The Lower Mount Rogers Formation is an excellent introduction to rock types. It is diverse with both igneous and sedimentary rocks. The stretch of road between Troutdale and Konnarock, VA (Rt. 603) is a great way to see these formations and rock types, especially during winter months when the vegetation has died down (including poison ivy) and the rock is better exposed.
The lowest part of the formation is called conglomerate. Conglomerate is a clastic sedimentary rock made up of rounded clasts (fragments of rock broken off other rocks by physical weathering). The spaces between the clasts are generally filled with sand and clay particles and bound together by a cement that is usually composed of calcite or quartz. A fine example of this can be found along Fairwood Rd (Rt. 603) in Grayson County, Troutdale, VA. Different types of rocks found in the Lower Mount Rogers Formation are represented in the cobbles in the exposed outcrops along the road. The rock types include rhyolite, granite, milky quartz, sandstone, and greenstone. This formation dates approximately 760 million years old.
Conglomerate
Gneiss is a foliated metamorphic rock identified by bands of varying mineral composition. It is a high-grade metamorphic rock in which mineral grains recrystallized under intense heat and pressure (thus metamorphic rock). This alteration increased the size of the mineral grains and segregated them into bands. Gneiss can form in several different ways. Most commonly, it begins with shale, a sedimentary rock transforming into slate, then phyllite, then schist, and finally into gneiss.
Cranberry Gneiss is a billion years old; the oldest rock of the Appalachian Mountains. In most of the Mount Rogers area, it is predominantly a metamorphosed granite, giving it a metamorphic layering that is representative of the building of the Appalachians during the Paleozoic. The Mount Rogers area is remarkable in that most of the rocks here have escaped the effects of the Paleozoic metamorphism, so the cranberry pieces preserve much of their original texture. If you see feldspar in a sedimentary rock like this conglomerate, it usually means that the source of the sediment is not far from its site of deposition. This is because the feldspar breaks down quickly once exposed to air and moved in water.
The Middle to Upper part of the Konnarock Formation rests above the Mount Rogers Formation and is overlain by the Unicoi Formation. Its age is unknown, but due to the placement with other formations, it is estimated to be between 570 and 760 million years old. Normally the Blue Ridge is extensively metamorphosed, destroying almost all of the sedimentary layering, but a well-preserved outcrop is visible along Rt. 603. This outcrop is indicative of a historic lake and grants us a rare window into the sedimentary history of the Blue Ridge.
Sedimentary layering
The maroon bands are made of mud. In order to be so fine, the water must have been very calm suggesting a lake. The grayish-green bands are composed of more quartz and indicate a current that is slowing down. This is consistent with the idea of a lake with streams feeding sediment into the lake.
Dropstones indicate slow, still water such as a lake
As you continue along Rt. 603 (now Laurel Valley Rd) just before the stop sign at Rt. 600 (Whitetop Rd), the Konnarock Formation there is similar to the last location but with large rounded stones that are isolated, not a part of a turbidity current like the first location. It is excellent evidence of glaciers in Virginia (not the recent ice age of the Pleistocene). Glaciers would have come down from the highlands and broken off into icebergs. At some point, the icebergs melted and the stones dropped down to the lake bottom. Evidence suggested that they were not transported by water currents but “dropped” in vertically through the water column. These are called dropstones. Dropstones are isolated fragments of rock found within finer-grained water-deposited sedimentary rocks. They range in size from small pebbles to boulders. Some of the dropstones appear to be pieces of Cranberry Gneiss!
If you would like to learn more about the geology of the Mount Rogers area, Radford University has a great online field trip accessible to the public. It is dated, but fortunately, the rocks don’t move. Check it out to learn more about the geologic significance of the Blue Ridge.
