Here's some images from my first post-master's-post-master's graduate class: "Bahama Montana," Dave Lageson's one-credit examination of carbonate sedimentology with a particular focus on some interesting features in the Bridger Range: Waulsortian-type bioherms. This field trip was on my fourth day in Montana this summer, following a two-hour lecture the previous evening. Unfortunately, the road to Fairy Lake still wasn't totally open, which meant that we had to add an additional three miles each way to our hike, which meant we didn't get to examine the bioherms themselves at close range. Oh well; next year perhaps...

The class hiking up to the summit of Sacagawea Peak:

(The green stripe on the left/west may look familiar from the satellite image I shared yesterday.)

Sacagawea Cirque, not looking especially "Death Cirque"-like* today:


The view south from Sacagawea Peak:


The class, looking east from the summit:


The elusive Waulsortian bioherms, off in the un-logistically-feasible middle distance:

...Interesting that they weather out in high relief, eh?

Dave instructs:






Some cool Columella stromatolites that I hadn't noticed on previous trips up Sacagawea:


More Columella stromatolites:


The class was a good example of how field trips have to be modified to fit local conditions. It was a bummer the road closure added six miles to our hike, but we were able to scour the talus slopes in Sacagawea Cirque for Mississippian fossils like crinoids, brachiopods, corals, and bryozoans. I got some sweet samples of fenestrate bryozoans, but saw none of the spectacular rugose corals that I collected on my first visit to this cirque 2 years earlier.

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* The "Death Cirque" moniker is one applied by my NOVA Rockies students the following week, for reasons I shall reveal in due time...

Labels: field trips, fossils, limestone, mississippian, montana, stromatolites

My third day in Montana this summer, Lily and I took a hike in the Bridger Range, going up the west side of the range via Corbly Gulch to a cirque opposite the "usual" route up Sacagawea Peak, which starts at Fairy Lake on the east side, then goes up into Sacagawea Cirque* and south to the peak. Instead, we went up Corbly Gulch and got a whole new look at Bridger stratigraphy. First, orient yourself with this topographic map:



The Fairy Lake route brings you to the ridge crest from the upper right (northeast), wheras the Corbly Gulch route brings you to the same ridge crest from the lower left (southwest). Now take a look at some satellite imagery:



The green line at upper right is the ridge crest; Sacagawea Peak is just off-screen to the right. It will not surprise you to learn that stratigraphic contacts strike NW-SE in this area. The forested left-hand part of the screen is underlain by Mesoproterozoic LaHood Formation, a coarse-grained formation in the Belt Supergroup. Then there's a little gap of grassy area and a thin line of trees atop a light-brownish layer. This is the Cambrian Flathead Sandstone, which is chock-full of interesting sedimentary structures and trace fossils. The prominent light-colored ridge-forming layer traversing the screen from upper left towards lower right is the Cambrian Pilgrim Limestone, which shows "fossil hurricanes" in the form of limestone-chip conglomerates.

Here's some of the trace fossils in the Flathead Sandstone:


Here's a limestone-chip conglomerate from the Pilgrim Limestone, which I interpret as a paleo-hurricane deposit: rip-up clasts from a carbonate bank tumbled and re-deposited together in a big jumble:


We hiked up to the ridge, and peered down into Sacagawea Cirque (getting pummeled by the wind!), but didn't feel like we had sufficient time to attempt summiting Sacagawea, since I had to be back on MSU's campus for an evening session as part of "Bahama Montana" class. More on that tomorrow...

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* The following week, my Regional Field Geology students proposed to rename Sacagawea Cirque as "Death Cirque," for reasons I will explain in due course...

Labels: cambrian, fossils, limestone, montana, msse, primary structures, proterozoic, sediment

Two examples of plumose structure, beautifully expressed in a small boulder of the Helena Formation (Mesoproterozoic limestone from the Belt Supergroup) in Glacier National Park, Montana. Photo by Bob L'Hommedieu.

Labels: field trips, limestone, montana, national parks, proterozoic, structure

I'm swamped -- absolutely sodden with responsibilities, of a dozen flavors. Stressed, harried, scatterbrained, and to top it all off, no time for proper blogging. But that doesn't stop me from getting excited about a new opportunity to do something cool... Even though I don't need any more credits for my MSSE degree this summer, I just found out about "Bahama Montana," an expedition into past carbonate environments of the Big Sky state, and their fossil inhabitants. Can't wait!

