Remains of a mud puddle

Last Wednesday, I took a field trip to the North Anatolian Fault in Turkey, but I got distracted by this fine looking display of sedimentary structures in  a dried-up mud puddle in an old quarry.

The coin, a Turkish lira, is about the same size as a U.S. quarter. What you’re seeing here are dessication cracks (“mud cracks”), and accompanying them are exquisite little raindrop impressions, the minute craters excavated by a light sprinkle of rain after the mud has already started to dry out and “gel.” (If the water which deposited the mud were still there when the rain fell, the standing water would have dissipated the energy of the drops’ impacts, and no craters would have been excavated.)

Here’s a slightly more oblique perspective, to give a sense of how the individual mud flakes are internally laminated, and curl along the edges, producing a concave-up shape.

Note too that the cracks bisect some of the rain drop impressions, and therefore the raindrops fell first, and then the dessication cracks propagated on through them, a nice example of cross-cutting relationships. In some cases, the propagating crack used the “crater rim” of the drops as a mechanical zone of weakness, fracturing there preferentially. Here, let’s zoom in on a couple of nice examples (one from photo #1, a second from photo #2):

If anyone wants a full-sized copy of any of these images for teaching purposes, let me know via e-mail, and I’ll send you one.

Dolly Sods

Over the long Labor Day weekend, my fiancée Lily and my friend Seth and I took a three-day backpacking trip in the Dolly Sods Wilderness area of West Virginia:

Dolly Sods is a unique place, a little patch of flora that is more typical of Canada. It sits atop the eastern Continental Divide, and most of the area drains to the Gulf of Mexico via the Ohio River. Parts of Dolly Sods are sparsely treed, and resemble Arctic tundra. It is the easternmost bit of the Appalachian Plateaus province. Many places reminded me of Alaska:

Rolling meadows and bogs occur in patches, interspersed with forest of spruce, hemlock, and aspen (yes, aspen!):

The area was used as a proving ground during World War II, and there are still some dangerous bits and pieces left over from that time:

Here’s our happy trio, ready to set off on Friday afternoon:

Very quickly, I clued into the wealth of small blueberries which were omnipresent in the “tundra” landscapes. I snacked on these continuously throughout the weekend:

A glimpse of two forms of power generation off to the north: Mount Storm on the left (a coal-fired electric generation plant) and a field of windmills on the right:

Here and there, outcrops of white rock rose up above the lichens and shrubs:

This is the Pennsylvanian-aged quartz sandstone of the Conemaugh Group. Occasionally, it outcrops as bedrock, and other times, you just get these clean boulder fields, surrounded by tundra vegetation:

So what do we see when we zoom in on these outcrops and boulder fields? Well, mostly, we see quartz sandstone:

…Although there is a regular smattering of quartz-pebble conglomerate, too:

Occasionally, primary structures jump out at the eye, like some graded bedding…

…or these cross-beds:

Annotated copy:

There were even some fossils, like these plant scraps:

Plant scraps compressed en masse make coal, and there are coal interbeds to be found in places in Dolly Sods, and bituminous coal can also be found as float, as with these chunks:

There was even some structure to observe!

Annotated version:

A bigger outcrop, right around the bend, showed even more pervasive distortion of the sedimentary layers:

Annotated version:

What’s going on with these folds? After all, the Allegheny Plateau isn’t known for pervasive structural shenanigans… I’m guessing this might be soft-sediment deformation: slumping and sliding of sedimentary layers before they got lithified… Any other thoughts? (chime in via the comments section below, if so).

Here is sand weathering out of the sandstone, the grains free and loose again for the first time in ~300 million years:

The plants were a joy:

Here’s the view at sunset from our third campsite:

Yesterday (Monday) morning, when we woke, we found that the temperature had dropped below freezing overnight, and a coarse layer of frost covered everything:

Detail of the frost crystals on my tent’s rain fly:

The sun rose, and starting melting off the frost and dissipating the fog:

Soon only the fog remained:

In the warmth of the new day, we hiked out, got apple dumplings at the Front Porch Restaurant across from Seneca Rocks, and drove back home along good old (new) Route 55. It was a great weekend away, just the right distance, in wild country, with great weather. I felt rejuvenated by the experience.

