Integrative Natural History of the Blue Ridge Province, Part 5: Balancing Act | Greenstone Overlook, Blue Ridge Parkway, Virginia, USA
The central island in the Greenstone Overlook's parking area contains an excellent exposure of one of the Blue Ridge's most extensive and interesting rock units, the Catoctin Formation.
The term greenstone is a somewhat informal name for what most geologists now prefer to call metabasalt. But the former moniker is certainly applicable here, where a low, tilting ledge of what I take to be bedrock has an unusual and lovely tint.
Whatever you prefer to call it, this rock was basalt—a mafic, extrusive igneous type—that was subsequently metamorphosed. In the process of transformation its predominantly black-toned mineral content was altered to include such green constituents as epidote, actinolite, and chlorite.
Atop the ledge sits an eye-catching example of what is often called a "balancing rock." This is a very hefty boulder, presumably Catoctin too, that after being detached by weathering or erosion slumped downhill to this spot a very long time ago. Either that, or some megamuscular Civilian Conservation Corps workers, in a Depression-era prank not recorded, hoisted it up here for decorative effect. And, also in the realm of alternative history, I'm tempted to start a meme about Ancient Astronauts or perhaps some far-wandering Phoenicians doing the heavy lifting.
Were this feature located farther north, in New England, I'd hypothesize in yet another direction, and guess that a melting Pleistocene ice sheet had delivered it to this precarious perch instead. At any rate, one day the big rock will lose its balance at last and continue its journey downslope.
And regarding the Catoctin Formation itself. Being metamorphic, it has two ages of note. The first, the latter part of the Neoproterozoic era, marks the time when this deposit was erupted in great sheets of lava that spread over both land and water. This was the time of the breakup of the supercontinent Pannotia, triggered in part by a hot spot fed by a mantle plume.
So the Catoctin is most likely the remains of an immense flood-basalt event. As such, it's the younger equivalent of the gigantic eruption that took place in the Lake Superior region half a billion years earlier.
The second benchmark in the Catoctin's history is the late Paleozoic era (and for the most part the Mississippian / Lower Carboniferous subperiod). It was then that the metamorphism took place, during major tectonic activity some Eastern-US geologists now attribute to an orogeny they call the Neoacadian.
In any event, this deformation that turned the Catoctin rock a handsome green occurred well before the Alleghenian mountain-building episode that marked the formation of the next supercontinent, Pangaea.
To dive into this ancient flood-basalt lava a little more deeply, see the other photos and descriptions of this series in my Natural History: Virginia album.
GT
Integrative Natural History of the Blue Ridge Province, Part 5: Balancing Act | Greenstone Overlook, Blue Ridge Parkway, Virginia, USA
The central island in the Greenstone Overlook's parking area contains an excellent exposure of one of the Blue Ridge's most extensive and interesting rock units, the Catoctin Formation.
The term greenstone is a somewhat informal name for what most geologists now prefer to call metabasalt. But the former moniker is certainly applicable here, where a low, tilting ledge of what I take to be bedrock has an unusual and lovely tint.
Whatever you prefer to call it, this rock was basalt—a mafic, extrusive igneous type—that was subsequently metamorphosed. In the process of transformation its predominantly black-toned mineral content was altered to include such green constituents as epidote, actinolite, and chlorite.
Atop the ledge sits an eye-catching example of what is often called a "balancing rock." This is a very hefty boulder, presumably Catoctin too, that after being detached by weathering or erosion slumped downhill to this spot a very long time ago. Either that, or some megamuscular Civilian Conservation Corps workers, in a Depression-era prank not recorded, hoisted it up here for decorative effect. And, also in the realm of alternative history, I'm tempted to start a meme about Ancient Astronauts or perhaps some far-wandering Phoenicians doing the heavy lifting.
Were this feature located farther north, in New England, I'd hypothesize in yet another direction, and guess that a melting Pleistocene ice sheet had delivered it to this precarious perch instead. At any rate, one day the big rock will lose its balance at last and continue its journey downslope.
And regarding the Catoctin Formation itself. Being metamorphic, it has two ages of note. The first, the latter part of the Neoproterozoic era, marks the time when this deposit was erupted in great sheets of lava that spread over both land and water. This was the time of the breakup of the supercontinent Pannotia, triggered in part by a hot spot fed by a mantle plume.
So the Catoctin is most likely the remains of an immense flood-basalt event. As such, it's the younger equivalent of the gigantic eruption that took place in the Lake Superior region half a billion years earlier.
The second benchmark in the Catoctin's history is the late Paleozoic era (and for the most part the Mississippian / Lower Carboniferous subperiod). It was then that the metamorphism took place, during major tectonic activity some Eastern-US geologists now attribute to an orogeny they call the Neoacadian.
In any event, this deformation that turned the Catoctin rock a handsome green occurred well before the Alleghenian mountain-building episode that marked the formation of the next supercontinent, Pangaea.
To dive into this ancient flood-basalt lava a little more deeply, see the other photos and descriptions of this series in my Natural History: Virginia album.
GT