This is how good it is to be a professor on summer break: Yesterday afternoon, after composing yesterday morning's epic account of my Massanutten trip, I toodled on over to the Palisades Museum of Prehistory to (a) drink beer and (b) talk rocks with the museum's curator, Doug Dupin.

The Palisades Museum of Prehistory is in far western Northwest DC, near the Dalecarlia Reservoir and Sibley Hospital. There, you'll find a neighborhood called the Palisades, and in the Palisades, you'll find Doug Dupin's house. In Doug's backyard, you'll find what appears to be a nice shed. Turns out, this is the museum. It's a long story, but basically it boils down to this: Doug was a cartographer, but a contract went sour, and so he was staying at home with a lot of time on his hands. He decided to grow some grapes to make wine, and store that wine in a self-dug wine cellar. He started digging the hole, and encountered arrowheads, pot sherds, and other artifacts. He got intrigued, and decided to showcase the findings atop the wine cellar in a self-made museum.

If you want more details, the Washington DC CityPaper profiled Doug in a 2006 article. A good read; I recommend it.

Doug is a great guy -- pursues what he's interested in, be it homebrew, viniculture, skateboarding (he once rode the length of the C&O Canal on a self-made board -- read about it in this New York Times Magazine article), or archaeology.

Doug attended my "Walkingtown, DC" walking tour of DC's geologic history, and brought along a few odd rocks for me to identify. At the end of the tour, he invited me over to see his museum. Yesterday, I finally got the chance to do that. We cracked open a couple bottles of Dogfish Head 60-minute IPA and started browsing his collection of found prehistoric objects. Doug was very interested in my analysis of rock types (apparently archaeologists use a different set of terminology for describing what rock types projectile points are made out of).

On his own property and in neighboring areas of the Palisades, Doug has found hundreds and hundreds of objects, many of them beautifully worked arrowheads of flint, quartzite, and rhyolite. There are also some oddballs that don't fit with the human prehistory theme: a 1791 coin bearing the image of Louis XVI, crystals of amethyst and gypsum, old glass bottles, rounded river cobbles, and anything else that caught his attention. One of the most astounding things I saw yesterday was a huge woolly mammoth tooth. Doug told me a friend of his found it in the Potomac River while canoing (I think he said near Seneca Creek, but that was a beer and a half in, so maybe I've got that wrong). But there it was, a fully ridged mammoth molar; unmistakable. I hadn't heard of previous mammoth finds in our area, but I guess it's not surprising they were here.

Anyhow, I had a great time, and I recommend that everyone in the DC area make an appointment with Doug to go check out his collection and support his project.

Labels: cenozoic, dc, history, mesozoic, museums, piedmont, tourism

Morning, folks. I awake to a challenge from Chris at GoodSchist, to show where my local bedrock was at the time of Pangea's incipient breakup. (I think Chris chose the late Triassic, 220 Ma, since Ron Blakely's map of that time shows New Zealand clearly in the south.) It's an interesting time for the rock beneath Washington, DC. After have just experienced ~50 million years of crunching between North America and Africa, DC's tortured bedrock is now being stretched as Africa begins to pull away again. A series of rift valleys mark the stretching of the crust, shown clearly in the map as a series of NE-SW oriented lakes along the axis of the Appalachian orogen.

DC's future location is between two of those rift valley lakes: one to the east, one to the west. If I owned DC real estate during the Triassic, I'd be very interested in this process, because one of those rift valleys is going to become a new ocean basin, and one isn't. The one that isn't is destined to stop opening and fill in with dirt. It will be a failed rift valley, an aulacogen of sorts.

The question is: which one is the weakest link? If the one to the west breaks open, that will be the new Atlantic Ocean basin, and DC will stay hitched to Africa. If the one to the east breaks open, that will be the site of the Atlantic, and DC will stay hitched to North America.