Another metamorphosed graded bed

Over the summer, when my blogging access was limited to my iPhone, I uploaded a photo (taken with the iPhone) of a metamorphosed graded bed on the summit of Mount Washington, New Hampshire.

Here’s another one that I saw, further down on the mountain, on the Auto Road (famous for its iconic bumper sticker):

Lens cap for scale. …And here’s the obligatory annotated copy:

Both of these images are enlargeable by clicking through (twice).

I think today’s photos are of better quality than the iPhone photo. This is the coolest freakin’ thing ever. What you have here are alternating beds of quartzite and andalusite schist. The boundaries between the two rock units are alternately crisp and gradational. Interpretation? Once upon a time, you had a turbidite sequence where the bigger, heavier grains (quartz sand) settled out first, followed by progressively finer and finer mud. The base of the graded bed is a crisp transition from mud to sand, but then as you go up through the graded bed, it grades from sand into mud.

Later, these distinctive primary structures were metamorphosed during the late Devonian-aged east coast mountain building episode called the Acadian Orogeny. The high temperatures and pressures cooked the rock. The sandy part, dominated by quartz, didn’t really change mineralogy much under the metamorphic conditions. The muddy part, on the other hand, was chock full of clay minerals which are not in equilibrium under elevated temperatures and pressures, so they reacted chemically. Their elements reorganized into new minerals: big honkin’ crystals of the mineral andalusite. They might just as well have reorganized into sillimanite or kyanite if conditions were slightly different, but temperature dominated over pressure, so andalusite was the mineral form that was stable (at equilibrium) under those conditions.

As a result, the “mud” was now coarser grained than the “sand.” The overall sense of grading had been flipped by the metamorphosis, yet the overall crisp/gradual pattern was preserved. This, my friends, is exquisite.

Rocks of Glacier National Park

This is the second of my Rockies course student projects that I wanted to share here on the blog: it is a guest post by Filip Goc. Enjoy! -CB

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The Rocks around Glacier National Park, Montana: Introduction to the formations

The geology around Glacier National Park is great for beginners because the area is structurally straightforward and formations are generally easy to distinguish. Still, there is a lot to be excited about.

The rocks exposed firstly from the top down are old sedimentary rocks of the Belt Supergroup. It is called “Belt” after Belt, Montana, and “supergroup” because it is immense. These rocks were deposited in a Mesoproteozoic (1.6-1.2 Ga) sea basin, and show little to no metamorphism despite their age. Below belt rocks that make up the peaks of Glacier NP, there lay Cretaceous (~100Ma) shales; which brings us to the structure. How can these young shales get underneath MUCH older Belt rocks? Yes, there is a major thrust fault, and it is called Lewis Overthrust.

Simply put, the Belt rocks were first deposited in the sea basin. Then, Paleozoic rocks were deposited, but they are not exposed in Glacier NP, as they have eroded away. Then, Mesozoic rocks, including the Cretaceous shales and sandstones of the Western Interior Seaway, were deposited. Around 150-80Ma, as a result of the Sevier Orogeny, a HUGE slab of Belt rocks hundreds of miles wide and 15-18 miles thick slid over the Cretaceous formations more than 50 miles east! Slabs just love to glide on shales with their weak planes. Mr. Maitland from our group would call it the Banana Peel Principle, although most geologists who love French as much as I do prefer a much more refined term décollement horizon (yes, it is essentially a banana peel).

Check it out in this photo. The Belt rocks of the Altyn and Appekunny formations comprise the cliffs. They are much more resistant to erosion than the weak Cretaceous strata underneath them (low hills covered in trees). The striking white layer in the Appekunny formation is a quartzite bed.

