Thanksgiving Day, Lily and I took a hike in Hickory Nut Gorge State Park, North Carolina, just south of the much-better-known Chimney Rock, which was closed for the holiday.



Just outside the park, on the public right-of-way, I collected this lovely chunk of granite gneiss which shows both foliation and lineation:


This is classic Blue Ridge province basement rock; it formed ~1.1 or 1.2 billion years ago during the episode of mountain building known as the Grenvillian Orogeny. We've got many of the same sorts of rocks (though slightly younger) up in Shenandoah National Park in Virginia. However, the thing that caught my eye about this one is the fact that it has such well-developed lineation. You're probably already familiar with foliation, the planar alignment of mineral grains in many metamorphic rocks. Lineation, a linear alignment of mineral grains, is somewhat less common, as it requires a different sort of stress field to form. In the scanned image above, you're looking straight at the plane of foliation. Within that plane of foliation is the pronounced lineation, which indicates that the maximum principle stress was directed perpendicular to foliation (plane of the screen), an intermediate principle stress was directed left to right, and the minimum principle stress was directed top to bottom, which is why the gneiss squootched out in that direction*. Hemmed in from the sides, smushed from the front and back, it had nowhere to go but "up." The strain ellipsoid here would be shaped something like a flatworm, or a baguette that had been run over by a steamroller.

* I'm assuming a monoclinic stress field.

Labels: blue ridge, granite, igneous, metamorphism, north carolina, structure