The largest meteorite (or maybe comet?... we don't really know which) impact crater in the United States is in Virginia, underneath the lower Chesapeake Bay. In the Eocene, a large bolide (unidentified space chunk) slammed into the Earth. Dating of microfossils found in the same sedimentary layers as impact ejecta have provided a date of ~35.5 Ma for the event. The impactor hit on the continental shelf offshore of Eocene Virginia, carving through the Atlantic-deposited sediments there and gouging into the crystalline bedrock beneath (igneous and metamorphic rocks like the modern Piedmont province, but buried beneath Coastal Plain layers).

The crater was discovered over a ten-year process that began with offshore sampling near Atlantic City, New Jersey in the mid-1980s. Those drill cores came up with a layer of ejecta (including shocked quartz and little beads of glass called tektites) among the late Eocene layers of sediments. Searching around, eventually the crater was seismically imaged by oil exploration in the Chesapeake Bay in the mid-1990s.

Centered on Cape Charles, Virginia, the crater is about 50 miles across, but appears wider as sedimentary layers adjacent to the hold have slumped inward along listric faults. The James, York, and Rappahannock Rivers all trend into this depression, and ultimately the crater is probably responsible for the Susquehanna River taking on its southerly course. When sea level rose and flooded the valley of the Susquehanna, the Chesapeake Bay was formed.

A similar impact structure offshore of New Jersey, the Toms Canyon Impact Crater, may have formed at the same time as the impactor broke into pieces before impacting.

The lead-off image to this post is by the team at the U-Haul trucking company, which performs a terrific public service by finding out interesting things about the different states (and Canadian provinces) and posting them on the sides of their trucks with eye-catching graphics. A great many of the topics they choose are about geology, from minerals to fossils to impact craters to cartography and canyons. A while ago, I wrote an article for Geotimes looking at their program.

More information on the crater:

Wikipedia's entry on the crater.
W&M Geology Department's page about the crater.
USGS team examining the crater.
National Geographic article (2001).

Labels: art, cenozoic, coastal plain, geology, meteors, new jersey, piedmont, rivers, virginia

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

Okay, so we've all heard what a stinker the new movie 10,000 B.C. is, right? I actually went to see it, on Geotimes' nickel, along with a couple of other scientists so we could assess the scientific validity of the film for the magazine. Afterwards, I went to enter my own "review" into Netflix (2 stars out of 5) and I noticed there was another "10,000 B.C." film in the Netflix library: "L.A. 10,000 B.C.," a program from the Discovery Channel that examined the natural history of the Los Angeles area during that same time. I decided to check it out, and last night I watched it.

My brief review: It's not really a traditional natural history program. Instead it takes fossil evidence and uses it as a starting point for a "reality T.V." style stunt program. They take three L.A. stunt actors and "train" them to be Ice Age hunter-gatherers. Then they build pneumatic robots to mimic the teratorn and Columbian mammoth, and the stuntmen and women have to battle them. No joke. This resulted in some cool visuals, though: the mammoth crushing a ten-pound can of tomatoes and having all that red spray everywhere (mimicking the head of a Clovis hunter). And the footage of the stuntman being tossed thirty feet through the air by the "angry" robot mammoth was kind of cool too.

But you can't really call that a nature program. There were some cool facts presented, but the majority of the film was devoted to sensationalism of the encounters between humans and these Pleistocene species. The film was also very repetitive, taking half an hour's worth of material and stringing it out into 1.5 hours. It appeared to have been designed so anyone channel-surfing could get an orientation as to what the program was all about regardless of when they tuned in. That's kind of lame if you're watching the whole thing from start to finish, methinks.

Labels: birdies, cenozoic, mammals, movies, tv