Birthing a litter of drumlins Quite appropriately, Glacial Till won the new the latest edition of “Where on (Google) Earth?”, hosted here yesterday. The location I picked is the subject of a new paper by Mark Johnson and colleagues appears in the current issue of Geology (October 2010). It shows a place in Iceland where a piedmont-style outlet glacier called Múlajökull is pooching out to the southeast from the Hofsjökull ice cap. Here’s a more zoomed-out view of the glacier’s terminus:


Here, I’ve jacked the contrast up a bit, so you can see what’s so cool about this location — note the radial array of elliptical meltwater lakes…


The other outlet glacier, seen just to the west, is Nauthagajökull. With this context established, we can take a look at Figure 1 from the Johnson, et al. (2010) paper:


The red ellipses are between the lakes I pointed out earlier. They are drumlins, elliptical hills of glacial till. Drumlins are examples of the sub-set of glacial geomorphology which includes features made by deposition of glacial sediment (till). They are taller at the upstream end, and taper out downstream, a shape something like an “upside-down spoon.” Long-term readers will recall the time that I shared the experience of visiting some drumlins in New York, where I learned that “spoon” analogy from Paul Tomascak.

There are a lot of drumlins left over from the Pleistocene glaciation, but we don’t totally understand how they form. That’s what’s so exciting about the recession of Múlajökull: it’s exposing the world’s only known active drumlin field for geologic scrutiny. Johnson, et al., have documented 50 separate drumlins emerging from beneath the ice. Their field works has yielded some new observations that may shed light on how these distinctive landforms develop.

First off, they note that Múlajökull is a “surge-type” outlet glacier, which means that it pulses forward rapidly (4 times in the past 60 years), which isn’t the case for other glaciers, like neighbor Nauthagajökull. See the comparison in Figure 1d — where Nauthagajökull is relatively smoothly retreating, but Múlajökull has fits and starts. This may be important: Nauthagajökull hasn’t produced any drumlins.

Second, they documented various aspects of the drumlins at Múlajökull. They have an aerial aspect ratio of about 3.0, which is similar to what we see in the drumlin zones of New York and other Pleistocene drumlin fields. So that makes uniformitarians happy — maybe the dynamics of Múlajökull are analogous to the Laurentide ice sheet! Another, more detailed study, was made of the internal structure and stratigraphy of the drumlins, as exposed in channels carved into the drumlin laterally by flowing meltwater. The guts of the drumlin show multiple till units, the most recent of which truncates the ones below it in a subtle but discernible angular unconformity.The uppermost till can be traced to the end-moraine produced by the most recent (1992) surge of the glacier, but not beyond it.

They also note the presence of orange-colored water-escape structures, cutting across the till units and filled with fine sediment, and a pebble fabric which is parallel to the drumlin’s long axis (and ice-flow direction).

A final class of data is gained by taking a look at what the glacier’s snout looked like before it revealed its internal drumlins. Here’s Figure 5 from the new paper, which overlays the traced drumlin boundaries from Figure 1 on an air photo from 1995, a time after the glacier surged forward in 1992, but before the most recent recession of the terminus that revealed the drumlins:


The authors note that the crevasse pattern on the 1995 glacier is clearly related to the location of the drumlins that have recently emerged. A V-shaped pattern of crevasses may be seen immediately upstream from many of the drumlins’ positions.

After the 1992 surge, the glacial ice at the terminus of Múlajökull has been essentially stagnant: there are no recessional moraines between the 1992 surge end-moraine and the current ice front. Without moving ice, the authors find it difficult to imagine how drumlins could be formed. They infer that the drumlins formed during the surging stage of the glacier’s movement. The erosional basal contact of the upper till unit seen inside the drumlins suggests that erosion (as well as deposition) is an important part of the processes which form drumlins. Stress differences under and between crevasses cause slight differences in the rates of erosion vs. deposition the glacier bed. More till builds up beneath crevasses, less till accumulates between them. Time goes by, the glacier surges, and a big batch of new till gets added to the top of the drumlins. Amplifying feedback enlarges the drumlins with each successive surge, mainly on the upstream end and the sides of the drumlin. The authors interpret the drumlin’s internal stratigraphy of multiple till units as the record of multiple surges.

