On his popular science blog Pharyngula, PZ Meyers has a regular series of posts called "I get email," (example) wherein he discusses e-mails he gets. I get e-mail, too (as I'm sure, so do other science bloggers of all stripes). Here's one I got the other day from Brian, a recent graduate from one of my many almae matres (oh yeah, I took Latin). I post it here in case anyone else is wondering the same thing:

I have a simple question for you... I was out at the Pimmit Run-Potomac
confluence collecting rock samples with that awesome chlorite/pyrite/garnet
assemblage and I encountered a couple pieces of unakite float. I'm just
wondering about its provenance. Your blogs seem to indicate that unakite is
typically found in situ farther west in the Shenandoah which would be a pretty
long way to travel (and pretty cool too!) although I believe there is Antietam
around Mather Gorge so I guess it's not impossible; unless it was
anthropogenically relocated which would be much less cool. A little insight
would be greatly appreciated so I can wow my friends when describing what is now the piece de resistance in my fish tank.

So I wrote back with this (links are additions, since I'm blogging it):

Yes, you could certainly have found some Blue Ridge unakite as float in the Potomac Gorge. I've seen many other Blue Ridge Formations as float on the bedrock terraces of the Potomac: Catoctin Formation, Harpers, Weverton, Antietam (like you mentioned), and something that looks a hell of a lot like the Old Rag Granite. I've found well-rounded bituminous coal cobbles, too! I've found unakite further out, in the Coastal Plain, as well as blue quartz (which is unique to the Blue Ridge). So I think it's quite likely you could have found some unakite.

Anyone else have any questions? Like PZ, I could make this a regular series. The more local and the more geo-centric, the better.

Labels: blue ridge, coastal plain, i-get-mail, metamorphism, piedmont, rivers, sediment

An article in today's Post explains more.

Labels: coastal plain, maryland, plants

And now, a few images from April's Environmental Geology class field trip. We made three stops: (1) a large coal-fired power plant in Maryland, (2) Westmoreland State Park in Virginia to look at coastal erosion, and (3) Prince William Forest Park in Virginia to look at pyrite emplacement and acid mine drainage.

Here's one of the bluffs on the Potomac River at Westmoreland:

Note the recent pile of breakdown in the middle of the bluff where all the water seepage is, and also the orange trail as soil from the uppermost bluff has marked another mass wasting event's passage down to the river.

These are Miocene-aged sedimentary layers known as the Calvert Formation, part of the Coastal Plain. In places, the gray clay has been altered along fracture surfaces, as shown by these orange stripes criss-crossing one another. My toes for scale:


The students spent some time searching for fossils: this is an area where lots of shark teeth are found. We didn't have much luck, but after a long cold winter, it was nice to be standing in the warm sunshine and water:


At Prince William Forest Park, we hiked down to the Cabin Branch Pyrite Mine to look at the massive denudation there due to acid mine drainage, and we also spent some time poking around for treasures, in this case chunks of pyrite:


We had better luck than at Westmoreland...




...But of course we were in a national park at Prince William, so we left the pyrite where we found it. (Westmoreland, in contrast, allows you to keep any fossils you find in loose sediment: that figures, eh?)

I'd like to say that the group of students I had in Environmental Geology this past semester was terrific, one of the best groups I've worked with in a long time. Maybe it was because the class was discussion-focused, or maybe it was the cookies we ate every Tuesday night, but it was a great experience for me, and I'm looking forward to teaching the course again. Thanks, everyone, for making it so much fun!

Labels: coastal plain, environmental, field trips, maryland, miocene, nova, ore, piedmont, virginia

I saw Ray Stanford, an enthusiastic amateur paleontologist, speak last month at a meeting of the Paleontological Society of Washington.

It was my first PSW meeting, and I got a warm welcome from PSW president and University of Maryland paleontologist Tom Holtz, who gave a specific shout-out to NOVA Geoblog, encouraging the ~30 attendees to check it out. (If you're arriving as a consequnce of that endorsement, welcome!) Four of my Honors students joined me for the talk. Just getting to go behind the scenes at the Smithsonian is a treat in itself. From the Easter Island moai in the Constitution Avenue lobby of the museum, we were escorted through labyrinthine passageways to the Cooper Room. Our route brought us past immense fossil collections, cossetted away in row after row of cabinets. It was enticing, and made me resolve to arrange a special tour there sometime for the Honors students.