Then erosion took the stage with its rivers and mass wasting. Finally, around 2 Ma the Ice Age came, and we derive the name of the park from the enormous glaciers that carved the peaks into their current shapes. The last of these huge glaciers melted ~12,000 years ago, and some people think the park should therefore be named Glaciated Park, ExGlacier Park, or just Glacier-No-More. The glaciers to be seen there today are young and tiny.

Glacier National Park is a great place to educate kids about geology because many formations can be identified by their colors. From old to young, there are Altyn (and Prichard), Appekunny, Grinnell, Empire, Helena (or Siyeh), Snowslip, and Shepard formations. Let’s get color-wise. First above the Lewis Overthrust are the light gray to white (or weathered into light tan) layers of limestone and dolostone of the Altyn formation. The Prichard formation exposed on the west side of the park is essentially of the same age as Altyn, but was deposited deeper, and is therefore darker as there wasn’t as much of oxygen in the depositional waters. It consists of dark gray to black argillite with slate-like appearance. (Argillite is slightly metamorphosed mudstone.) Then there is light green or burgundy argillite of Appekunny formation. Both versions have the same iron content, but the purple version is more oxidized. It was deposited closer to shore than Altyn. The Grinnell formation is the one dominated by burgundy argillite. The Empire formation is a transitional formation between Grinnell and Helena. Helena consists of medium to dark gray dolostone and limestone, often covered with honey-colored weathering rind. The Snowslip formation features red or green argillites, shades of orange or yellow, rusty colors, purple tones, white quartzite… pretty much “rainbow rock.” The Shepard formation is similar to Helena, gray limy rocks with orange buff.

Now for the fun stuff:

The Prichard formation is exposed on the west side of the park; is essentially of the same age as Altyn, but was deposited deeper, and is therefore darker as the increased pressure produced biotite. It consists of dark grey to black argillite with slate-like appearance. Aren’t these potholes beautiful? Also notice the joint sets. The diameter of the larger pothole is ~55cm.

There are great features to be seen in the Grinnell formation.
In this picture from McDonald Creek, there is cross-bedding in white quartzite, and a cross-section of ripple marks! A stream dumped sand onto muddy shore, ripples were created, and then mud leveled them out! Quarter for scale.

Annotated:

Close-up:

In this outcrop there are some mud cracks filled in with sand, exposed in cross section view:

There’s also cross bedding, and mud chip rip-up clasts. When the muddy shore gets exposed to the sun (low sea level), the mud dries up, loses volume, contracts, and cracks. That’s when a shot of sand came, probably with some rainstorm. Quarter for scale. As you can see in this picture of present-day drying mud, in the next stage mud cracks curl up:

Streams or waves can easily carry those “chips” away, and deposit them with sand. That’s the mudchip rip-up clasts. Very cool outcrop!

These are just another batch of nice mud cracks in Grinnell formation. Boot for scale.

This boulder has mud cracks overprinting ripple marks. Two in one! Swiss Army knife (11 cm long) for scale.

This one has it all. Cross bedding, mud chip clasts, ripples, and mudballs. Field notebook for scale.

These strange ripples in the Shepard formation are called interference ripple marks. They form when two currents go against each other at ~90°. Field notebook for scale.

Folded argillite and quartzite of the Grinnell formation with preserved ripple marks. Car keys for scale.

A bit more of the structure within the Grinnell Formation. This beautiful faulted fold lies on the way to Grinnell Glacier. Field notebook for scale.

Here’s a fold that hasn’t yet been breached by a through-going fault. Width of field of view is about 30 cm.

Note on prominent RED color in Glacier National Park: These red beds above St. Mary Lake are Grinnell formation.

Within the Helena formation, there is a conspicuous layer of diorite with contact metamorphosed rock above and below it – the Purcell Sill.

Part of this piece of Purcell Sill diorite has been altered to make the green mineral epidote. The horizontal field of view is ~80cm.

