8.0

Fast approaching are the Blue Ridge Mountains, whose heavyweight bulk dwarves even the sizable stretch of Massanutten Mountain visible now from the rear view mirror. The Blue Ridge Province represents a tremendous tectonic achievement. It is an overturned anticline whose horizontal width along Route 66 measures about thirty miles. At the core reside the Grenville basement rocks. These igneous and metamorphic rocks are 1.1-1.8 billion years old, and represent the oldest known rocks in Virginia. Both anticlinal limbs reveal lava flows of the Catoctin Formation and sedimentary rocks of the Chilhowee Group. At 570-600 mya, the age of these flanking rocks indicates a large gap in the rock record between the depositional time of the Grenville basement rocks and Catoctin/Chilhowee rocks. A cross-sectional view of the Blue Ridge Province shows the distribution of rock units in the anticline1.

Route 66 traverses from younger rocks on the western limit of the fold, to the older core rocks, and then again to younger rocks on the eastern limit of the fold. It is important to remember this age progression when tracing the roadside geology of this area. Click on the cross section for an illustration of the Blue Ridge Province. 10.0 Your car may perceive the changing geology better than you as resistant Chilhowee sedimentaries change the flat topography to gas-guzzling foothills. Visible at the right treeline just before mile marker 11 is an intriguing outcrop about four feet high. Here is the first of many opportunities to examine closely the structure of the Blue Ridge Province. Clues in this outcrop reveal the active depositional history of the Chilhowee Group sedimentaries.The Weeverton Formation is a matrix supported quartzite conglomerate, which suggests its deposition occurred in a fluvial rather than talus (rock slide) environment. The adjacent photograph shows that clast diameter sizes vary from .5-10.0 inches, indicating instability in the stream's ability to transport material. In Geology of the Shenandoah National Park, Gathright indicates that the matrix composition shows that the stream's erosional surface incised the Catoctin, Swift Run, and Pedlar Formations at local topographic relief of at least 100 feet (26). The Weeverton formation is early Chilhowee and as erosion continued, terrestrial land settled nearly to sea level. Behind and ahead of this outcrop is mostly Harpers Formation, a 2000 foot stack of metamorphosed shales (rocks compacted from mud and baked by heat and pressure). That this Weeverton exposure exists as only a thin layer within the Harpers formation suggests that this location is a transitional stage between the two formations; this prediction should yield more Harpers Formation further along 66. 11.2 More impressive than the Weeverton outcrop is the rock visible through Mannassas Gap. Reaching heights of more than thirty feet, the Harpers Formation is unmistakable on bothsides of the highway. lackish-green and platy, these rocks are primarily metamorphosed shale (slate and phyllite) with thin meta-sandstone beds at irregular intervals. This combination, and the presence of trace fossils, suggests a different environment than the terrestrial river suggested for the quartzite conglomerate outcrop. This strata originated more proximal to a stable coastline, and probably represents a near-shore river. Common to this formation are large singular quartzite beds. A fine example is visible in this outcrop as a large (5 feet) bed that traverses both sides of the highway. Notice that folding has tilted this bed from its original horizontal depositional position. 12.5 A speedy glance at the median reveals the Catoctin Formation in a few patches of non-vegetated, crumbling outcrop. Although the occurance is poor here, this basalt unit measures at a maxium thickness of 2000 feet. These thick sheets are the product of sea floor spreading, illustrated below. About 600-700 million years ago, Africa and North America were joined together as a stable supercontinent. As the plates shifted, however, a rift developed at the continental margin. The continents fell away from each other slowly and a Protoatlantic oceandeveloped between them, filling several thousands of kilometers. This ocean basin grew as basalt lava flows from a central rifting source spread laterally in both directions. Sedimentary rocks of the Chilhowee group deposit in and atop the Catoctin formation, furthering the idea of marine emplacement. The characteristic green of this rock type exists because the dominant mineral assemblage present, epidote-chlorite- actinolite, is green. Click the mineral you want to see and the James Madison University online mineral museum will provide you with a specimen. The green color, and its resistivity, makes the Catoctin formation unmistakable, even in the poor flashes available as you proceed East on 66. 17.0 Pastures on the north (left) side of route 66 harvest two sizable outcrops. A trek to the base of them reveals that the anticlinal core is nearly beneath you. Lichen covered, the rocks here are quartz monzonite augen gneisses. A lengthy name, this describes the minerals present in the rock. Quarz is present in small quantities, but the primary constituents are feldspars. Augens, small eye-shaped inclusions, of potassium feldspar are also visible. A gneiss is a metamorphic rock, and shows evidence of severe heat and pressure. This one appears slightly banded, and the pink color visible even from the highway belies a high orthoclase feldspar content. Click on the mineral name to view a picture of it. The clock by which geology ticks is too great for our momentary lives to grasp, and this truth is evident in the two seconds you will have to glance from your speeding car at the contribution of nearly one billion years. This outcrop reveals the core of the Blue Ridge anticline, and some of Virginia's oldest rock.