The authors of the new paper conclude by examining the two principal models for drumlin formation: a subglacial bed-deformation model from Boulton (1987), and a meltwater model proposed by Shaw (2002). They point out the truncated stratigraphy they observed inside the Múlajökull drumlins as evidence for the Boulton model, and a lack of sufficient meltwater to support the Shaw hypothesis.

Right now, Múlajökull is our only functional modern analogue for drumlin formation in the Pleistocene, but others may soon emerge. The authors also predict that as glacial recession continues to play out all over the world, we may someday observe other active drumlin fields, and gain further insights into what’s happening beneath continental glaciers.


Boulton, G.S. (1987). A theory of drumlin formation by subglacial sediment deformation, in Menzies, J., and Rose, J., eds. Drumlin symposium: Rotterdam, Balkema, p. 25-80.

Johnson, M., Schomacker, A., Benediktsson, I., Geiger, A., Ferguson, A., & Ingolfsson, O. (2010). Active drumlin field revealed at the margin of Mulajokull, Iceland: A surge-type glacier Geology, 38 (10), 943-946 DOI: 10.1130/G31371.1

Shaw, J. (2002). The meltwater hypothesis for subglacial bedforms. Quaternatary Interational, v. 90, p. 5-22. DOI: 10.1016/S1040-6182(01)00089-1.

Photos from Virginia Geological Field Conference

For the second year in a row, more exotic travel plans meant that I wasn’t able to attend the superb Virginia Geological Field Conference. I see that they have now posted some photos on the group’s Facebook page, so go check them out to see what we both missed last weekend. Here’s a taste:

Sheared meta-conglomerate:

Metamorphosed mantle (?) xenoliths:

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.

Champlain thrust fault


Over the summer, I went up to Vermont to visit my friends the Clearys. Joe Cleary is a college friend and a talented luthier. He and his wife Tree and their children Jasper and Juniper have settled in Burlington, a lively town with a lot of cool stuff going on. Joe took time out one morning to show us a superb example of a thrust fault on the shore of Lake Champlain. It is on private property, but Joe got permission for us to hike there first. Our group that day consisted of Joe, Lily, and me, plus by a stroke of good luck, my pal Pete Berquist was in Burlington at the same time, with his friend Amy. The five us were Team Burlington for the day.

There are two rock units involved in the faulting at this location. Consider the first:


This is the Dunham Dolostone. It’s early Cambrian in age. It’s resistant to erosion, and stands up in cliffs above Lake Champlain. The distance from my ten little piggies down to the water is probably fifty feet. Below the Dunham Dolostone, you can find the Iberville shale. It is actually younger than the overlying dolostone. (We know this from unfaulted stratigraphy elsewhere in the region.) The Iberville shales are Middle Ordovician in age. They are relatively weak (‘incompetent’) rocks, and have been sheared out by the faulting. Here, Team Burlington demonstrates the sense of shear, by leaning over in the direction that foliation has rotated towards:


Looking in one direction along the base of the fault to show the differential weathering of the two units:


Flip it around 180°, and you see the same thing in the other direction:


Pete, Joe, and I crawled underneath the ominously overhanging dolostone to check out the detailed structure of the fault. Here’s Pete tickling the sheared out shales, looking for little sigmas…


The shales had nice veins of calcite running through them, and the high contrast of light and dark reveals some lovely folds, like this one:


Pete goes into professor mode, gesticulating and using the verb “shmoo” to describe the reaction of the shale to a gazillion tons of dolostone sliding over top of it:


Another nice fold (little tiny blue Swiss Army knife, 5.7 cm in length, for scale):


And another nice fold:


This fold is transitioning into a shear band:


Here’s my favorite part of the outcrop, a big fold with little parasitic folds all over it, showing opposite senses of shear on the opposite limbs of the big fold:


S-folds on the upper limb, Z-folds on the lower limb. Sweet, eh?