So when one hikes in the area at or above Purcell Sill, all the Grinnell is way down in the stratigraphic column. The red mudstones exposed around the center of the park ( like around Logan Pass) are mostly of Shepard formation. They look a lot like Grinnell, but they are younger. The width of the rock in the foreground is ~ 1m.

Mudcracks in Shepard formation near Hidden Lake. The trail to the Hidden Lake has one of the thickest Shepard exposures in the park.

There is one more red formation, even younger than the Shepard. This is the Kintla formation. Most visitors don’t encounter the Kintla. It can be seen as a red cap on the tops around Logan Pass (even above Shepard.) There are also exposures around Waterton Lakes on the north side of the Canadian border, and, of course, around Kintla Lakes and Hole-in-the-Wall on the northwest side.

This cross-section of mudcracks at the Boulder Pass are possibly within Kintla formation or Shepard formation. It is really hard to tell without a precise geologic map. At any rate, it is NOT Grinnell. The width of the rock in the foreground is ~ 70cm.

Since we are at Boulder Pass, there’s also plentiful of typical Snowslip at the trail to Hole-in-the-Wall. This sample shows why the formation is called Snowslip (at least as far as I figure it). The glacial striations show what direction the “snow slipped.”

Sometimes there are areas of low oxidation called reduction spots.

As the red color of Grinnell formation is caused by oxides of iron, non-oxidized Grinnell has a different color: the greenish Appekunny tone or shades of orange. There are whole greenish beds of reduction within Grinnell. The cool thing is that iron content is roughly the same throughout the rock! GAME for you: What do you see when you look at this reduction zone?

I see a very specific animal, and I know exactly what it is doing in there:

It is a bunny rabbit, and he is looking for stromatolites, or so-called “cabbage heads”!

So what is a stromatolite? Did the bunny choose the right formation to dig in? If not, what formation would be better?

Read on.

(Stromatolite layer along Going-to-the-Sun Road. The stromatolite layer is ~60cm high.)

Stromatolites are blue green algae or cyanobacteria that thrived on Earth in the Precambrian. The oldest stromatolite fossils on Earth are around 3.5 Ga. Stromatolites persist in the modern world in places where they are protected from grazing predators like snails. They were one of the first abundant photosynthetic organisms. They essentially remove CO2 from ocean; use the carbon for themselves while causing precipitation of calcium carbonate, and release the oxygen. They cover their cells with protective slime. When the slime gets too covered in sediment, they just grow a new layer, which results in dome-shaped layered “cabbage heads.” Stromatolites used to be so abundant that the sheer volume of oxygen they produced significantly changed the composition of our atmosphere. Stromatolites made our planet suitable for organisms like us!

Stromatolite beds are within many of Belt formations. The major stromatoliferous bed in Glacier National Park is the Helena formation. Some beds are in the Altyn and Snowslip formations also host stromatolites.

One of the best exposures of enormous stromatolites is at the Grinnell Glacier. Those honey-colored guys in Helena formation were ground down by the glacier so we can admire their cross-sections of their colony from the top. But first, a side view:

Here is our little group hanging out with the Helena stromatolites.

Notice the easily visible glacial striations!

This awesome 1.8m diameter stromatolite cracked in half!
In fact, this one was quite AGGRESSIVE, and had a DEADLY appetite.

Exposed at the Boulder Pass. Stromatolite in Shepard formation, as viewed from underneath. Stromatolites grow dome-shaped, but this is bowl-shaped. Therefore it is upside down. Field notebook for scale.

This exposure near Hole-In-The-Wall cirque I named “Stromatolite Wall.” All those columns you see are stromatolites. The wall is some 8m high (exposed).

Looking up the wall:

This stromatolite weathered into a three-dimensional column! One can easily see the separate slime layers.

