Soapstone Valley, DC
I've been meaning to go check out the Soapstone Valley for years, but finally got around to it on Memorial Day. The park is a valley that shoots off to the east from Rock Creek Park, with an eastern terminus at Connecticut Avenue:
I didn't have far to walk before I found my first cobble of soapstone. It felt soapy in my hand, and was easily scratched by my fingernail. (Fingernail = 2.5 on the Mohs scale of hardness; talc = 1) I found it interesting that the soapstone cobbles had less algae growing on them than the other cobbles in the stream... Hmm. Because they slough off their outer layers more easily? Or because there's something chemical going on that prevents algae growth?
Why does anyone care about soapstone? Well, people who care about prehistory are interested in soapstone because it was easily carved to make various artifacts. As a geologist, I'm more interested in it because it's a metamorphic rock that implies an ultramafic protolith. In other words, as the various rocks that would become DC's bedrock were squished and squeezed and heated during Taconian mountain-building, one of the ingredients in the mix may have been a peridotite. As the graywacke around it metamorphosed to metagraywacke, the putative peridotite metamorphosed into soapstone.
The stuff I found in Soapstone Valley is a talc schist with porphyroblasts or relict phenocrysts of something dark and chunky in it:
Here's a close-up. The big crystals were dark green, like augite, but they had a texture that looked more like hornblende. Not sure as to their identity. I'll put one under the microscope later to try and suss out the relationship between the cleavage planes.
They're definitely mafic though! Here's an example where the large crystals are rusted out:
So there was plenty of soapstone float, but no bedrock outcrops. At first, I was in the highly foliated metagraywacke schist of the Rock Creek Shear Zone...
...but as I headed upstream I found boulders of the Kensington Tonalite, implying exposures of the KT further up the valley...
... and sure enough, that's what I found. This is the Kensington Tonalite, a late Ordovician granitoid.
Where I first crossed the contact, I thought it looked a little odd, and then a later look at the geologic map of the Washington West quadrangle (Fleming, et al., 1995):
Fleming, et al., list it as a sheared biotite tonalite of the Georgetown Intrusive Suite, which I guess explains its appearance as distinct from the Kensington Tonalite.
When I got up to the eastern edge of the park, I saw the source of the stream:
The valley widens out here, almost as if the rock is weaker... And where concrete has been poured (to stabilize the slope??) the underlying rock is etched away: it's the super-soft soapstone...
Here's a boulder of soapstone (my fingernail scratches it to demonstrate that it's soft):
Here's the geologic map of the area. You can see Soapstone Valley cutting an east-west swath across the strike of the structures. ("ss" means "soapstone"...)
My annotations on Tony Fleming's map (reference below).
Reference:
Geologic map of the Washington west quadrangle, District of Columbia, Montgomery and Prince Georges Counties, Maryland, and Arlington and Fairfax Counties, Virginia. Anthony H. Fleming, Lucy McCartan, and Avery Ala Drake. U.S. Geological Survey (Reston, VA), 1995.
_________________________________________________________________
A quick tangent to note a milestone: this is my 700th post on NOVA Geoblog. Thanks to everyone for reading. Looking forward to 700 more...
Labels: appalachians, dc, granite, igneous, metamorphism, minerals, ordovician, piedmont
7 Comments:
The diversity of your area continues to astonish me! Ultramafics (and rock derived from them) tend to be pretty enriched in Ni and Co, which are both toxic to plants. That's my guess as to why the algae is less abundant on the talc cobbles. It's a serious problem with forestry in ophiolitic terrain in OR and CA- once cut (or burned), they're very difficult to reforest.
BTW- been meaning to get back to you on your wish list. I can probably set you up with some very nice pillow basalt, and maybe some of the sed structures. I have neither car nor money, so it's contingent on my powers of persuasion to convince friends they want to go look at rocks. But I'm often pretty good at that.
Lockwood, if you score me a pillow basalt, I'll (a) be your best friend forever, and (b) treat you to a geology field trip of your choice next time you're in the DC area.
C
Callan,
Congratulations on your blogging milestone.
You do great job of clearly explaining and illustrating many of the basics of geology. The photos and videos are outstanding.
Look forward to your next 700 posts
I've got a couple of people interested. I'm going to push for a couple of weeks from now (early July), since that's when the "spring" wildflowers do their thing up on the peak. I wanted to know if you're interested in any of the following samples I could pick up:
hyaloclastite- breccia composed of glassy basalt (sideromelane) and palagonite. There are a variety of basaltic breccias to be had, but this one is very cool looking.
This beastie.
granophyic gabbro This is an oddball chemically and petrographically; I'll try to track down a reference later.
hornfels- spotted in some places, and with remnant bedding in others.
rip-ups in arkosic turbidites
I can probably find some zeolites too, though Marys peak isn't the best locality for those.
Hey Lockwood,
Any and all of those sound superb. Love the graded beds; I was covetous of them when I first saw them on your blog.
C
Here's the paper on the petrography of the Marys Peak Sill (~6 Mb PDF). I realized I misspelled "granophyric," but basically, this freak has olivine, and intertwined k-spar and quartz, and what strikes me as a ridiculous amount of apatite (2-3%) all in the same thin section! The paper is from 1953, a lot of the terminology is archaic or obsolete, the "Burpee Formation," is now the Tyee (the arkosic turbidites), but for this sill, I don't know of anything more recent. There's a thesis abstract that puts these Oligocene intrusions in a broader context, but I think the Roberts paper, despite its age, is kind of mind boggling.
Interesting stuff. That pipe at the upstream end of the valley extends some distance uphill, to Tenleytown, following the pre-urban course. Unfortunately for geology, the landscape was covered over in the early 1900s to make the land developable. (Shameless plug: I wrote something somewhat related to this topic)
As for the concrete, at the top of that hill, there's an unfinished grade for a road, Audubon Terrace, that may have been abandoned precisely because of slope instability.
Post a Comment
<< Home