Eventually I'll use this credit... right?

Labels: fossils, limestone, montana, msse

Today, I would like to share with you some spectacular images that I took using a new toy, a Nikon binocular dissecting microscope with digital camera mount. These photos show a limestone in which you can find a large number of the single-celled foraminifera called "fusilinids." These benthic forams are about the size and shape of grains of rice, and here you will be looking at them in cross-section, seeing the spiral shape with numerous internal chambers that helps support their cytoplasmic bulk. Remember that these are macroscopic, not microscopic: each fusilinid is a single cell the size of a rice grain!




Now, you might be wondering why I'm so keen on showing off fossils. After all, this isn't a paleo blog... But there's more than just fossilization going on here. These fusilinids have also been squeezed. The weight of overlying sedimentary strata has compressed this rock perpendicular to the bedding plane, (top to bottom, in all these photos) and some of the fusilinids got crammed against their neighbors. Now, fusilinids make their skeletal material from the mineral calcite, and calcite can go into solution when the pressure is high enough. In places, you can see where one fusilinid has penetrated into its neighbor, dissolving the neighbor away as it intrudes. The following two images are close ups of the upper image. Photo #2 is from the lower left of the first image; Photo #3 is from the upper right of the first image:






In both, you can see where the edge of one of these internally-spiraled, ellipsoid-shaped fusilinids has dissolved its way into a neighboring fusilinid, disrupting the neighbor's internal architecture and symmetry. Insoluble minerals like dark-colored clays build up along this dissolution horizon. Here's one more photo for good measure:




Pretty cool, eh? The fossils serve as strain markers, hinting to us about how much of the rock's calcitic volume has been lost.


I would like to thank the Nikon representative who demonstrated the camera for me, Stanley M., for taking the time to show how the device works, and for allowing me to make some images before I had officially bought the thing.

Labels: fossils, limestone, structure

My friend Mark Tyra (a College Park geology master, like myself) just alerted me to a cool site which explores the presence of meteorites preserved in Ordovician limestones in Sweden. Check it out!

Labels: limestone, meteors, sweden, websites

On the way back up from the VGFC this weekend, we briefly detoured off the interstate (81) to go up Route 11, and across a singular natural feature in Virginia: Natural Bridge. This is a span of limestone going over a creek (and because it spans a watercourse, it is thus not an arch, but a bridge).

Unfortunately, this is all we saw of it:


The bridge is privately owned, and it's fenced off from view from Route 11, in spite of the fact that the road actually goes over the bridge. So we drove across it, but we couldn't really tell. And we didn't feel like stopping and paying the $$ to get in to see it from underneath.

In spite of that disappointment, what's pretty cool about the area is that it shows up well in this Google Maps "terrain" view:


Kind of wild: a natural bridge that's actually used as a bridge...

Labels: caves, limestone, valley and ridge, virginia

Yesterday I noted that there's an interesting pattern to be seen as one crosses DC's Duke Ellington Bridge:

After sharing these photos yesterday, I posed question for you: What's up with the coloration of these exposures? Why are they black on top and white on bottom? It's the same rock (Indiana limestone), so why the difference in color?

The answer has two parts. First, the calcite (calcium carbonate) which comprises the limestone is sitting out there in the air, and is subject to rain and what-not. Some of that rain has sulfuric acid in it, and that dilute sulfuric acid reacts with the calcite, producing a thin layer of gypsum (calcium sulfate). Those itty-bitty crystals of gypsum have bladed habits, and those bladed crystals are really good at trapping soot and dust. So while the calcite underneath isn't as effective as a soot-trap, the thin layer of chemically-altered gypsum on the surface of the blocks rapidly accumulates dark-colored particulate matter.

So that explains the dark color, but what about the lighter-colored lower portions? Is it simply that they aren't exposed to as much acid rain? Perhaps because they're further down on the "outcrop"? Nope... though that's clearly a consideration (note the thin white vertical lines below some of the stars), it wouldn't explain the abrupt transition from dark colored above to light-colored below. So: what gives?