The point of the talk was Stanford's immense collection of fossil dinosaur tracks (and at least one apparent mammal track which is quite large: raccoon-sized at least, with apparent dinosaur skin impressions right next to it). It used to be thought that Maryland only had Triassic/Jurassic fossil tracks, from the Newark Supergroup rift valleys that opened up during the breakup of Pangea / opening of the Atlantic Ocean. Stanford has made a real scientific breakthrough by demonstrating that there are early Cretaceous-aged tracks in the area too.

None of his Cretaceous-aged tracks are collected in situ. Instead, he finds them all as "float" (weathered-out loose blocks) in streams draining exposures of (what I infer to be) the Patuxent Formation. (He didn't specifically mention source formations that I heard during the talk.)

He's found a ton of stuff! Actually,if I'm being literal, he's found tonS of stuff! And he stores it all in his living room! He recently had the foundations of his house reinforced because he has so much STUFF. Hundreds of tracks, and other fossils, too. Whoa! This guy does not play by the same rules as most folks.

There were a lot of coprolites mentioned, including:

• a 98-pound coprolite (!)
• a coprolite with a dinosaur footprint in it
• a dinosaur footprint with a coprolite in it

He also shared what he claimed were skin textures preserved in tracks. Some were self-evident, and I readily accepted them as valid. However, others weren't visible to the naked eye, and he only "demonstrated" them with Photoshopped images wherein the contrast dial was turned up to 11 -- I think this "technique" generated patterns that resembled skin impressions, but when I looked at the fossil itself, they were nowhere to be seen. I am dubious about this particular claim.

The talk gave me lots to think about, but not so much about dinosaur lifestyles or anatomy so much as the role of amateurs in science. Here's a guy with boundless enthusiasm, and he's finding stuff that the books literally said didn't exist. His efforts have resulting in expanding Maryland's Mesozoic paleontological record into the Cretaceous, and he's found all sorts of stuff that's super-duper interesting, like that mammal track.

Stanford was profiled last year in Geotimes magazine, before it switched its name to EARTH. Discovery News also ran a story about his findings. Interestingly, when Googling his name for this blog post, I also came across some other wacky stuff he's involved in, including UFO's. This definitely jibes with the lack of scientific rigor that I perceived in his presentation. (Quote from the interviewer: "In the 1970s, Stanford was the moving force behind the Association for the Understanding of Man (AUM) and Project Starlight. The former an attempt to decipher the UFO enigma by psychic means, the latter using advanced scientific instruments.")

So, having learned this, what do I make of his paleontological data? The best I can come up with is to trust my own eyes and view his claims open-mindedly but with the traditional scientific filter of skepticism. I accept the coprolite data; I found it self-evidently convincing. The skin-texture data? Not so much. The UFO stuff? Don't get me started...

Labels: coastal plain, cretaceous, culpeper basin, fossils, maryland, meetings, mesozoic, museums, psw, science and society, smithsonian

Strange Maps has an interesting piece up today about where West Virginia came from (as a state): turns out it was all about the Civil War. The accompanying map shows the original proposed name for West Virginia, "Kanawha," as well as a proposed demarcation between Virginia and Maryland that trended along the western margin of the Blue Ridge physiographic province. If this boundary had come to pass, Virginia would have gotten the Valley & Ridge province, but Maryland would have retained the Blue Ridge, Piedmont and Coastal Plain.

Labels: blogs, blue ridge, coastal plain, history, maps, maryland, piedmont, valley and ridge, virginia, west virginia

Yesterday was the Geological Society of Washington's fall field trip. A group of about twenty of us went down to George Washington Birthplace National Monument, a stretch of land in the Virginia Coastal Plain, about an hour east of Fredericksburg. The trip was lead by Wayne Newell of the USGS in Reston and Rijk Morawe of the National Park Service.

Here's a map of the Monument, adjacent to a small bay formed as the valley of Popes Creek flooded with post-glacial sea-level rise (essentially the story of the entire Chesapeake Bay in miniature):


Wayne and Rijk are studying the coastal processes here in an attempt to use the Popes Creek as an analogue for Chesapeake Bay processes in general. One of the reasons they really like it is because unlike other small bays in the area, it has a spit (almost a baymouth bar) protecting it from the ravages of the tidewater Potomac (which it flows into). Here's the spit heading southeast across the mouth of Popes Creek Bay:


This rotted old wooden seawall was erected along the coast in the 1960s. This is on the Potomac, just upstream from the Popes Creek Bay. Effectively, this seawall serves as a "before" line, a marker which conveys the shoreline's former position. You can see how much erosion has taken place since then:


I'm less interested in these coastal dynamics, though, than I am in the bedrock geology. There were some bluffs along the river which exposed the Miocene Calvert Formation (clay-rich lower unit) topped by a foot-thick diamictite unit, and then well-rounded river gravels on top of that:


Here's Merily (sp?) from AGI checking out the sequence of strata:


My favorite part of the trip was looking at the variety of cobbles on the beach. These cobbles are derived from all of the mid-Atlantic's physiographic provinces within the Potomac River's watershed (Valley & Ridge, Blue Ridge, Culpeper Basin, Piedmont, Coastal Plain). All those physiographic provinces have been weathered to produce the sediment that the Coastal Plain is made of. In spite of their diminutive size, they give insights into the geologic history of Virginia over the past billion years. So if you're familiar with Virginia geology, you will see some familiar rocks here.

For instance, there were a lot of these Skolithos-bearing quartzite cobbles. These are pieces of the Antietam Formation, a meta-quartz-sandstone that crops out in the Blue Ridge province, many many miles upstream:


Skolithos is the name given to vertically-oriented cylindrical burrow trace fossils, which start showing up in the Cambrian period of geologic time, indicating the evolution of vascularized bodies among animals. They are usually interpreted as worm burrows. This cobble shows several different diameters of Skolithos tubes:


Here's a cobble of another distinctive Blue Ridge rock. This amygdular meta-basalt is a piece of the Catoctin Formation, a sequence of (mainly) mafic lava flows that erupted as the supercontinent Rodinia was breaking up in the Neoproterozoic era of geologic time. The white spots you see are amygdules: vesicles that have been filled in by mineral deposits. When lava erupts, it degasses. If the lava cools into extrusive igneous rock before the bubbles have a chance to pop, little round holes are preserved in the rock, like Swiss cheese. We call these "vesicles." When vesicles get filled in with deposits of minerals (from groundwater passing through the rock), they are called "amygdules," from the Latin for "almond," which I guess they resemble in an ellipsoidal sort of way:

(I showcased a very similar cobble here in March of this year.) Like the Antietam Formation cobbles, this Catoctin Formation cobble originated in the Blue Ridge province, and has tumbled dozens of miles downstream to end up out here on the Coastal Plain.

Here's one from even further away! This is a cobble of flint from one of the limestone units out in the Shenandoah Valley, the easternmost valley of the Valley & Ridge province. (I've previously posted on those rocks, too.) While the limestone which originally hosted this flint nodule has weathered away, the flint is microcrystalline silica: very hard, very chemically stable. It's a common cobble to find surviving out here in the Coastal Plain:

We also found some rocks that are distinctive occupants of the Culpeper Basin, a Triassic-Jurassic rift valley upstream. Here's a chunk of the Manassas Sandstone Formation, another rock that has been previously mentioned on this blog:


The rock I spend most of my time thinking about is the metagraywacke of the Mather Gorge Formation. (For one mention on NOVA Geoblog, click here.) Here's a piece of it that looks identical to the rocks you'll see near Chain Bridge, DC, or along the Billy Goat Trail (Potomac, Maryland):

This rock was metamorphosed ~460 million years ago, in the late Ordovician, although the original sediments are older than that: perhaps Cambrian or late Neoproterozoic in depositional age. This sample even had a little bit of hydrothermal quartz stuck to it, a common feature of Piedmont metamorphics...

Having covered clasts derived from the Valley and Ridge province, the Blue Ridge province, the Culpeper Basin sub-province, and the Piedmont province, there's nothing left in the Potomac River watershed except for the Coastal Plain itself. And sure enough, we saw Coastal Plain clasts too. Here's a chunk of the Calvert Formation that GSW Field Trip Chair Bill Burton found: He cracked it open and found a shark tooth fossil inside:

This is the first time I've ever seen a tooth preserved as a carbon film. Except it wasn't really just a film, it was more a three-dimensional external mold with a carbon film, and little nuggets of carbonaceous material rattling around inside. Shark's teeth are pretty common in Miocene deposits on the Coastal Plain, including C. megalodon teeth, but this style of preservation was pretty novel for me. If you're into fossil collecting, don't go to George Washington Birthplace National Monument, because collecting isn't allowed there. However, nearby Westmoreland State Park offers legal fossil collecting opportunities. It's about ten minutes further south.

I'd like to thank the field trip leaders and Bill Burton for organizing the trip. I enjoyed the excursion!

