1 Billion years ago...

Our "big picture" view of the geologic history starts with the Cranberry Gneiss that formed approximately one billion years ago. These rocks are part of the "Grenville" Province of rocks that are found up and down the eastern seaboard of North America. These rocks are generally granitic, and represent the deeply eroded roots of an ancient large chain of mountains. Long belts of granite are associated with collisional plate boundaries and this leads to speculation that perhaps such a collision happened here. This ancient North American continent (called Laurentia) was part of a larger continental landmass (called Rodinia) that was built during the fusing of two plates. Rodinia was not the more famous "Pangea" supercontinent, but a prior supercontinent that existed some 700 million years earlier than Pangea, one plate tectonic cycle earlier.

A sketch of what the area looked like just before 760 m.y. Heating of the lithosphere has already started and thermal uplift has resulted. Magmas were created in the upper mantle (mafic) and the lower crust (granitic). The host rock in the continetal crust is the Cranberry Gneiss. As a result of the uplift, the Cranberry Gneiss was eroded extensively over a wide area.
Between 1 billion years ago and 760 million years ago

Little is recorded in the geologic record between the formation of the Grenville rocks, and the Mount Rogers Formation. However, the fact that that the Cranberry is commonly found as pebbles in conglomerates in the Mount Rogers and Konnarock indicates that much of this time period was spent in the erosion of the ancient Grenville Mountains. It is interesting to note that every rock that rests directly on top of the Cranberry rests "unconformably" on top. In other words, the formations that rest on the Cranberry rest on an erosional surface, as if the Cranberry served as a large floor that was eroded for a long time, and later rocks were deposited on top. If you could go back in a time machine and observe the scenery, the mountains would look quite barren - this was before the evolution of land plants, so the land would have look ashen gray.

At 760 m.y. ago, rifting of the crust resulted in the creation of a rift valley. This was a fault-bounded valley (heavy black lines show then-active faulting), creating a lowland surrounded by mountains. This was a perfect place to deposit the sedimentary part of the Mount Rogers Formation (orange). The felsic and mafic intrusions broke through to the surface creating the rhyolite and basaltic volcanics. The volcanic rocks are layered with the sediments, and the volcanic rocks and the surrounding uplifted basement served as sources of sediment. Toward the latter stages of the Mount Rogers formation, the rhyolite volcanoes really began to dominate, and a huge outpouring of lava resulted. In the diagram, the intrusions (in light blue) are of the "Crossnore Complex" and the volcanics (light blue and green) are of the Mount Rogers Formation.
760 million years ago

Starting at 760 m.y ago and continuing for approximately another 20 m.y., the Grenville basement rocks were intruded by a new set of felsic and mafic rocks in the Blue Ridge. These are the rocks of the Crossnore Complex (Rankin, 1970) which includes the Mount Rogers formation. As modeled by Rankin (1976), this concentration of igneous rocks marks a cycle of rifting of the ancient continent. The presence of sedimentary rocks in the lower Mount Rogers formation, especially the coarse conglomerate on display at Stop 1, indicates a fairly high relief such as in a rift valley, with the basement rock and rhyolite piles serving as material being eroded. It should be emphasized however, that two cycles of rifting are recorded in the Blue Ridge: the 760 m.y. cycle and a later one that is recorded in the Ashe Formation which is found east of Mount Rogers. The Ashe contains an extensive deep-water sedimentary-volcanic set of rocks which, towards the Charlottesville area, is overlain by a prominent metabasalt called the Catoctin Formation. The Catoctin has been radiometrically dated at 570 m.y. old (Badger and Sinha, 1988). Geologists believe that this second group represents the actual formation of Iapetus, some 190 million years after the eruption of the Mount Rogers Formation. The Mount Rogers and Crossnore Complex represents, apparently, an earlier failed attempt at rifting.

During "Konnarock time", the rift valley stil existed creating a location for a lake. The highlands were glaciated, and glaciers brought sediment to the lake. At first, much of the lake sediment was varve-like. Eventually, the lake filled with sediment and the glaciers themselves began to encroach onto the lake margins depositing the diamictite.
After "Konnarock Time", there was a period of erosion. By the time the Unicoi Formation was deposited it is likely that much of the rugged relief of the Mount Rogers and Konnarock times was greatly reduced. The diagram above shows a flatter topography with the old Crossnore intrusions still present in the crust, and rocks of the rift valley (Mount Rogers and Konnarock) still preserved by virtue of being lower in elevation.
Between 760 million years ago and 570 million years ago

