In the Company of Plants and Rocks: periglacial

Showing posts with label periglacial. Show all posts

Tuesday, January 3, 2017

More Story-telling Rocks

Curiously-arranged stones say it used to be much colder, and not that long ago.

Laramie winters are cold, with daily highs generally below freezing. Winds blow hard, often 30 to 40 mph, gusting to 50, 60, even 70. When arctic air masses visit, temps drop well below zero (F) and we post numbers and pictures on Facebook to show what we have to endure. But we shouldn't brag. Our local rocks and dirt tell us that Laramie weather could be much worse.

Twelve thousand years ago—a mere blink of a geologic eye—an ice sheet covered North America as far south as today’s central Montana, North Dakota, and eastern South Dakota. It never reached Wyoming, but we weren’t exempt from the bitter cold. Most of our basins contained permafrost, with only the uppermost soil melting in summer. When it refroze, expansion cracked the surface, year after year, creating networks of polygons. Now they’re buried under soil, except where fortuitously revealed by disturbance.

Relic frost polygons exposed during road construction at Crescent Junction, Wyoming (Mears 1987).

Frost wedges (polygons in cross-section) at the Rawlins city dump (Mears 1987; arrows added).

In the mountains, it was cold enough that snow survived summer melting. It accumulated, turned to ice, and began to flow, as glaciers. In the Medicine Bow Mountains west of Laramie, alpine glaciers merged into an ice cap across most of the crest, except for the highest part—the Snowy Range, a long steep-sided ridge of hard quartzite. Glaciers scratched and polished the rock only at the base; snow and ice didn’t accumulate above, at least not enough to form glaciers.

The Snowy Range stood above the alpine ice cap (SH Knight 1990).

Field assistant cavorts on the gently-rolling crest of the Medicine Bows; Snowy Range behind.

Though unglaciated, the Snowy Range was not immune to cold. Frigid conditions left their mark in other ways, most obviously the abundant felsenmeer (rock seas)—massive accumulations of boulders created by water! Bit by bit, expansion with freezing shattered solid quartzite into huge jumbled piles of angular blocks.

The trail to the summit of Medicine Bow Peak repeatedly crosses felsenmeer, fortunately via paths built by trail crews. The summit itself is a giant rockpile.

My old pal Sparky takes a break en route to Medicine Bow Peak.

Final section of trail to the summit. See any patterns in the plants and rocks in the foreground?

Emmie summits Medicine Bow Peak, her first ascent.

Just before the final push to the summit, the trail crosses a saddle underlain by a basalt dike (older sources call it diorite). Basalt is softer than quartzite and weathers more easily, explaining the low area with soil. Short alpine plants form turf among lichen-covered rocks—a meadow of sorts.

View of meadow from summit trail, in late August.

Meadow on basalt dike; older sources call it diorite.

Dike from below, near Sugarloaf trailhead.

From the right perspective, one sees that rocks in the little meadow are not randomly scattered. They outline polygonal patches of vegetation. Sound familiar? Yes, these are permafrost features—stone nets. Rocks and dirt were sorted, by annual freeze-thaw. Currently, active stone nets are common in Alaska, northern Canada, and other places with permafrost.

Above, stone nets seen from the trail to Medicine Bow Peak, looking south (Mears 1962). Below, roughly the same view in 2016; the nets are much harder to see. The illustrator emphasized rocks over plants, for clarity.

In the early 1960s, researchers dug a trench to study structural details. Larger rocks had accumulated along the sides of the polygons, with slabby ones often oriented on-edge. A dark (organic) layer of soil supported vegetation; plant roots extended as deep as three feet. When the study was over, they refilled the trench.

These stone nets are easiest to see from the ground, looking west slightly downhill (e.g. photos below). On gentle slopes, rocks have been sorted into lines that anastomosed into polygons. On steeper areas they form stripes.

As far as I could tell, common plants included alpine avens, moss campion, short sedges and a nearly-prostrate alpine willow. I need to return when things are blooming!

At the time of Mears’s 1962 publication, no one understood the mechanism behind frost sorting. Since then, computer modeling has suggested that with severe freezing, wet rocky soil expands enough to lift larger rocks and push them aside. Finer materials remain because they flow in response to frost heaving (more details here).

Sorted rocks are covered in lichen, suggesting they haven’t moved in awhile.

Developed soil, vegetation, long plant roots, and lichen-covered rocks indicate these stone nets are no longer active. But that could change. If another glacial episode were to override our global warming, stone nets and frost wedges would grow again. And here in Laramie, it may not have to be all that much colder!