Another absolutely stunning stromatolite bed is in the Snowslip formation. It is not a thick one, but special. The Snowslip formation was deposited closer to shore than the Helena, and the algae had trouble living there. There was quite some amount of organic material and mud periodically dumped in. Stromatolites caught the mud with its load of minerals into the slime layers, and those minerals later stained the fossils. The result: RAINBOW STROMATOLITES (my term). Next time somebody whines about how stromatolites are boring blue green grandpas, sitting around for billions of years doing nothing, just show them these playful buddies.

Oh yeah! A close-up:

One more (for good luck):

Elsewhere in the Helena Formation, you can see halite casts:

These are features that form when evaporation concentrates the dissolved Na & Cl ions so that they begin to bond together and crystallize salt. Later, when the water level rises again, the halite dissolves away and mud can fill in the empty cubic mold:

There’s one more interesting feature in the Helena formation. OOLITIC LIMESTONE. I made it uppercase because most people don’t see this in the park. It is exposed just next to the Going-to-the-Sun Road in the western part of the park.

In the photo, the gray beds are limestone, the brown ones are sandstone.

Oolites (also called ooids or ooliths) are little (0.25 to 2mm) round balls of limestone that for in warm shallow marine environments. The grain of limestone is gently rotated around by waves, and so the limestone precipitates in layers around the center…

If you look closely, you should see the oolites. They are ~0.5mm in diameter. Quarter for scale.

Although Glacier National Park is primarily famous for its jagged glacial landscape (and for a good reason!), the rocks that make up its horns and arêtes are remarkable as well. Despite having been displaced ~50miles east, they retained many of their primary sedimentary features. It’s common to spot beautifully preserved ripple marks or mudcracks. The abundance of fossil algae – stromatolites – is striking as well. Glacier National Park offers arguably one of the best Belt rock exposures in the US, which also makes it extraordinarily colorful. The deadly combination of colorful strata, white snow, and jagged peaks ensures the park is the one of the most scenic places around. It is just gorgeous up there.

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Filip now leaves NOVA; my Rockies course this summer was his final NOVA class. Now he’s off to the University of Virginia. Good luck, Filip! —CB

Geology of Massanutten Mountain, Virginia

Here’s a new video from Greg Willis, the same guy who brought us a fine video on Piedmont geology. In this new opus (20 minutes), Greg details the geology of the Massanutten Synclinorium (Shenandoah Valley, Massanutten Mountain, and Fort Valley) in western Virginia. WordPress isn’t letting me embed it here, but you should go and check it out!

Pristine stratigraphy vs. bioturbated

Beautiful fiancée for scale.

Metamorphosed graded bed

This is the coolest thing I’ve seen this week: a graded bed metamorphosed via Acadian mountain building:

The graded bed starts at the Swiss army knife at left, where you see an abrupt transition between coarse grained metamorphic porphyroblasts (“pseudo-andalusites”) and finer grained quartzite. This was once a mud to sand transition when these were loose sediments in the Kronos Sea, but with elevated temperature and pressure, the clay minerals in the mud reacted to grow elegant porphyroblasts of andalusite and sillimanite. The sand was made of quartz: less reactive stuff, and all it did was fuse together when metamorphosed. I love this: the coarse to fine relationship in the original graded bed is flipped on it’s head by metamorphism. Follow the bed “up” (to the right), and you will see the quartzite grade into andalusite-dominated former mudstone. Pretty sweet, eh?

This example is from the summit area of Mount Washington, New Hampshire. More fun stuff when I get back to my home computer next Monday.

Uniformitarian

Heat-stressed map of the Chesapeake Bay / Washington, DC region, as seen at Kenilworth Aquatic Gardens. Looks like mudcracks, eh?

Similar stresses; similar strains.

New “primary structures” display at NOVA

One of the things I managed this week was to fill up a new display case in our Student Study area with a structural geology display. On one side is primary structures, both igneous and sedimentary….