Here, a sort of S-C fabric has developed, with foliation tipped over the the left, and then near-horizontal shear bands running along through it:


Here’s something weird. Perhaps a reader can explain it. Here’s a shot of some of the veins, with the same 5.7 cm knife for scale:


Now we’ve zoomed in, and you can see some detail in the vein:


What are those lines? Is that more “S-C” fabric? I mean, it can’t be cross-bedding in a vein… but I’m having trouble visualizing what process of shearing the vein could yield such a delicate, even distribution of dark material amid the vein fill. What the heck is going on here?

Okay, now that you’ve twisted your brain up thinking about that, you can relax with a structure whose meaning is obvious. Some artistic and romantic previous visitor (not a member of Team Burlington) had arranged pebbles weathered from the two rock units into a bimodal icon of love:


Displacement along the Champlain Thrust is estimated at 30–50 miles (48–80 km). These dolostones started off near the New Hampshire border, then crossed Vermont, almost but not quite making it into the Empire State! The Champlain Thrust is the westernmost thrust fault that has been associated with the Taconian Orogeny, a late Ordovician episode of mountain building associated with the docking of an island arc with ancestral North America. Looking up at the fault trace:


A final glance at the thrust outcrop, looking north and showing the fault’s gently-inclined easterly dip:


Joe, thanks for taking the time to bring us out there!

Aeolian sand in Hampton, VA

This video was produced by my friend Pete Berquist. It shows rapidly moving “sheets” of sand saltating down Grandview Beach in Hampton, Virginia, during high winds associated with Hurricane Earl.

What do you notice here? A couple of things jump out at me, but I’d be curious to hear what this video makes Mountain Beltway readers think about…

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:dollysods_04

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:dollysods_24

Rolling meadows and bogs occur in patches, interspersed with forest of spruce, hemlock, and aspen (yes, aspen!):dollysods_04

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:dollysods_05

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:dollysods_06

Here and there, outcrops of white rock rose up above the lichens and shrubs:dollysods_11



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:dollysods_07

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:dollysods_26



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:dollysods_31

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


Soon only the fog remained:dollysods_32

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.

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


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.
blogpostrockies2010-1-2 (Custom)

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:

blogpostrockies2010-1-4 (Custom)
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.
blogpostrockies2010-2-2 (Custom)

blogpostrockies2010-2-2 annotated (Custom)

blogpostrockies2010-3-2 (Custom)

In this outcrop there are some mud cracks filled in with sand, exposed in cross section view:
blogpostrockies2010-4-2 (Custom)

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:
blogpostrockies2010-1236 (Custom)

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.
blogpostrockies2010-19 (Custom)

This boulder has mud cracks overprinting ripple marks. Two in one! Swiss Army knife (11 cm long) for scale.
blogpostrockies2010-24 (Custom)

This one has it all. Cross bedding, mud chip clasts, ripples, and mudballs. Field notebook for scale.
blogpostrockies2010-5 (Custom)

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.
blogpostrockies2010-03032 (Custom)

Folded argillite and quartzite of the Grinnell formation with preserved ripple marks. Car keys for scale.
blogpostrockies2010-3 (Custom)

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.
blogpostrockies2010-7 (Custom)

Here’s a fold that hasn’t yet been breached by a through-going fault. Width of field of view is about 30 cm.
blogpostrockies2010-8 (Custom)

Note on prominent RED color in Glacier National Park: These red beds above St. Mary Lake are Grinnell formation.
blogpostrockies2010-6 (Custom)

Within the Helena formation, there is a conspicuous layer of diorite with contact metamorphosed rock above and below it – the Purcell Sill.
blogpostrockies2010-30 (Custom)

Part of this piece of Purcell Sill diorite has been altered to make the green mineral epidote. The horizontal field of view is ~80cm.
blogpostrockies2010-11 (Custom)

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.
blogpostrockies2010-43 (Custom)

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.
blogpostrockies2010-36 (Custom)

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.”
blogpostrockies2010-34 (Custom)

Sometimes there are areas of low oxidation called reduction spots.
blogpostrockies2010-22 (Custom)