Blizzards often close I-80, filling Laramie with trucks. Would life go on if glacial times returned?

• • •

Thirty-three years have passed since we lined up along a ditch to admire relic frost wedges, but the memory is still vivid. It was early October and snowing Laramie snow—horizontal with the wind. Doc Mears spoke of Pleistocene times 12,000 years ago, his strong baritone easily penetrating the howling wind, while we took notes as best we could with wet cold shivering hands. “I estimate the average annual temperature need drop just 10º C for a return to full periglacial conditions,” intoned Mears. None of us doubted it!

Dr. Brainerd “Nip” Mears, Jr. at Hanna Junction, 1970. Mears’s discovery of relic ice wedges was a major contribution to our understanding of Wyoming paleoenvironments. Photo by Wayne Sutherland.

Sources (in addition to links in post)

Knight, SH. 1990. Illustrated geologic history of the Medicine Bow Mountains and adjacent areas, Wyoming. Geological Survey of Wyoming Memoir 4. PDF

Mears, B, Jr. 1962. Stone nets on Medicine Bow Peak, Wyoming. Short note, University of Wyoming Contributions to Geology (p. 48).

Mears, B, Jr. 1987. Late Pleistocene periglacial wedge sites in Wyoming: an illustrated compendium. Geological Survey of Wyoming Memoir No. 3. PDF

Mears, B, Jr. 2001. Glacial records in the Medicine Bow Mountains and Sierra Madre of southern Wyoming and adjacent Colorado, with a traveler's guide to their sites. Geological Survey of Wyoming Publ. Info. Circ. No. 41. PDF

Saturday, March 31, 2012

My most important geology teacher -- Dr. Brainerd “Nip” Mears, Jr.

This post is my contribution to Accretionary Wedge #44.

1986 meeting of Friends of the Pleistocene, with University of Wyoming
geologists against a backdrop of Quaternary volcanic deposits,
Yellowstone National Park. Nip Mears kneeling on left, in red hat.

My profession is field botany, but I love things geological, especially landforms and the stories behind them. In fact, I’m a bit obsessed. I take geo-vacations -- I pick some part of the western United States, learn about the regional geology and then go geo-touring. Much of my appreciation comes from classes in geomorphology -- the Queen of the Earth Sciences -- that I took while a Botany grad student at the University of Wyoming. All were taught by Dr. Brainerd “Nip” Mears, Jr., and so Doc Mears faces no competition for this honor. Still, the knowledge, skills, enthusiasm and wonderful field trip memories that he provided me are evidence enough that the title "most important geology teacher” is fully deserved.

Fossil frost polygons exposed in highway construction; Mears 1987.

Mears’s research was on Pleistocene environments and the abundant evidence left behind in Wyoming mountains and basins -- stone nets, periglacial wedges, relic permafrost. But I believe that for him teaching was more important than research. In 2003, Mears received the University’s Outstanding Former Faculty Award; in his acceptance speech he stated that although he was of course grateful to receive the award, his many fine students “were reward enough”. Two-thirds of the bio in the printed program talked about his teaching excellence, for example the following from Kent Sundell, geology professor at Casper College (Wyoming):

“Dr. Mears was the most sincere, caring instructor. His door was always open and he was always willing to give his students his most valuable possessions: time and knowledge. He could have closed his door, written more papers, and received more grants and research accolades. Instead he chose the unselfish path of doing research and writing during evenings and weekends”.

In one of Mears’s last publications, Glacial records in the Medicine Bow Mountains and Sierra Madre of southern Wyoming and adjacent Colorado, there are three “Forward”s -- one by a colleague, one by a student and one by the editor -- all tributes. The colleague described Nip’s pioneering work on Pleistocene environments in Wyoming, including his discovery of periglacial wedges in the basins. His student mentioned his devotion to “Geomorphology, the Queen of the Earth Sciences”, and anointed Mears “King of the Earth Sciences”. The editor, also a former student, wrote: “Nearly every field trip with him was a memorable experience, as he taught his students the importance of keen observation and rigorous scientific method as well as the necessity for field geology”.