The Konnarock Formation indicates a different geologic environment. Miller (1994) details an environmental model of glaciation. The lower part of the Konnarock Formation contains "fine laminites" which resemble seasonal varves that would have formed in a lake that was periodically frozen. Coarser laminites were likely deposited by turbidity currents on the lake bed. Dropstones were formed from icebergs that carried pebbles, which then melted and dropped the pebbles onto the sediment of the lake's floor. The upper part of the formation contains the massive diamictite which formed at the margin of the lake where the glacier itself flowed into the lake, and sediment from the bottom of the glacier was deposited as the ice melted. Alternatively, debris flows could have also deposited this material. The overall trend of a shallowing of the depositional environment indicates that sediment gradually filled in the lake, and the glaciers advanced into the lake.

Is glaciation during this time period found elsewhere in North America? Yes!! Glaciation during the latest Proterozoic is also known in Massachusetts and Newfoundland. However, the fact that the clasts in the diamictite are only from the locally derived Cranberry Gneiss and other nearby formations argues against having a large continental ice sheet here and instead argues in favor of local alpine glaciation. Miller (1994) speculates that a fairly rugged relief must have existed, possibly enhanced by rifting. It is interesting to note that few clasts in the Konnarock are derived from the earlier rhyolite volcanoes, but instead are derived from the older Cranberry Gneiss. This supports the idea that the volcanic period of the Mount Rogers was over and time a period of erosion intervened between the deposition of the Mount Rogers Formation and the Konnarock Formation. The rhyolite in the highlands may have been eroded away and the remaining rhyolite in the rift valley was buried by the Konnarock itself.

At 570 m.y., the Blue Ridge was caught up in the rifting that opened Iapetus ocean. Faulting returned, creating a more mountainous topography, as evidenced by the fast-moving streams and alluvial fans of the lower part of the Unicoi Formation, now seen as conglomerate. Quartz and feldspar-rich grains in the Unicoi attest to the active erosion of the basement rock. New mafic magmas were responsible for the basalt flows observed in the middle part of the Unicoi.
The top of the Unicoi contains clean quartz sands that were deposited in a shoreline environment. As Iapetus continued to open and the shoreline drifted away from the central rift, the continental margin subsided and Iapetus flooded the coastline. This created a broad continental shelf where sand and mud dominated the environment.
570 million years ago

The Unicoi Formation, represents the next episode in our history. At this point, the opening of Iapetus was imminent, and a new cycle of rifting had begun. The coarse conglomerates of the lower Unicoi are evidence of stream deposits with the pebbles representing remnants of stream gravel bars and alluvial fans in the rift valleys. The sandstones and conglomerates that contain feldspar, and the coarse grain size of the conglomerates indicate a rugged topography, possibly due to faulting associated with the rifting. The mafic basalt flows in the middle of the Unicoi are thought to be the remnants of lava flows associated with the rifting. These volcanic rocks are probably similar in age to the Catoctin Formation of central Virginia, which have been dated at 570 million years old (see above). In the upper part of the Unicoi, the nature of the rock changes from land to continental shelf. These rocks are notably sandy and crossbedded, indicating migrating megaripples or underwater sand waves, possibly in a near-shore tidal current. At this point in the history of the Mount Rogers area, we can see the physical evidence for Iapetus in these rocks.

Later in the Paleozoic

The Unicoi marks the end of the highly fragmented geologic record where rock formations are separated by several hundred million years of erosion. Later rock formations in the Paleozoic Era, although far from complete, record a far more continuous history. Aided by better fossil preservation, Paleozoic rocks record the further opening and stabilization of the Iapetus ocean and the Laurentian continental shelf. There was a long period of reef-building and limestone deposition during the Cambrian and Ordovician periods that is now seen as the thick belts of carbonate now found the the Valley and Ridge. (In Virgnia, I-81 travels along this belt from Winchester to Bristol!) The closing of Iapetus started in the late Ordovician and the final collision with Africa which formed Pangea and which built the ancient Appalachian Mountains, occurred 230 million years ago in the late Pennsylvanian and Permian periods. The results of this collision are seen throughout the field trip as faulting, folding, and metamorphism of the rocks all throughout the Appalachians. The faulting sliced the crust into a series of thrust sheets which then telescoped the rocks, causing a repetition of the stratigraphy. Subsequent to the formation of Pangea, the geological record becomes fragmentary again, and aside from present erosional landforms, no really large features have been preserved in the geologic record since that time in the Mount Rogers area.

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