It's here that context plays an important role. This is an urban location, an outcrop in the city. Like many flat surfaces in the city, it's subject to being tagged with graffiti. Periodically, the City sends along a crew to power-wash the bridge's graffitied surfaces. When they do this, they strip away not only the spray-paint, but also the gypsum and its trapped soot! Because graffiti artists can only reach so high, the city only power-washes so high, and the upper portion of the bridge "outcrop" is both unmolested by graffiti and uncleaned by the City. It records a continual accumulation of gypsum and soot, but the lower portion has its proverbial slate cyclically wiped clean!

I'm on a field trip this weekend (I wrote this post on Thursday and set it to publish while I was away), so I don't know who won the prize (a "GEOLOGY ROCKS" bumper sticker!) but as soon as I get back, I'll settle up with the clever winner. In advance, I'll congratulate you: Nice job!

Labels: contest, dc, limestone, minerals, sediment

To walk from the Woodley Park neighborhood of DC to my neighborhood (Adams-Morgan), you have to cross the deep gorge of the Rock Creek Valley. To do this, walk east on Calvert Street over the Duke Ellington Bridge.


My question for you: What's up with the coloration of these exposures? Why are they black on top and white on bottom? It's the same rock (Indiana limestone), so why the difference in color?

First person to post the correct answer in the comments section below gets a "GEOLOGY ROCKS" bumper sticker as a reward. Full answers tomorrow in a separate post...

Labels: contest, dc, limestone, sediment

Today, some shots from my time in Yellowstone National Park last summer. Here's Mammoth Hot Springs:

Close-up of the travertine deposits at Mammoth:

Me advertising my brother's company at Mammoth:

Norris Geyser Basin, slime:

Norris Geyser Basin's loneliest tree:

More slime, this time two colors:

Nasty patch of slime. Looks like snot:

Bison herd:

Columnar jointing in basalt:

Me showing you where the columnar jointing is. (I'm pointing at it...)

Strata exposed in the Tower area:

And here they are again, labelled:

Lastly, heading north out of Yellowstone back to I-90 and Bozeman, here's a weathered-out Eocene dike in the Paradise Valley. The dike is more resistant to weathering than the rock it cuts through, so it stands up as a "wall"-looking feature.

Labels: basalt, field trips, hot springs, limestone, montana, primary structures, stratigraphy, travel, yellowstone

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

A year or so ago, I picked up this nice sample of limestone in the Shenandoah Valley of Virginia (easternmost valley of the Valley and Ridge physiographic province). It was a cobble in a stream, not in situ, but it can't have come very far (by natural means anyhow) since it's quite angular. I liked it because of the alternating colors of its layers. I was not totally sure why they are different colors, but I strongly suspected it had something to do with different reactions to weathering (perhaps different calcite / aragonite ratios, or an increased silica / clay content in some layers?). I also liked the patterns of sedimentary layering, thinking back to undergraduate discussions of Flaser bedding and the like, but not remembering the details clear enough to interpret this one. Perhaps one of the sedimentary geologists can help clue me in? Still, I suspected it had something to teach me, so I brought it back to my lab at NOVA. Side view:

There, I sawed it in half. Top view:

To my chagrin, but not my surprise, the interior showed the layering less clearly. In the sawn section, I could clearly see where the weathering "front" had penetrated a short distance into the rock along the lighter-colored layers. While they were yellow-tan on the face of the sample, they were merely light gray in the interior. I decided to try and create a little weathering of my own, and reached for one of the students' acid bottles. I dropped about ten drops of acid on the sawn face, let it fizz for a bit, rinsed it off, and repeated the acid application. Almost instantly, the different layers jumped out into high contrast. The light-colored one was much more reactive than the dark colored one. Here's a view from the scanner which offers a comparison between the un-acid-treated sample (left) and the one I gave the brief acid bath to (right):

Not only does the layering jump out at you, you can see some micro-faulting too. Here's another view, from the camera, of the two samples, one stacked atop the other. I'm astonished at how 30 seconds of acid produced such a remarkable difference in their appearances:

As soon as I had documented the efficacy of the technique, I treated the second sample the same way as the first. One is now in the NOVA teaching collection, and the other is a proud new member of the CB office deskcrop collection.

Labels: limestone, nova, teaching, valley and ridge