This week we went exploring south of Great Falls, along a tributary that cuts through Precambrian metamorphic rocks, before joining the Potomac River.
Difficult Run twists its way through a mass of hard rocks that we have met before, Precambrian Schist and gneiss, forming a series of quiet pools (Fig. 3) separated by resistant, rocky sections (Fig. 4).
It was a shady walk beneath a tall canopy of mature hickory, ash, and other temperate forest trees. There was plenty of evidence of the recent spring floods. Large trees were jammed up on rocky outcrops and among the trees covering the floodplain. After a short hike, we came to what looked like an abandoned quarry (Fig. 4), which can be identified by the bright spot in the relief map of Fig. 2, just above the label for Site “2”.
I was able to examine several slabs of the rock exposed in the quarry and along the river bed (see Fig. 1 for appearance) at Site 2, revealing foliation and inclusions similar to other exposures of this rock (Fig. 5).
We’ve seen the structures displayed in Fig. 5 before, in this same rock, at Great Falls and other locations along the Potomac River. I’m presenting these examples to give the reader some idea of the scale of these processes. For example, the juxtaposition of foliation in Fig. 5A suggests a frenzy of activity, like in a pan of boiling water; that analogy is reasonable if we adjust the viscosity, temperature, pressure, and time scale from water on the stove to rocks buried deep beneath the surface, but heated from below–just like the pan of water. I’m speculating here but, just to get an idea of what I’m talking about, the crazy structures in Fig. 5A probably took on the order of ten-million years to form.
It might help to see the problem from a more god-like perspective.
As you might expect, the exposure of such a deformed and mineralogically diverse set of lithologies along the Potomac’s course produces features like Great Falls, as well as what we’re examining today.
There is more to these rocks than metamorphic structures, including folding, foliation, and inclusions. All of those were formed between 1000 and 500 million-years ago. After deep burial (maybe 15 miles) beneath an enormous mountain range, these rocks hardened and were exhumed by erosion of the overlying rocks. They were brittle and, as isostatic pressure relaxed, they cracked just like a cooling pumpkin pie, forming joints.
Confused by what I had seen so far (i.e. Figs. 4, 5, and 8), I followed the trail to the confluence of Difficult Run and the Potomac river (Fig. 9).
Looking across Difficult Run to the south at Site 4 (see Fig. 2 for location), I was once again bewildered.
It is tempting to assume that the overlying rocks in Fig. 11 are sedimentary, deposited on an erosional surface in the underlying metamorphic rocks (angular unconformity); however, the geologic map (Fig. 12) reveals that these are similar in age and lithology.
I summarized the geologic history of this area in a previous post, so I’d like to wrap up by demonstrating how pervasive deformation is, in this post. Imagine the deformation seen in Fig. 5A scaled up several orders of magnitude, to the scale of a bluff (Fig. 11). We also saw evidence of rotation of porphyroblasts at another location along the Potomac and again, more than a hundred miles to the south, in Lynchburg. This deformation actually extends to the microscopic scale, but we had neither proper samples nor a microscope to demonstrate it for these rocks.
Think of metamorphosis and ductile deformation as being like a peach pie, the contents trapped between the bottom of the pan (deeper, more resistant rocks) and the pie crust (overburden); the filling is boiling in the oven, overturning, even displacing smaller pieces of fruit. That is what’s happening miles beneath the mountains, on time scales of millions of years rather than minutes.