Great Falls is a wonderful place for a field trip because its history spans more than 500 million years, including three orogenies, a few ice ages, and even historical alterations to the land. One of the latest geologic maps published by Southworth and Fingeret of the USGS (2000) can be found here, with more detailed descriptions of some of the units I'll be discussing. (If I get anything wrong, it's because of my own rusty memory, and not Callan - he gave a stellar tour!)
The units we encountered on the hike included metagraywacke, granite, lamprophyre, amphibolite, and various visitors from the Blue Ridge. Here we go:
Some sexy folding in Lower Cambrian metagraywacke. Graywacke is basically a "dirty" sandstone: instead of being purely quartz grains (as in quartz sandstone), or quartz and feldspar (arkosic sandstone), it contains clay. When it's metamorphosed, the clay becomes muscovite or biotite mica, which shows up in the darker stripes. This unit was deposited by turbidity currents in an ocean basin (you can see graded bedding sequences in some places), and during the Taconian orogeny was smashed between the North American continent and an incoming volcanic island arc. The compressional stresses created the prominent folding seen here.
I didn't realize just how much I missed going on geology field trips - or how easy it is to let your mind drop out of "geology mode". I'm glad for the refresher, and for the chance to get used to thinking on my toes again. (I am not so glad that I managed, somehow, to acquire a dozen itchy spider bites. I'm not dead, so I'm assuming they aren't poisonous, but about half of them are in places on my back that I can't reach, and they're driving me nuts. Guess I should start marinating myself in DEET again.)
By the way, Callan will be leading a few more of these hikes, so if anyone's in the DC area, check out the schedule and sign up!