4 June 2008
Posted by Jessica Ball
I went on a short hike recently to find samples of Virginia rocks to fill out my collection and use for a presentation I’m going to be making at a career fair. Because I didn’t feel like trekking all the way out to the mountains by myself (not at all environmentally conscious!) I went looking for them in the stream beds near my house.
I live in a pretty generic suburban area, but much of the housing in my neighborhood was built in the 50s and 60s, and it seems that they weren’t so keen on destroying all of the natural environment back then. As a result, there are a lot of wooded areas and a number of small streams that cut through Coastal Plain sediments and drain into the Potomac. A geologic map of the area calls the deposits “Qte: Low-level fluvial and estuarine deposits (Pleistocene).” Mainly, it’s a lot of sand with pebble-to-cobble sized clasts and lots of of iron oxide chunks – overlain, naturally, by every polymorph of poison ivy conceived of by nature. It’s an area I’m familiar with, having spent a great deal of time there when I was young, getting mucky and probably exposing myself to any number of diseases and bacteria (I didn’t actually go IN the stream, but even with wading boots, it’s hard to avoid getting a little wet. Hooray for rubbing alcohol and soap.)
The streams themselves are supposedly spring-fed, and there are a few roads and subdivisions with “Springs” in the name. I suspect more water comes from runoff these days, since there are a lot of paved surfaces in the area, but I can vouch for the existence of the springs – one part of my yard and the sidewalk adjacent are constantly wet, no matter what the weather has been like, and none of the drains in our house connect to the sewers in that area. Wet enough, in fact, that’s it’s created a great example of differential erosion in the concrete curb where the water drains to the street.
Anyway, back to the rocks. Here are some of the clasts I picked up:
Antietam metasandstone: An old friend, and one I was expecting to find in abundance (which I did). Very distinctive Skolithos trace fossils (worm burrows), which make it pretty easy to spot. This is a Chilhowee Group rock (shallow marine and fluvial siliciclastics deposited in the late Proterozoic and early Cambrian), and makes up one of the resistant layers in the BR. (Screw is ~1 in.) I don’t call this quartzite, as some publications have done, because individual quartz grains are still visible. (My undergrad adviser had a major pet peeve about this, and I’ve inherited it.) It’s very well cemented and may have been metamorphosed to some extent, but in every sample I’ve looked at, the sand grains are still distinguishable, and haven’t been recrystallized enough to call it a quartzite. That degree of recrystallization would also have destroyed the Skolithos tubes, which hasn’t happened in these samples.
A closeup of one of the tubes.
Sandstone with cross-stratification: Lots of this, surprisingly. The cross-stratification ranged from mm to cm-scale layers (in the largest cobble I picked up). I’m betting Chilhowee Group again, possibly Weverton (braided stream deposits) or another part of the Antietam.
Coarse-grained sandstone: I was hoping this would be an arkose when I cleaned the mud off it, but it looks to be just a sandstone with some oxides in the cement. No feldspar cleavage surfaces, as far as I can tell, and it’s not really close enough to a source area for feldspar to have survived. Could be another bit of the Weverton – I wouldn’t call it Antietam, because the Antietam sandstone tends to be made of very pure, very white quartz, and I’ve never seen it with oxidized cement.
Chert: Again, unsurprising; this stuff shows up a lot in stream beds. I thought at first it might be micritic limestone, but it won’t scratch or fizz, so chert’s the verdict. It probably came from a carbonate unit like the Lincolnshire Formation (middle Ordovician cherty limestone).
Mystery rock! N
ot really what the story on this is – it looks like a siltstone with inclusions of little clay flakes.
Anthracite coal: No photos of this, but I’m sure everyone can guess what it looks like. I was a bit surprised to find it, as this particular stream doesn’t run anywhere near railroad tracks, which is usually where I find stray coal in my area. I have doubts that it would have survived the trip from western Virginia to the Coastal Plain, so I don’t think it arrived here naturally. Perhaps it was dumped or washed in from someone’s yard – the land around my house used to be part of a large farm, so maybe it’s historic coal.
I really want to show off the cross-bedding in the large cobble I collected, but I haven’t had a chance to clean it properly just yet. I’ve also found a few other samples that I’m not completely sure of, but again, haven’t had time to sit down with the handlens and camera, so they’ll have to wait until the weekend.
Some good resources for Virginia geology:
USGS Geolex Database: This is a great website – it allows you to look up recognized geologic units by age, name, location (down to the county level), rock type, even by author citation. Units are displayed with alternate names, “areal extent”, type localities, subunits, and a history of the names and descriptions that have been applied to the unit, even back into the 19th century, with the authors and publications they’re found in.
Geology of Virginia (College of William & Mary): A very good basic overview of Virginia geology, with generalized province and geologic maps to download, and a selection of outcrop and sample photos.
Geological Evolution of Virginia and the Mid-Atlantic Region (JMU): Oddly organized, in my opinion, but useful for quick reference.
Virginia Department of Mines, Minerals and Energy, Division of Mineral Resources: Archived issues of the Virginia Minerals newsletter, some open-file reports, and a source for info on diamonds in Virginia.
Your bottom picture looks like a metatuff.About quartzite: It’s OK for a quartzite to display original sand grains, as long as the rock fractures across them and not between them. Such is called an orthoquartzite.