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?
blogpostrockies2010-9 (Custom)

I see a very specific animal, and I know exactly what it is doing in there:
blogpostrockies2010-9 cartoony  (Custom)
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.

blogpostrockies2010-2 (Custom)
(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:
blogpostrockies2010-16 (Custom)

Here is our little group hanging out with the Helena stromatolites.blogpostrockies2010-12 (Custom)

Notice the easily visible glacial striations!blogpostrockies2010-15 (Custom)

This awesome 1.8m diameter stromatolite cracked in half! blogpostrockies2010-17 (Custom)
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.
blogpostrockies2010-35 (Custom)

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).
blogpostrockies2010-26 (Custom)

Looking up the wall:
blogpostrockies2010-27 (Custom)

This stromatolite weathered into a three-dimensional column! One can easily see the separate slime layers.
blogpostrockies2010-28 (Custom)

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.
blogpostrockies2010-40 (Custom)

Oh yeah! A close-up:
blogpostrockies2010-41 (Custom)

One more (for good luck):
blogpostrockies2010-42 (Custom)

Elsewhere in the Helena Formation, you can see halite casts:
blogpostrockies2010-13 (Custom)

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:
blogpostrockies2010-14 (Custom)

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.
blogpostrockies2010-39 (Custom)
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.
blogpostrockies2010-37 (Custom)

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.


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!

The Purgatory Conglomerate

After my thesis defense at the University of Maryland, my mentor and friend E-an Zen asked me if I had ever heard of the Purgatory Conglomerate. I had not. Over the years, E-an has been a great source of new ideas and information to me, and so when he raises a notion, I pay attention.

In my thesis, I had done some strain analysis on volcanic clasts in a meta-ignimbrite that had developed foliation and lineation in Mesozoic shear zone in California’s high Sierra, and that reminded E-an of a rock he had once seen which was screaming for similar treatment: the Purgatory Conglomerate.

On my summer travels this year, I finally had the opportunity to swing through Newport, Rhode Island, and check it out in person. To me as a structural geologist and Zen devotee, this was like nirvana. Check it out:



I was very excited to be there. Here’s me enthusiastically embracing a watermelon-sized clast:


Looking along the trend of the stretching lineation (which is pretty much non-plunging):


Most of the clasts are quartzites of various flavors… Depositionally, it’s a relatively mature conglomerate.


Here’s looking “down the barrel” of the stretched clasts in a big boulder sitting atop the outcrop:


Here’s a really big clast, with local beer cans for scale (not mine, I swear):

Here’s a really long clast:
Recall that my Swiss Army knife is 11 cm long, but even without the specific unit, you can see that this clast has an axial ratio (on the plane of the outcrop) of roughly 7:1.

Here’s another long one with an axial ratio of 7:1, with a bonus feature. It displays internal bedding (of the sandstone it was originally derived from):


This is totally awesome. These cobbles, boulders, and pebbles have flowed into elongated shapes! We can use the geometric term “prolate” to describe their cigar-like or hot-dog-like forms.

It’s not all conglomerate there. There are some meta-sandstone bodies too. Here’s an example of sandstone meeting conglomerate:

Even cooler: here’s bedding: lenses of conglomerate within sandstone:

Annotated copy of that same photo:purgcong_37

(I once showed you something similar from the Sierra Crest Shear Zone: check photo C of this archived post.)

So how did the Purgatory Conglomerate get so distinctively deformed? Close examination of the rock suggests the main mechanism was pressure solution:

purgcong_10In the photo above, look below the Swiss Army knife for a triangular clast, and trace out its boundaries. You will see that it impinges on the hot-dog-shaped clasts immediately next to it. This triangular grain is encroaching on its neighbors’ territory! Now, one way to interpret this is that the original clasts had shapes which, jigsaw-puzzle-like, were perfectly formed to accommodate their neighbors’ shapes. But that seems rather unlikely, especially when you consider the ten gazillion clasts in this outcrop, all perfectly locked together.

