Hiker's Notebook: Massanutten Sandstone

Geologic Formation: Massanutten Sandstone, Clinch Sandstone, Tuscarora Sandstone

Massanutten is an Indian word meaning basket and is given to Massanutten Mountain located in the center of Shenandoah Valley because it is shaped like a basket. Massanutten Sandstone is named according to standard geologic methods for the region where the formation was first described, Massanutten Mountain. However, geologic formations generally extend over large areas and may only be exposed in remote and unique circumstance according to the localized effects of uplift and erosion and may therefore get parochial names. Clinch Sandstone was first identified on Clinch Mountain in Tennessee in 1856 and Tuscarora Sandstone was named for Tuscarora Mountain (after the Tuscarora Indians) in central Pennsylvania. As both names were already well established when an onomastic system for geological names was formalized, the combined name of Clinch-Tuscarora Sandstone was adopted for the formation in 1974. Nonetheless, it is also known as Massanutten Sandstone.

Sandstone is a clastic sedimentary rock that contains particles that range from one sixteenth millimeter to two millimeters in size. If the grain size is smaller it is mudstone; if larger conglomerate. A clastic rock (from the Greek klastos meaning broken) is one that is formed by fragments of preexisting rock, suggesting an erosive provenance. The constituent rock fragments that predominate in a sandstone serve as a means of classification. Arkose sandstone contains pink feldspar, Graywacke is bonded with gray clay-sized particles and is dark in color, and Orthoquartzite is predominantly comprised of grains of silicon dioxide, or quartz, and is nearly pure white. There is some confusion in terminology concerning the use of the terms sandstone, quartzite and quartz. Quartz is a mineral comprised of silicon dioxide (SiO₂), and is a constituent in both sandstone and quartzite. It is second only to the aluminum silicate feldspars as the most common mineral of the earth's crust. Quartzite is geologically defined as metamorphosed sandstone; orthoquartzite is defined as a sedimentary rock that is comprised almost exclusively of quartz and in some texts is considered a type of sandstone. In many cases, quartz-rich sandstone and quartzite occur in the same area, as they have the same littoral origins but may have been subjected to different metamorphosing temperatures and pressures. The only conclusive test is to examine a fractured surface. If the crack goes around the quartz grains, it is sandstone and if it goes through them, it is quartzite.

The white, fine-grained, uniform texture of Massanutten sandstone indicates that it is orthoquartzite, or very quartz-rich sandstone. This composition reflects the geologic origins of the formation. It was deposited in the Silurian era (named for the Silures, a Celtic tribe of Wales conquered by the Romans in 80 CE; characteristic rocks were first identified in Wales) about 430 million years before the present (a more convenient unit for "million years before present" is Mega-annum or Ma). It was subsequently uplifted during a sequence of mountain building disturbances called orogenies (from the Greek oros meaning mountain) during the Permian era (named for the city Perm, just west of the Ural Mountains in northern Russia) about 250 Mega-annum. The deposition and uplift process is best explained using the theory of plate tectonics.

The sequence of events that led to the deposition of Massanutten sandstone begins about 1.2 billion years ago (1200 Ma) with the proto-Appalachian mountains were created by the collision of the North American and Eurasian Plates to form the supercontinent called Rodinia. Old Rag Granite and the Pedlar formation of the Blue Ridge Mountains are its vestiges. When Rodinia broke up about 600 Mega-annum, the edges of the continental plates moved apart, allowing magma to extrude onto the surface where it cooled to form masses of basaltic rock, such as the Catoctin formation. This began the process of sediment erosion from the highlands of the proto-Appalachian Mountains and its deposition in the lowlands of the Appalachian Basin to the west. Ultimately, this would result in the accumulation of nearly 40,000 feet of sedimentary rock, more than seven times the distance between the top and the bottom of the Grand Canyon.

For most of the Paleozoic Era, the Appalachian Basin was under water, collecting the sediments that washed down rivers that flowed from west to east. From the middle of the Cambrian (Cambria is the poetic name for Wales which derives from Cumbria, the Celtic name for Wales) to the late Ordovician (the Ordovices were another Celtic tribe in Wales), a period of some 70 million years, this deposition consisted largely of carbonates, creating what is commonly called the Great Carbonate or Great American bank. The carbonates combined with either calcium to form limestone or calcium and magnesium to form dolomite, generating a rock layer up to 10,000 feet thick that created the geosyncline that is the basis for the Shenandoah Valley of Virginia, the Great Valley of Maryland, and the Cumberland Valley of Pennsylvania. A geosyncline is defined as an elongate belt of the earth's crust that subsided due to the weight of the sediments that had accumulated. Since limestone is subject to hydraulic erosion, a geographic region known as a karst (named for an area near Trieste with characteristic topography) was created. The many caves, caverns and underground rivers of the Shenandoah Valley are the result of the karst topography inherent to limestone strata.

Near the end of the Ordovician era, the movement of the North American Plate and the Eurasian and African Plates reversed, ending the breakup of Rodinia and leading ultimately to their recombination as Pangaea (from the Greek pan and ge, literally all earth). The ultimate collision of the three plates occurred over 150 million years in three separate orogenies: the Taconic, the Acadian and the Alleghenian. It is hypothesized that the Taconic orogeny resulted when the North American and Eurasian Plates first collided. This resulted in one of the plates moving under the other, a phenomenon known as subduction; the plates are larger than the continents that are located on them. The Acadian orogeny occurred when the land masses riding on the plates ultimately collided. The Alleghenian orogeny resulted when the African Plate collided with the other two. The basis for this hypothesis is that the first two orogenies affected primarily the northern Appalachians, whereas the last was primarily in the south.

The Taconic orogeny was named for the Taconic Mountains of New York, where the effects of the collision were more pronounced. However, a general uplift occurred in the Shenandoah Valley. This ended the carbonate deposition as the sea became shallower, an environment that resulted in the deposition of fine silt and mud. This created the Martinsburg formation, which consists of many beds of sandstone that gradates to mudstone and shale over a thickness that ranges from an inch to over a foot. At the beginning of the Silurian era, sand was washed westward from the uplifted Taconic highlands and deposited on top of the Martinsburg Formation. Large quantities of pure quartz require a source of sand, the absence of other materials, and a means of sorting the grains according to size. The pre-existing sand was available from the proto-Appalachian Mountain sedimentation (like those of the Chilhowee Group). It had already been cleaned of contaminants and the sedimentation process sorted the grains. The pure sand sediments lithified (changed to stone) to become Massanutten sandstone.

Over the course of the next 150 million years, numerous sediments were deposited on top of the Massanutten sandstone as erosion continued. The Acadian orogeny had little effect, as it occurred to the north. The Alleghenian orogeny that occurred at the end of the Paleozoic era about 250 Million years age caused the uplift of the Blue Ridge Mountains and the folding and faulting of the Ridge and Valley Province. The compression of Shenandoah Valley resulted in the formation of a syncline in its center. A syncline is a geologic formation that results in the strata being concave up such that the youngest rocks are in the center, much like a bowl. Following the Alleghenian orogeny, the continental plates reversed and Pangaea began to break up, eventually forming the Atlantic Ocean and the current configuration of continental land masses. The syncline that formed as a result of the orogeny was subject to millennia of erosion which removed the softer shales and mudstones, leaving behind the harder sandstones, quartzites, and granites. The Massanutten sandstone that was deposited in the Silurian era gradually became exposed, forming the crest of Massanutten Mountain.