell River (dark red line, at left), presumably through Big Stone Gap, to the foot of Little Stone Mountain. Benge’s party moved more slowly up and over the rugged crest of the PVA, allowing the rangers to set an ambush in a stream valley today known as Hoot Owl Hollow. Benge was killed in the first shots of the exchange. Benge apparently intended to trade speed for secrecy, and it did not work out on this occasion.

    Benge’s Path from the Holston River (bright red, at right) was circuitous but presumably intended to avoid other people. He was further slowed by one of his older captives, who struggled in rough terrain. The ranger party from Yokum’s Station (a corruption of the name “Joachim”) was able to quickly move up the Powell Valley and ambush Benge as he descended from the highest topography.


    Benge supposedly led his party down from the vicinity of High Knob towards the ridges at left. This is the most reasonable route, avoiding the large cliffs at right. Benge’s Gap is just above the leader line pointing to Hoot Owl Hollow; Benge’s Branch is another valley towards the top of the image. Materials I find online offer differing reports of exactly where Benge was ambushed, and it’s possible that no one specifically knows anymore. In any case, he was obviously “funneled” into the ambush area by surrounding topography. The ranger groups took this into account in planning where to meet Benge and his captives.

Why do gaps form in ridges? The simple answer is that a gap develops where the rock that supports the mountain ridge is easier to erode. Greater ease of erosion may result from the rock being broken and weakened by faulting. It may also result from localized differences in sedimentary layer thicknesses or patterns. Gaps occurring within plunging (tilted) fold sets form where weak layers crop out between harder rocks above and below in the sedimentary sequence. Some examples are shown below.

Cumberland Gap and the Pine Mountain Narrows have developed where a minor fault cuts from the PVA across the Middlesboro syncline to the Pine Mountain Fault. Two small ridges, oriented “north-south) in the image below between the Narrows and Cumberland Gap, reflect movement and deformation on this fault. The major ridges are not significantly offset by this fault, but it obviously weakens the rock sufficiently to localize erosion and form gaps.

The role of minor faulting in the formation of Cumberland Gap and the Pine Mountain Narrows is reflected in the two small ridges running north-south. Note the large, round Middlesboro Impact Structure just west of Cumberland Gap. Its age is unknown.

Moccasin Gap, southeast of the PVA, is another example of a gap formed on a minor fault oriented nearly perpendicular to the ridge. The offset on the fault is minimal; in the top image below, the gap in the ridge is obvious, but the two ridge segments do not appear offset. The geologic map overlay shows the results of field mapping of the location. The ridge segment on the right is offset towards the observer a few hundred feet, providing a natural weak point to localize erosion.

Moccasin Gap has developed on a very minor fault that breaks the ridge-forming section sufficiently to promote localized erosion. At the center of the bottom image, the purple and pink layers are offset from the their continuation on the left ridge segment. US 58 uses this gap today.

North Fork Gap on the southeast edge of the PVA has formed due to erosion of a tilted sedimentary section. The LiDAR hillshade below shows development of the gap in weak Devonian shales between strong, ridge forming Silurian and Mississippian sandstones. Tilt of layering is down to the right in this image.

North Fork Gap is formed on easily eroded Devonian shale layers within this tilted section of rock.

Library of Congress 1859 map of Virginia:


This post was originally published on The Geo Models blog