Birthing a litter of drumlins

ResearchBlogging.org 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:

ice_drum

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…

ice_drum_2

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:

ice_drum_3

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:

icedrum4

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.

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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.

Giant ground sloths

In the American Museum of Natural History:

These mylodontids reminded me of Puerto Natales

Glacial striations, southern Central Park

New York City has some cool geology: Paleozoic metamorphics scraped by Pleistocene glaciers.

The Ghosts of Evolution, by Connie Barlow

Over Snowmageddon, I read Connie Barlow’s book The Ghosts of Evolution: Nonsensical Fruit, Missing Partners, and Other Ecological Anachronisms. [Google Books; Amazon]

Barlow isn’t a scientist, but she’s got a scientist in her pocket: Paul Martin of the University of Arizona. In 1982, Martin and Dan Janzen of the University of Pennsylvania published a paper in Science in which they postulated that a lot of the features of some modern plants are best explained by co-evolution with Pleistocene megafauna (mammoths, gomptotheres, glyptodonts, ground sloths, rhinoceroses, horses, etc.). As those animals are now extinct or extirpated from North America, the plants lack their “disperser” partners. As a result, their “over-sized” or “overly-protected” fruits don’t get dispersed, and tend to rot on the ground. Gravity is the main modern dispersal agent, and so they tend to be quite common in floodplains, but not upland areas.

North American examples of these so-called “ecological anachronisms” are honey locust, osage orange, gingko, pawpaw, Kentucky coffee tree, persimmon, and potentially desert gourds. Another great example, from Central America, is the avocado. These plants bear fruits (or fruitlike growths, if they’re not true angiosperms, like the gingko) which are either very large, very tough, or have very large seeds that are not swallowed by modern animals. Barlow claims that these plants are “haunted” by their departed ecological partners, an evocative analogy that gets repeated many times, long after it’s worn out.

The book is interesting and it held my attention. More importantly, it made me look at the trees around me and wonder at the evolutionary forces that sculpted them, forces now absent, though their sculpture remains. These plants surround us, and one you learn to spot them, it’s hard to pass them on the street without pondering their species’ history.

Walking back to my car after structural geology class at George Mason University last week, I saw some of the pods of the honey locust — big leathery things 15 cm long, 3 cm wide, and 1 cm thick. Some had been cracked open by the pounding action of undergraduate footsteps, and I saw inside the green pulp that Barlow described in the book. I remember she described it as “sugary,” so I grabbed an unmolested pod, and stuck it in my bag. That night, at home, my girlfriend and I cracked it open and tried some. It was sweet! Kind of mango-gummy, I’d say. However, the shell is quite bitter, and so if you try it at home, don’t lick the shell. After I accidentally grazed the shell with my lip and then my tongue, I had to spit and rinse my mouth out. It was nasty.

The Ghosts of Evolution isn’t a perfect book. One criticism I would offer is that it’s a bit repetitive, where the original thesis (a fresh, interesting idea) gets beaten into the ground with endless reiteration. Guns, Germs, and Steel fell victim to the same lack of editorial excision, in my view.

Another problem is that Barlow illustrates her plants with a series of “arty” photographs. The composition of these photos is symmetrical and balanced. They convey beauty, but they aren’t really scientific. Also, the sense of scale she provides is a honey locust seed. This may seem an appropriate sense of scale to a North American botanist, but most of us do not have an intuitive sense of the size of a honey locust seed, even if we are told it’s “about 1 cm long.”

Finally, I would say she needs to be more precise about where the science stops and her own enthusiasm for the idea takes over. The desert gourds she discusses are an exemplar of this: She runs with the idea of ecological anachronisms, and tries to apply its principles to a plant she sees in her own New Mexico neighborhood, but it is unclear how much of this discussion is her own extrapolations and how much has been rigorously researched by botanists. On the other hand, she does things that are beautiful without being scientifically significant: like rubbing a honey locust pod over a mastodon tooth in the American Museum of Natural History, and reflecting how that’s probably the first time that’s happened in more than 10,000 years!