Memorable experiences indeed! One of the more vivid field trip memories I have is of an October trip in the Laramie Basin to look at fossil periglacial wedges. At a ditch especially rich in wedge cross-sections, we were busy taking taking notes while Nip lectured in his distinctive baritone about Pleistocene basin environments ... when it started to snow. He kept going, waving his fist in the air for emphasis. He was talking about strong winds that kept basin surfaces largely snow-free, allowing rapid freezing and cracking ... when the wind picked up as if on cue. Then he mentioned that if the mean annual air temperature were just 5º C cooler, the Laramie Basin would return to periglacial conditions. I didn’t doubt it at all, in fact the return seemed to be upon us at that moment! But no matter, Mears kept talking enthusiastically, an enthusiasm that was contagious. It was so neat to be able to “see” what the Laramie Basin was like during glacial times, and via something as mundane as an irrigation ditch.

Fossil sand wedge in cutbank at Sodergreen Lake,
Laramie Basin, Wyoming. For more, see this post.

Mears was a stickler about writing, an enthusiastic editor, and definitely “my most important teacher” of technical writing. Drafts would come back loaded with red editorial marks, comments in his characteristic scrawl and the faint smell of cigar smoke. I now know he also had an appreciation for non-technical writing about geology. In preparing this post, I found a collection of essays, The Nature of Geology, compiled by Mears “specifically for beginning students of geology ... intended to demonstrate that geology is a dynamic living science.”

Looking back, what impresses me most about Doc Mears was his compassion for students. At first glance he seemed rather tough -- a crusty old cigar-smoking field geologist who had served as a Marine in World War II. But I saw many examples of his concern for students, even in my limited time around him. There were little things, like working on learning students’ names and interests during downtime on field trips, and making sure everyone was involved in field discussions (including the botanist). And there were major things -- taking the time to edit my writing in detail with constructive comments, encouraging my interest in plant geography and paleoenvironments, and serving as the outside member on my thesis committee, even asking the most interesting questions during my defense.

When I travel around Wyoming, I always spot geomorphological features introduced to me by Nip Mears, and I can still hear his baritone voice and see him waving his fist in the air. He has had a huge impact on the way I view landscapes. When Mears retired in 1989, I sent him a letter, including the following:

“In my wanderings about Wyoming in pursuit of rare plants [my job at the time], your ideas and bits of wisdom surface continually -- when I see a hogback or a high-level erosional surface, or when I am standing on “hallowed ground” such as the hummocky-bumpy moraine below Fremont Lake. I find myself marveling at superimposed drainages, steeply-dipping strata, broad-backed anticlines and features of topographic reversal. Thanks for making the landscapes of Wyoming so fascinating.”

A gift from Anne and Nip Mears was matched by the State of Wyoming
to establish a $1.4 million endowment for the UW Geology Museum.
Photo courtesy StoryCorps, 2008.

Nip Mears is included also in one of my earliest posts -- The Inconvenience of Vegetation.

Selected Readings

Mears, B., Jr. 1970. The nature of geology; contemporary readings. New York: Van Nostrand Reinhold Co. Essays and excerpts, ostensibly for beginning students of geology but suitable for anyone who feels the awe inspired by earth science; selected to illustrate the human side of geology -- appreciation, profession, battles, environmental responsibilities and philosophy.

Mears, B., Jr., Eckerle, W.P., Gilmer, D.R., Gubbels, T.L., Huckleberry, G.A., Marriott, H.J., Schmidt, K.J., and Yose, L.A. 1986. A geologic tour of Wyoming from Laramie to Lander, Jackson, and Rock Springs. Laramie, WY: Geological Survey of Wyoming. My only geo-publication; the product of a course in advanced geomorphology that culminated in a field trip through western Wyoming.

Mears, B., Jr. 1987. Late Pleistocene periglacial wedge sites in Wyoming: an illustrated compendium. Laramie, WY: Geological Survey of Wyoming. Everything you ever wanted to know about the fascinating fossil periglacial features of Wyoming basins.

Mears, B., Jr. 2001. Glacial records in the Medicine Bow Mountains and Sierra Madre of southern Wyoming and adjacent Colorado, with a traveler's guide to their sites. Laramie, WY: Wyoming State Geological Survey. A southeast Wyoming geo-tour -- a great way to spend a weekend or short vacation.

Thursday, November 24, 2011

In search of relic periglacial wedges

Yesterday was sunny, cold, quite windy and there was little snow-cover in the basin -- probably much like it was here during the late Pleistocene. It was a good day for a field trip in search of periglacial relics.

Wyoming is composed mainly of high mountain ranges and broad basins. The ranges were glaciated during the Pleistocene and mountain glaciers sometimes extended several miles from canyon mouths, but the basins generally were ice-free. These basins are fairly high, 5000 to 8000 feet elevation, and periglacial environments appear to have been the norm, with permanently frozen ground subject to intense freezing cycles and other permafrost processes.

