23 January 2018
My family and I took a hike this past weekend in the George Washington National Forest. We hiked up the hill from our house and explored along the trail system that runs along the crest of Massanutten Mountain, and then exited through Veach Gap. Along the way, my five-year-old son made lots of little discoveries.
Here, he examines some interesting smooth lines on the face of a block of Massanutten Formation quartz arenite (quartz sandstone):
These lines are smooth to the touch, and all parallel to one another:
They are slickensides, little polished grooves that develop on a fault surface when one block of rock grinds against its neighbor. So this side of this cobble is a scrap of a fault surface!
There’s still a tiny piece of the opposite side of the fault attached to this block, right underneath my son’s fingertip. You can see the slicks emerging from beneath that little wedge:
In our region, most of the rock units are pretty susceptible to weathering and erosion, and the most typical lithology to be found is this indurated Massanutten quartz arenite. Occasionally it yields neat features like cross beds or pebbles, but mostly it’s massive. Honestly, it’s not all that sexy. But when it is improved with the addition of a bit of structure, it can hold my attention (and the attention of a five-year-old boy!) for a moment or two.
The age of the sand’s deposition is Silurian, somewhere between 440 and 417 (±10) million years ago. But the age of these slickensides must post-date that: It is, after all, impossible to fault something that doesn’t yet exist. Furthermore, the slickensides must also post-date lithification (turning the sand into sandstone). While unlithified sand can be faulted, it won’t leave slickensides behind as a signature. So I suspect, based on the age of other deformation in the Valley & Ridge province, that these slicks are late Pennsylvanian in age: that’s the time of the Alleghanian Orogeny, an ancient mountain-building event that accompanied the assembly of Pangaea. At this time, this part of the Appalachian system got seriously stressed out. It was compressed from the east/southeast, causing the stratified rocks to crumple and fold. In places, the stresses were too grade for the ductile folding to accommodate, and there were faults that ruptured through the rock, grinding one piece into a new position relative to its neighbor.
So we’re looking here at a miniature Appalachian geologic history: first deposition of sediment, and then later deformation of the resulting sedimentary rock. And also, I suppose we might add in the effects of differential weathering, breaking down the limestones and shales and whatnot, and leaving the tough quartz arenite poking out proud of the landscape to make mountain ridges. In this single cobble, we can extract information that sketches a rough outline of the entire mountain range’s history.
Lagniappe: later in our hike, we passed the celebrated anticlines of Veach Gap, evidence of a more ductile form of Alleghanian deformation:
Thanks for joining us on this little jaunt. I hope you found it as slick as we did!