The Mount Rogers area is widely regarded by geologists as containing some of the most fascinating and unusual rocks in Virginia. The rocks represent a nearly half-billion years of late Proterozoic and early Paleozoic history during the period prior to and during the breakup of the supercontinent of Rodinia and the opening of the Iapetus Ocean. Adding to the facination with the area is a prominent pile of volcanic rock, chiefly rhyolite, which is very unusual for the southern Appalachians. In addition, there is evidence of glaciation, not from the Pleistocene, but from the late Proterozoic. The distinctively laminated varve-like rocks, dropstones, and diamictites are unique to the Mount Rogers area and are not found elsewhere in the Blue Ridge.
The Mount Rogers belt of rocks extends for approximately 45 km along a NE-SW strike from near the tri-state junction of Virginia, North Carolina, and Tennessee northeastward into Virginia. The rocks of this field trip straddle the Iron Mountain-Stone Mountain fault system, with the rocks on the northwest in the Shady Valley Thrust Sheet and the rocks on the southeast in the Blue Ridge Thrust Sheet. The area of study for this field trip lies within the Konnarock and Whitetop Mountain 7.5 minute quadrangles.
The Mount Rogers area contains some of the most rugged terrain and spectacular scenery in Virginia. Mount Rogers, because of its hard, resistant rhyolite bedrock, is the highest point of elevation in the state - 5729 feet above sea level. The region differs climatically from the surrounding area. It is not uncommon to see snow in April on the mountain tops. Mount Rogers lies within the Mount Rogers National Recreation area and, with the nearby Grayson Highlands State Park, attracts many hikers, campers, horseback riders, scientists, and naturalists. Mount Rogers itself is not readily accessible to the public, so we will view the rhyolite on top of nearby Whitetop Mountain, and we will spend a part of the day on the Appalachian Trail which will provide a good look at both rocks and the breathtaking (and we mean that literally at one mile elevation!!) view. Because of the changeable weather conditions and high winds, it is advisable to wear warm clothing.
The foremost authority on the geology of the area is Douglas Rankin who recently retired from the U.S. Geological Survey. He was one of the leaders of the U.S.G.S. team that mapped the Winston-Salem 1:250,000 Quadrangle in the mid-1960s, and produced an important field guide (Rankin, 1967) that is a "must-have" item for anyone who is interested in learning more about this area. He recently returned to the area to wrap-up his three-decades of research and published two definitive works: U.S.G.S. Bulletin 2029 (1993), and an updated field guide published as Virginia Tech Guidebook Number 10 that was run as a field trip in conjunction with the Southeastern Section of the Geological Society of America meeting in April, 1994. The present author (Jon Tso) attended the 1994 field trip and much of what follows in this guidebook is based Rankin's work, as well as from his own numerous trips over the last twenty years.
Other key researchers in the area include Julia Miller, of Vanderbilt University, who studied the glacial deposits in the Konnarock Formation, and Ed Simpson, of Kutztown University, who studied measured sections of the Unicoi Formation while completing his doctorate at Virginia Tech. For an overview of the stratigraphy and structure of the region from this time period of the Blue Ridge, reference is made to the work of Rankin (1971), Rankin and others (1972), Rankin and others (1973), and Wehr and Glover (1985), and the state geologic map of Virginia, published by the Virginia Division of Mineral Resources in 1993.
Please see the reference page for complete reference citations.
The field trip area is within the Blue Ridge geologic and physiographic province. In this part of southwest Virginia, the Blue Ridge consists primarily of two main sequences of rocks: 1.1 year-old (Grenville) crystalline continental basement rock, and late Proterozoic to early Cambrian sedimentary and igneous rocks. To the west, lies the Valley and Ridge Province which is composed of folded and faulted Paleozoic sedimentary rocks. To the east lies the Piedmont Province which contains a complex amalgamation of metamorphosed plutonic, volcanic, and sedimentary rocks that occurs on many thrust sheets. Some of these are interpreted to be ancient volcanic arcs and sedimentary packages that originated in the Iapetus Ocean and were later accreted (thrust faulted) onto North America (Laurentia) during the closing of Iapetus prior to North America's joining with Pangea.
A closer look at the Blue Ridge of southwest Virginia shows that its overall structure is that of a series of thrust sheets. The most prominent of the thrust faults is the Fries Fault system which has been mapped from the central Virginia area (where is it known as the Rockfish Valley Fault) to the Grandfather Mountain area of North Carolina. In very broad terms, in rocks northwest of the Fries Fault which include the Mount Rogers rocks, the Paleozoic metamorphic grade of the rocks is low, and prominent areas of Grenville (1.1 billion year old) basement rocks are exposed. These crystalline floored thrust sheets are called "massifs", and the Mount Rogers area lies within the area of basement rock called the "French Broad Massif". On top of the basement lie various younger cover rocks of late Proterozoic and early Cambrian age (including the Mount Rogers, Konnarock, and Unicoi Formations which will be visited on this trip). The contacts between the basement and cover rocks are nonconformities or faulted unconformities. In general, the metamorphic grade of the cover rocks is low, usually in the greenschist facies, and often sedimentary and igneous structures are still preserved. To the southeast of the Fries Fault, however, basement rock is less common, and higher grade (amphibolite facies) metamorphosed sedimentary and volcanic rocks of late Proterozoic and early Cambrian age dominate. It is believed that many of these rocks were deposited during or after the rifting that formed Iapetus, either in marginal rift basins or perhaps on the oceanic crust of Iapetus itself!
It is important to keep in mind that all of the rocks seen on the field trip in the Mount Rogers area were formed prior to the formation of the Appalachian Mountains and our focus will be on its earlier history of rifting, volcanism, erosion, and glaciation. However, all of the rocks bear the evidence of the great collision that closed Iapetus, built Pangea, and raised the Appalachians during the Paleozoic. Regional metamorphism is pervasive throughout the Blue Ridge and estimates for the age of metamorphism begin at 450 million years ago, during the Taconic Orogeny, one of the smaller collisional events that served as a warm-up to the main collision with Africa approximately 250 million years ago.
All of the rocks on the field trip have been transported by thrust faulting. Thrust faults have telescoped the rocks and are responsible for the overall structure of the Appalachians as we see it today. The history of thrust faulting is long and complex. Some of the thrust faults, such as the Hayesville Fault in North Carolina predate the metamorphism because texturally, the fault rocks are themselves metamorphosed. However, in most places, the thrust faults appear to be later Paleozoic features because they displace the main metamorphic isograds. During the course of our field trip, we will cross over one of the local faults (the Catface Fault) along the Virginia Creeper Trail and observe some of the shearing texture that is common as a result of the thrust faulting in the region.
As you follow the field trip, keep in mind that all of the rocks formed in a tectonic setting quite unlike what we see now. Imagine yourself transported back in time three-quarters of a billion years ago. We were part of a supercontinent called "Rodinia". The landscape was rugged with rift valleys, streams, volcanoes, and eventually, glaciers. This was before land plants and animals existed, so the landscape was quite barren and exposed. Without protective vegetation, streams and glaciers were able to accomplish a terrific amount of erosion while depositing a tremendous amount of sediment on the rift valley floor. The rocks from that time are uniquely exposed in the Mount Rogers area. These rocks record the events and environments that happened. Now click over to either the geologic column, field trip route map, road log, or geologic map to begin your journey back in time.