As it turned out, the eastern rift was the one that connected up with other rifts to the northeast and southwest, and became the young Atlantic. The western rift, known as the Culpeper Basin, stopped stretching open, and got filled in with sediment. DC stayed attached to North America, and that's the way it is.

Labels: culpeper basin, dc, geology, mesozoic, piedmont, plate tectonics

The Culpeper Basin is a Mesozoic (Triassic/Jurassic) rift valley in northern Virginia.

As Pangea was breaking apart, a series of normal-fault-bound basins stretched open in an NW-SE direction (giving them long axes that run NE-SW). Some of them connected together in a NE-SW direction, and kept spreading further and further open. Through continued seafloor spreading, these became the Atlantic Ocean basin. Some did not keep opening, and essentially filled in with dirt. Those are the ones that are still preserved up on the North American continent today, including the Culpeper Basin. These basins vary in size, but they run up and down the coast of eastern North America, from Newfoundland down at least into the Carolinas (presumably there are more buried beneath Coastal Plain layers even further south than that). Collectively, these basins are referred to as the Newark Supergroup. They are characterized by immature sedimentary rocks and mafic igneous rocks.

Here's an E-W cross section through the Culpeper Basin, by Chuck Bailey at W&M:

Structurally, then, the basin is a graben, bounded east and west by normal faults.

The igneous rocks in the Culpeper Basin are mostly diabase, but there are some basalt flows too. The sedimentary rocks are a motley mix, including arkose, red siltstones, and lake deposits including siltstones and anoxic black shales. Along the eastern and western boundary faults, we also find coarser sediments that have been lithified into conglomerates. Sediments flowed into the basin from source areas both to the east and west, so you would expect the conglomerates along each edge to look a little different. Indeed, they do!

A modern analogue for the Culpeper Basin is the Afar Triangle region of northeastern Africa (Ethiopia, Eritrea, and Djibouti). Note the sedimentary influx from both the east and the west. Note the lakes, and note the mafic extrusions:

Back to the Old Dominion: I've mentioned the Culpeper Basin's eastern boundary fault before, back in March, when I posted this picture of the conglomerate that outcrops in Clifton, Virgina. It is characterized by lots of clasts of highly-foliated metamorphic rocks (derived from the neighboring Piedmont).

...But I haven't talked about the western boundary fault much. And since I visited it yesterday, today's the day to talk about it.

One of these western Culpeper Basin conglomerates is kind of famous. It's the Leesburg Conglomerate, and it outcrops near Leesburg. It's mostly limestone cobbles and gravel, with some quartzite, too, set in a red matrix. It's a beautiful rock. Here's a couple of field photos taken on Route 15, a mile or two north of Leesburg proper:

The Leesburg Conglomerate was used in the awesome columns in the U.S. Capitol's Hall of Statuary (topped by the much less interesting Carrara Marble of Italy).

Yesterday, NOVA adjunct geology instructor Chris Khourey headed out to Thoroughfare Gap (see map below) to check on a couple of field sites. Thoroughfare Gap is a water gap in the eastern limb of the Blue Ridge Anticlinorium, and it's also the western boundary of the Culpeper Basin. Both Interstate 66 and Route 55 pass through this striking landscape feature:

Note how different this looks as compared to the Leesburg Conglomerate. One thing that immediately jumps out at you when you see an outcrop of it is the large proportion of the cobbles that are pieces of the Catoctin Formation basalt (see more photos of the Catoctin in Monday's post on rocks of Shenandoah National Park). Here's a couple of close-up shots of such cobbles, bearing distinctive amygdules (filled-in vesicles):

But there's also plenty of limestone cobbles and gravel in there too, as this photo shows:

As with the Leesburg Conglomerate, the Waterfall Conglomerate's limestone inclusions are likely coming from the Cambrian & Ordovician carbonates exposed today in the Shenandoah Valley and other valleys of the Valley and Ridge province. More on that later this weekend, when I'll post some shots from the Massanutten Synclinorium.

Labels: basalt, culpeper basin, limestone, mesozoic, sediment, virginia