Labels: coastal plain, field trips, flint, fossils, gsw, rivers, sediment, virginia

I was reminded this morning to update the list of activities for the coming week, including the Billy Goat Trail hike scheduled for Friday:

Wednesday evening: GSW. Free and open to the public.

Thursday: James Ussher's "birthday of the Earth." How will you be celebrating?

Friday afternoon: I'll be leading a public geology hike along the Billy Goat Trail, starting from the Great Falls Tavern Visitor Center (C&O Canal NHP) at 12:30pm, going til 4:30pm or so. Maybe 5pm. Free and open to the public.

Saturday: GSW Fall Field trip: "Tidewater Geomorphology at George Washington's Birthplace National Monument, Westmoreland County, VA." RSVP.

Labels: coastal plain, field trips, gsw, meetings, piedmont

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

On one of my field trips last week, I collected this cobble of sandstone (penny for scale):

And here's the other side:

There's a delicate but telling geopetal indicator here in this sandstone: it shows us which way these sandstone layers were oriented in space when they were deposited as loose sand. Geopetal indicators give us "paleo-up," sometimes called the "younging direction." Classic examples include graded bedding, cross-bedding, and mudcracks. Here, it's a bit more subtle: small erosional "divots" in the layers of sand. These "divots" may be caused by something pushing down into the sand (the trace of an organism's trail), or may be caused by small amounts of scouring erosion. We only get to see them in two dimensions, so it's unknown whether they are simple pinpoints in three dimensions, or linear features -- perhaps even branching linear features. Reviewing the cobble's many layers, I've found three types of "divots":

Type 1 is a simple deflection of the the dark layers. It is more likely that the layer is deflected downward, but there is no guarantee: it could be a little lump of sand poking up from the bottom, too. In other words, Type 1 is not a completely compelling clue for paleo-up. Type 2 is more convincing as a geopetal indicator: here a lower layer or two has been actively scoured, and then an upper layer is draped over the scoured-out hole. Type 3 can also be seen though, and it's a weird one: I'm having a hard time coming up with a reason why two successive beds would both have a "divot" in the same location. Is this a squishing downward effect? For instance, were I to go stand on my bed, my weight would push downward on my comforter, but also the sheets underneath. They would both deflect from the bed's horizontal surface in the same downward direction. (Would this be a "duvet divot?")

See if you can find examples of all three in the photos above.

Labels: coastal plain, primary structures, sediment

Last week on one of the many field excursions, I found a nice cobble of amygdular basalt. Amygdules are vesicles (bubbles in degassing lava that didn't get the chance to pop before the lava solidified into igneous rock) that have been filled in with mineral deposits. In the mid-Atlantic, most amygdules are found in the Neoproterozoic lava flows of the Catoctin Formation, from which my cobble was presumably derived. The amygdules are typically filled in with zeolites, quartz, and jasper. This one doesn't show any jasper, but the basalt still appears to be basalt, too -- whereas the Catoctin typically is metamorphosed to greenstone / greenschist. I've noticed an association between jaspery amygdules and epidote formation in the metaingeous rock.

As with Skolithos-bearing Antietam Formation quartzite cobbles, clasts of the Catoctin deposited in the river gravels atop the Piedmont/Coastal Plain unconformity indicate a Blue Ridge provenance for the cobbles, and therefore a eastward-flowing river to deposit them 100 million years ago.

I took the cobble back to the lab and sliced it open on the rock saw. The brown circle in the background is a penny for scale.

Here's what the sawn surfaces look like after I sanded them down a bit and then scanned them:

Right purty, ain't it?

Labels: basalt, blue ridge, coastal plain, primary structures, sediment, virginia

Walking around the mid-Atlantic Piedmont (my home territory), we find a lot of these fellows lying around. They are cobbles of the Antietam Formation (a Cambrian quartzite from the Blue Ridge) which were weathered out and transported eastwards (~60 miles or so, as you can probably deduce from their rounding). They were then deposited as part of the Potomac Group (Cretaceous river gravels draped over the metamorphic rocks of the Piedmont; preserved today on Piedmont hilltops and as the basal layer of the Coastal Plain). The cobbles display the vertical trace fossil "Skolithos" (sometimes spelled "Skolithus"), usually interpreted as a worm burrow. Each burrow is 2-3 mm in diameter. Here I've got a few photos: a cross-sectional view, a "plan" view, and a shot of one of the boulders in a stream in Arlington, VA.

Labels: blue ridge, coastal plain, fossils, piedmont, sediment