Rocks exposed in the Commonwealth of Virginia record the advance of ancient shorelines, the collision of continents, the tracks of long-extinct dinosaurs, and over one billion years of geologic history. Virginia has a diverse landscape that extends from the high rocky summits in the Blue Ridge Mountains to low-lying barrier islands exposed to the Atlantic Ocean. Virginia has five different geologic regions: Coastal Plain, Piedmont, Blue Ridge, Valley & Ridge, and Appalachian Plateau.
Coastal Plain
The Virginia Coastal Plain is mostly unconsolidated or partially consolidated sediment deposited along a coastline not significantly different from that which exists on the coast of Virginia today. These sediments rest on an eroded surface of Precambrian to early Mesozoic rock. The majority of this wedge is comprised of late clay, sand, and gravel which were stripped from the Appalachian mountains, carried eastward by rivers and deposited in deltas in the newly formed Atlantic Ocean basin. If we could see the coastal plain sediments in a cross section they would look like a wedge, thick to the east and thinning to a feather-edge on the west.
Piedmont
The Piedmont is bounded on the east by the Fall Zone, which separates the region from the Coastal Plain, and on the west by the mountains of the Blue Ridge province. The region is characterized by gently rolling topography, deeply weathered bedrock. As a result of the Piedmont's humid climate and bedrock is generally buried under a thick (6 - 65 feet) blanket of saprolite (weathered rock).
A variety of igneous and metamorphic rocks make up the bedrock of the Piedmont. Most of these rocks range in age from Proterozoic to Paleozoic and form the internal core of the ancient Appalachian mountain belt. Triassic sedimentary rocks, diabase dikes, and basalt flows are present in a number of grabens and half-grabens that formed during the early stages of rifting associated with the opening of the Atlantic Ocean. Rivers and streams carrying sand, silt, and mud flowed into these lowland rift basins burying swamps and marshes, later producing small coal deposits.
Blue Ridge
The Blue Ridge province includes both the Blue Ridge mountains (Skyline Drive and the Blue Ridge Parkway), and the strip of land to the east running through Galax, Charlottesville, Culpepper, and Warrenton. Structurally the Blue Ridge province is a large, eroded anticline overturned to the west. The core of the anticline is composed of igneous and metamorphic rocks collectively known as the Grenville, although there are also late Proterozoic intrusives and sediments present too. They are the oldest rocks in the state at 1.1 billion, and a protolith (earlier rock now modified to something else) back to 1.8 billion.
Valley & Ridge
This province lies east of the Allegheny Plateau and west of the Blue Ridge; its western boundary is known as the Allegheny Front and marks an abrupt change from the flat lying rocks of the plateau to the folded/faulted rocks in the valley and ridge. On the ground these ridges can be easily seen looking west from Skyline Drive or the Blue Ridge Parkway. The row after row of parallel mountains and valleys (beginning with the Shenandoah Valley) gives the province its name.
The rocks here are sedimentary, folded into anticlines and synclines, lower and middle Paleozoic in age (Cambrian through lower Mississippian). The ridges exist because they are underlain by hard rock that erodes less easily than the softer rock in the valleys. The lowest part of the stratigraphic section is mostly Cambrian and lower Ordovician carbonates (limestones and dolomites) deposited in tidal flat and coastal environments. Most of the rest of the sedimentary rocks are sandstones and shales deposited in deep marine basins during times when the Piedmont region was a large mountain rather than its present flatness.
Appalachian Plateau
This province is found only in the southwest part of Virginia, although much of West Virginia is composed of the plateau. The rocks here are sedimentary, flat lying, and topographically high above sea level relative to the rest of the state.
The exposed rocks are late Paleozoic in age (Devonian, Mississippian, Pennsylvanian, and in parts of West Virginia, Permian). These rocks were deposited at a time when most of Virginia to the east contained major mountain ranges (Taconic, Acadian and Alleghenian orogenies). Underneath the Devonian rocks are additional flat lying sedimentary rocks of Cambrian, Ordovician and Silurian age. Finally at the bottom of the pile are igneous and metamorphic Grenville Basement Rocks (1.1 billion years and older.)
Sources
http://web.wm.edu/geology/virginia/
http://csmres.jmu.edu/geollab/vageol/vahist/PhysProv.html
Cache Requirements
To claim this cache you will need to email the following information. Please do NOT include answers to #1 and #2 in your log.
1) Of all the rocks, which is the largest? Include both the number and rock type (granite, shale, etc.), along with an estimate of its size (volume).
2) Of all the rocks, which is the smallest? Again give the rock’s number and mention whether it is igneous, sedimentary or metamorphic? How did it form? How is this rock used commercially?
3) Of all the rocks, which is your favorite? Where is this rock found in Virginia and why is it your favorite?
Including a picture of yourself (or GPSr) with the sign or your favorite rock in the background, while not required, is much appreciated.