Instead, the idea is that high pressure points (the edge of one round cobble touching an adjacent round cobble, for instance) are sites where certain minerals will go into solution. Quartz is both a common mineral and a mineral which will dissolve under high pressure and re-precipitate under lower pressure. Calcite pulls the same trick — that’s where stylolites come from. [Many nice examples of stylolites and other pressure-solution features here.]

Here’s another nice example showing how the individual clasts lock together with one another, suggesting part of their outer edge has dissolved away:purgcong_13

Here, too. See if you can pick out a few examples of where one clasts impinges on its neighbor. A refresher course may be found here.purgcong_15

Time for a different perspective. Unlike most of the previous pictures, this one is taken looking along the long axis of the clasts.


Zooming in for a closer look at that same photograph, the yellow areas highlight areas where one grain impinges on a neighboring grain:purgcong_zoom

If there are particularly large clasts, they may shelter smaller neighbors in their “pressure shadow,” immediately adjacent to them. Think of a building collapsing during an earthquake, with a strong central pillar. If you stand next to that pillar, you’re less likely to have the ceiling collapse on your head, since the pillar is protecting you. With that in mind, examine this part of the outcrop:

Since the long axes of the clasts runs left-to-right, that suggests that they were squeezed top-to-bottom. Therefore, the area immediately to the left of the giant clast would be “protected” from the highest pressures by the bulk of its large neighbor. If we zoom in there…

purgcong_28…we see clasts whose long axes are not aligned with the rest of the outcrop. They are pointed in several other directions, and/or are not as prolate as the rest of the clast population.

Here’s an annotation of the zoomed-in view, showing the orientation of the long axis of the grains in this outcrop plane:purgcong_28

The implication is that these “protected grains” were less subject to pressure solution than the grains which weren’t lucky enough to have a giant neighbor immediately “next door” (along strike).

In addition, it seems that the strain (deformation/stretching) of the clasts was more severe in some locations, and less severe in other locations. Here, my hands bracket a zone of less deformed (more spheroidal, less prolate) clasts within the overall outcrop of strongly deformed clasts:purgcong_29

The Purgatory Conglomerate is preserved at a spot called Purgatory Chasm. Here’s a shot of the chasm itself, cutting through the conglomerate outcrop down to the Atlantic Ocean. I’d guesstimate that it’s 10 m deep or so:

Here’s the Chasm, further out where the rock is clean of vegetation:purgcong_12

For a nice perspective on the whole area, check out this Quicktime 360° view.

So why is the Chasm there? We may get some insights by taking a look at another feature seen at the outcrop: a joint set which runs ~perpendicular to the long axis of the clasts:purgcong_25

A wider view, showing the same orientation of joints cutting across the conglomerate, ~perpendicular to the stretching lineation. Bikini babes at upper left for scale:

Now let’s go back to the Chasm, and take a look into it. Windsurfing board (washed out in daylight beyond the cleft) for scale:purgcong_34

There are a whole lot of joint faces there, all (a) perpendicular to lineation, and (b) parallel to the Chasm. You can see them all as parallel lines to the left of the Chasm. The large concentration of fractures in the area of the Chasm suggests that the Chasm was eroded out along a zone of more pervasively fractured rock. As you stand there and peer in, waves will come in and slosh towards the back end of the Chasm. But why is it so fractured here? I’m not sure.

In other places, you can see fractures that have “healed” into quartz veins:purgcong_23

This is where some of that dissolved quartz ends up, sealing shut these cracks. But not all the quartz veins I saw were perpendicular to lineation; there were some that were ~parallel to it, as in this photograph:


As a totally gratuitous bonus, the Purgatory outcrop also features glacial striations. In both of the following two photographs, the striae run from upper right to lower left:

Another spot, showing the same thing:purgcong_16

Annotation showing which lines are bedding traces internal to the quartzite cobbles, and which are glacial striations:purgcong_16

I’ve  got a few more photos of my visit to Purgatory in this Flickr photo set.

So: Thanks, E-an, for another great idea! What a cool place; I can’t wait to bring students back here…

Pristine stratigraphy vs. bioturbated

Beautiful fiancée for scale.


Get every new post delivered to your Inbox.