All in all, worth reading if you’re into ecology, evolution, the Pleistocene, or botany. Has anyone else read it? If so, what did you think?

Crucifix Site 1: Sediments

On the September 2009 GSA field forum in the Owens Valley, the final stop of our first day was to check out the so-called “Crucifix Site,” along Chalk Bluff Road (north of Bishop, California, at the southern margin of the Volcanic Tableland). It’s called the “Crucifix Site” because there is a metal cross erected there:chalk_bluff_stuff_13

This is the site of some pre-Bishop-Tuff volcaniclastic sediments. The place is interesting on several levels, including the sediments themselves, and the subsequent deformation they have experienced. Here’s a look at the outcrop:chalk_bluff_stuff_03

Some annotations help to call one’s attention to primary sedimentary structures and interpretations:chalk_bluff_stuff_03anno

Lots of the sediment itself was made of little beads of obsidian, usually surrounded with a “chalky” weathering rind:chalk_bluff_stuff_07

A cool little channel cross-section was visible, too:chalk_bluff_stuff_04

For some reason, this is what pops into my mind when I run into a well-exposed Cheshire channel cross-section:
chalk_bluff_stuff_04anno

Birds and wasps had tunneled into the softer layers, resulting in horizontal rows of holes. I tried to ignore these modern bioturbations so I could focus on the ancient tale in the rocks themselves. Some cool soft sediment deformation was visible, like these flame structures (upper part of the central gray layer):
chalk_bluff_stuff_10

Zoomed in on a pair of flame structures, and the down-sagged material between them:chalk_bluff_stuff_11

chalk_bluff_stuff_11anno

…And another set:chalk_bluff_stuff_12

chalk_bluff_stuff_12anno

This was odd: The lowermost layer (upon which my field notebook rests) is unperturbed, but the layers above it are all churned up in one small area (center), flanked by a couple of bird holes:
chalk_bluff_stuff_08

Zoomed-in on the area in question:chalk_bluff_stuff_09

Annotated, for your viewing pleasure; green is top of undisturbed layer; red shows boundaries of zone of disrupted sediment:chalk_bluff_stuff_09anno

I would be pleased to hear from anyone who knows more about sedimentology than me about the wavy bedding in the second (& third) photo, and this weird sediment disturbance in the lowermost photo. Also: with the flame structures, it looks like coarser material in the lower layer (gray) is the less viscous participant, while finer-grained (white) material is sinking downwards. Isn’t this the opposite of the way it usually works?

Faulted moraine

Continuing with the recounting of geological sights in the Owens Valley, California, area… This one is in the Pine Creek area. Take a look at this photo:

eastern_sierra_01

No, that’s not just a portrait of Jeff Lee and his awesome handlebar mustache. Look behind Jeff, on the hillside above.

See the little step down that the hill takes? Let’s zoom in:

eastern_sierra_02

Still can’t see it? Here, allow me to annotate it for you:

eastern_sierra_02b

That’s a fault! A normal fault, with the Jeff side of the landscape dropping down relative to the mountain side (in the distance). Great, you might think. A subtle fault scarp. Big deal.

Oh, but you should not be so quick to dismiss it! After all, the material that the fault cuts across turns out to be a significant clue to the timing of when this fault happened.

This Google Map shows these two very well-developed lateral moraines extending out of Pine Creek Canyon:

In the Pleistocene, a valley glacier glided down out of the Sierran highlands into the Owens Valley to the east. As it flowed, it brought ground-up Sierran rocks down with it, depositing the sedimentary debris as glacial till. The fault above cuts through the northern lateral moraine. The moraines (made of till) are therefore Pleistocene in age, and since the fault cuts across the moraines, it must be more recent than the Pleistocene.

This is not a shocker: the boundary between the Sierra Nevada and the Owens Valley is well known to be a normal fault, as are most of the recent faults in the Basin and Range province. But being able to say “ten feet of offset have occurred on this fault since the Pleistocene” is a significant piece of data.

Cool, huh?

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