Mountains and basins of Wyoming. Courtesy Wyoming Geological Survey.

Relic permafrost features have been found at many locations in Wyoming basins. The most common are polygons and wedges created by repeated cold thermal-cracking of permanently frozen ground. Most have been discovered fortuitously during excavation and other kinds of dirt-work.

Surface polygons are exposed only rarely, for example when an area is bladed off; these exposures are especially short-lived.

Sketch of relic frost polygons exposed on road cut along Interstate 80,

Walcott Junction, WY. Fill is pale fine sand derived from a mantle of

aeolian deposits that was bladed off during construction.

Sand wedge; courtesy LIC.

More commonly an area of thermal-cracking is sliced through, producing longitudinal sections through the cracks. Thermal cracks narrow with depth and so longitudinal sections generally are wedge-shaped, hence the names “sand wedge” and “ice wedge”. Fresh cuts in quarries, roads, ditches and dumps are the best places to find wedges; these exposures also are ephemeral, eventually obscured by erosion, slumping and vegetation.

Most of the relic wedges reported from Wyoming basins are sand wedges, indicating cold dry permafrost conditions. “Sand wedge” is a misnomer because fill material is not restricted to sand -- it can be finer or coarser. The name is used to differentiate these wedges from ice wedges, which form in wet environments.

Ice wedges grow as water fills narrow cracks next to the existing wedge. With post-glacial warming, the ice gradually melts and the casts are filled with material that falls or washes in. Often the fill is layered, and may include clasts too large to have fallen into the original thermal cracks. In contrast, sand wedges form in dry environments where thermal cracks are immediately filled with whatever blows in. The wedge grows with repeated cracking and filling, and the fill may have a vertical texture. Larger clasts are absent.

It was cold and very windy as we drove the back roads of the basin looking at road cuts and irrigation ditches. There was a lot of disappointment -- all the road cuts were slumped or vegetated and many of the ditches were filled with snow. But the last stop was a good one. At one of the plains lakes, wave action had cut a nice exposure of sand wedges.

Sand wedge (red arrow) in Niobrara shale.

The host material is the Niobrara Formation -- gray and orange especially-gooey limey shales. [This is the Niobrara shale being advertised as the next big oil shale play, in the Denver Basin at depths of 3000 to 14,000 ft.] The fill is sand, sometimes with slightly coarser fragments. The Niobrara’s thin layers of alternating colors make it easy to see deformation of host material by the wedge.

Sketch showing wedge from photo above with sandy fill, longitudinal crack;
layers in host material have been warped upward by the wedge.

The sandy fill is more prone to cracking than the sticky Niobrara shale. Many of the wedges were penetrated by a narrow crack, and in several places, chunks had spalled off along an old frost crack/sand wedge.

Periglaciologist Fred once again in his Eureka pose
(Fred is 3" tall when kneeling).

Periglaciologist Fred made his debut back at the end of October, having found some wedges in a ditch in the west part of town filled with unusual unsorted unknown material. Here he is thrilled to have found something he can explain.

Sometimes the more resistant wedge stands in relief to the
softer Niobrara shale. Fred kneels near base of wedge.

Closeup showing sandy texture with occasional slightly coarser clasts.

Deformed Niobrara shale adjacent to sand wedge.
Here the host material is warped downward.

Google Earth: 41.160436° -105.933083° (Sodergreen Lake)

Sunday, October 30, 2011

Periglaciologist Barbie? ... no, Fred!

I couldn’t do it, couldn’t bring myself to buy a Barbie doll. When I was a kid Barbie made her debut and both of the other girls on the street had Barbies. I had no Barbie doll -- what I wanted more than anything was a baseball mitt. But not wanting to be left out of Accretionary Wedge #39, I present to you ... (fanfare)

... Fred the Periglaciologist! Living where he does, his is a profession of choice.

Periglacial sand wedge, southeast Wyoming.

The source of the spectacularly-colored fill

awaits elucidation.

Fred is in his eureka pose, having just come across a spectacular exposure of sand wedges in a ditch in the west part of town. “Sand wedge” is a misnomer; fill material is not restricted to sand -- it can be finer or coarser. The name is used to differentiate these wedges from ice wedges, which form in wet environments. These that Fred found are relics of cold dry Pleistocene times when there was permafrost in the Wyoming basins. Thermal-contraction cracks filled up with whatever blew in -- loess, sand, miscellaneous debris.

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