Conglomerates and Dropstones

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.

Fairy Stone's for Young Explorers

Seven eager young explorers ventured to Fairy Stone State Park in search of these mysterious stones. The day was gray but comfortable and we hoped the rains would stay away long enough for us to fill our little baggies with treasures. 

Fairy stones are staurolite, a composition of iron aluminum silicate that forms only under extreme heat and pressure, as built up by the Blue Ridge Mountains. The mineral commonly occurs as twinned, six-sided crystals that sometimes intersect at 90 degrees to form a cross, or more commonly at an intersection angle of 60 degrees. The most common shape is a St. Andrew's cross, which looks like an X. Roman crosses are shaped like a plus sign. Maltese crosses, the rarest, have crossbars at the ends. 

Once on site, we didn't quite know how to begin. We asked a local store owner, and he obliged us with the strategies of finding the special stones. We scattered among the forested area checking washes and root wads looking for them. It didn't take long for the young, fresh eyes to find them and spread the word to the others. Everyone found treasures and the rock hounding ended in success. We were dirty (always a good sign) but not wet! It was a happy day!

December...A Great Time to Study Geology!

BRDC's Natural Heritage Program offered geology programs to the students of Grayson County Public Schools during December.

First, fourth and fifth grade students gained knowledge of geology through a series of hands-on activities. First graders read books and learned about fossils. Fourth graders studied the rock cycle using play-dough as their muse. They created sediments then turned those sediments into sedimentary, metamorphic and igneous rocks in quite the colorful fashion. The fifth graders took it a step further and explored the layers of the earth, plate tectonics and continental drift. The best part for them was the milky way candy bars used as a demonstration tool for plate tectonics....the EATING of the candy bar of course!

These programs enhance current SOL's and give the students additional reinforcement on these subjects. It is a pleasure joining forces with these outstanding teachers and having a positive impact on these wonderful students!

Geology Lessons along the Virginia Creeper Trail

Galax Middle School students braved a very cold day in order to be enlightened about the geology of the Blue Ridge. Dr. Arthur Merschat with the USGS joined us at Green Cove station to begin the hike along the Virginia Creeper trail towards Creek Junction. He came equipped with rock hammers, topographic maps of the area, and colored pencils. The rest of us were bundled up and excited to learn about the geology and how to create a geologic map.

Stopping at outcrops along the way, we would determine the rock type found there, observe the changes in the rocks from previous stops and then decide the color we wanted to use for that rock type. Brown was selected for sedimentary conglomerates; yellow for sedimentary sandstone; purple for sedimentary rock showing foliation; and finally green for igneous rock, a mafic basalt with "pillows". The end result was a beautifully colored map along the section of trail we covered.

We found a sunny location to warm up and have lunch. Thermoses of tomato soup and hot chocolate filled our bellies and kept us going for a little longer. In the shady locations, the stream alongside the trail was iced over and icicles hung from the rock outcrops. It was a beautiful, crisp winter day that ended too soon. We will need to return to finish the rest of the trail and our colorful map. Thank you, Arthur, for braving the cold and sharing the wonderful activity.