Falls of the James III: river work

In today’s post, I’ll finish up with my geologic discussion of the falls of the James River in Richmond Virginia, south of Belle Isle. Previously, we’ve examined the bedrock at this location (the Petersburg Granite) and a series of fractures – some faults and some extensional joints – that deform that granite. Now we come to the final chapter in this story — the story of the river carving up these rocks as it incises downward along the Fall Zone.

Unlike my native Potomac River, there is no gorge carved along the James at the boundary between the metamorphic & igneous rocks of the Piedmont and the overlying Coastal Plain strata to the east.

But there is still some cool stuff to see. In my first post on Belle Isle, I mentioned the diversion dam that keeps the river-bottom bedrock (mostly) dry and available to geological scrutiny. That dam diverted some of the James River into a mill race which led to a hydroelectric power plant that was abandoned a half-century ago. Here’s a map showing the dam, mill race, and some other key features:

You’ll note that’s a Google Earth image that I’ve rotated 90° clockwise to fit it into this vertical-friendly blog space. North is to the right. I’ve highlighted the trends of the NNE- and ENE-oriented fracture sets that I discussed here yesterday, as well as the quarry pond on the north side of Belle Isle. You’ll also note a Δ-shaped logjam at the intake for the mill race. The mill race itself is choked with mud (the bigger debris is strained out at the inlet; but the mud makes it through). There are some mudcracks visible there:

Chuck is trying to talk a colleague into drilling a sediment core through this deposit — easily two meters thick. It might potentially provide an interesting sedimentological (and geochemical) account of the last 50 years.

Let’s zoom in further to the ~dry area of the river bed south of Belle Isle; the part the dam makes accessible to sunbathers, dope-smokers, fisherman, graffiti-artists, and geologists:

Again, north is to the right. You’ll note the obvious trend of the two dominant fracture sets, as well as a large number of elliptical ‘dots.’ These dots are potholes, semi-cylindrical holes that get drilled into the bedrock when water currents maintain vortices (plural of vortex: think a liquid tornado) in one place over an extended period of time. Some of these potholes line up in rows — like perforations at the top of a checkbook, these aligned potholes create planes of weakness that make it easier to pop out large slabs of rock in a quarrying process not unlike what humans do with their ‘plug & feather’ techniques.

In the close-up Google Earth image, at the lower left (southeast), you can see that I’ve placed a pair of black arrows pointing to a train of four potholes. One has a big boulder in it, and then there are three others with an elongated axis in the NNE-direction. We visited this particular chain of holes, and saw something interesting.

Here’s Chuck standing in the second pothole, with the boulder-containing pothole in the foreground:

You can just barely make out the two more northerly potholes in the far distance, but here’s a second shot showing them from the vantage of the northerly tip of the second pothole. The backpack and power plant should provide orienting landmarks:

Now take a look at that first photo again — take particular note of the magmatic schlieren in the bedrock. Recall that schlieren are curtain-like zones of more mafic minerals in the granite. You can see that the schlieren wrap around these potholes. Here, I’ll trace them out (crudely) for you:

Potholes are recent geomorphological features imparted by the river. The schlieren formed ~320 million years ago — how could the older structure wrap around the younger carving? Chuck interprets this to mean that the potholes were etched out from some weaker/less stable rock type — perhaps some of the mafic-composition xenoliths that pepper the Petersburg Granite in this area. Certainly, we can see that the xenoliths often appear in long trains, strung out along the plane of magmatic foliation, and the schlieren wrap around those. If the river exploits the outcropping xenoliths as areas where it’s easier to drill, the ancient positioning of the xenoliths could lead to the modern positioning of the potholes. I’ve seen something very similar on the Billy Goat Trail (Potomac River, downstream of Great Falls), so it wasn’t too difficult for me to buy into this interpretation.

Have any of my readers seeing compositional variations (like xenoliths) controlling river geomorphology elsewhere? Do tell!

Finally, thanks again to Chuck for taking the time to show us around last Friday. Belle Isle is a cool place on many levels, and I’m glad I got the chance to check it out in person.