Thursday, July 17, 2014

Folded Land

Google Earth view of the "Heartland of Laramide Tectonics" in south central Wyoming. Red rectangle encloses area of photo below.  Click on images to view details.

Last month I took a short tour of the Heartland of Laramide Tectonics.  I had several geology guidebooks with me, so it was a fascinating trip.  The timing was good too.  If I had gone 75 million years earlier, it would have been really boring.
Traveling across south central Wyoming 75 million years ago.
At that time, the sea that had covered much of Wyoming for so long had retreated, leaving behind scenery of little relief.  But then the land was folded.  Now it’s a much more interesting place to visit.
Traveling across south central Wyoming in 2014.  Bold arrows point to selected attractions.
This folding happened during the Laramide Orogeny -- the period of mountain-building that produced the Rocky Mountains.  Most of Wyoming is dominated by Laramide structures.  They’re distinctive and usually easy to spot (it helps that the climate is semi-arid and vegetation often sparse).
Wyoming, with prominent Laramide mountain ranges in red; adjacent basins aren't labeled.
The creation of Laramide ranges and basins involved moving, tilting, folding and breaking (along faults) broad blocks of continental crust all the way down to basement rocks.  What?!  Is it possible to fold and break the basement of a continent?  Is it safe?
Basement (geology): rocks below a sedimentary platform or cover, or more generally any rock below sedimentary rocks or sedimentary basins that are metamorphic or igneous in origin.
 “Basement” is one of those terms we understand and use, but find difficult to define.  Part of the problem is that usage varies by continent.  In North America "basement" generally refers to igneous and metamorphic rocks of the craton -- the old stable part of the continent that has been around since Precambrian time.  The North American craton underlies much of Wyoming.
Source.
Actually, Wyoming was once on its own craton, before it was sutured to some others to form the North American craton.
Source.
So, as we were saying ... during the Laramide Orogeny “the Rocky Mountain foreland was fractured by deep-rooted reverse and thrust faults that uplifted broad blocks of Precambrian basement rocks, separated by deep basins" (Snoke 1993).  For example rocks from the continental basement are now exposed high in the Wind River Range, uplifted along a steep reverse fault (below).  A short distance west in the northern Green River Basin, these same rocks are 45,000 feet lower -- "staggering structural relief" (Snoke 1993).
Basement rocks form the high peaks of the Wind River Range, east of Pinedale; source.
The Laramide Orogeny was a big event in both time and space.  It lasted roughly 30 million years; start and stop times are debated and varied from north to south.  A large part of western North America was deformed, from Canada south into Mexico.  Actual Laramide-style deformation, involving basement rocks, characterized the tract from southernmost Canada to central New Mexico.
Extent of Laramide-style deformation, showing exposed Precambrian basement rocks (stippled); modified from Snoke 1993 (from Hamilton 1988).  Heartland outlined in red.
The area around the North Platte, Sweetwater and Medicine Bow Rivers in south central Wyoming has been called the Heartland of Laramide Tectonics (Lillegraven and Snoke 1996).  After my trip I asked one of the authors about the Heartland concept.  He attributed it to the other’s propensity for colorful language, but also emphasized that the region is filled with exemplary Laramide structures.

It certainly is a folded land.  My trip crossed multiple folds and faults, passing through scenery made of rocks ranging from Precambrian basement to mid-Tertiary basin fill.  Their stories are fascinating.  It’s so wonderful that by looking at rocks and landforms we can glimpse past worlds ... 30 million, 100 million, even a billion years ago.
Small anticline reveals Triassic redbeds, deposited when much of Wyoming was a muddy coastal plain.
Who walked here 170 million years ago?  Click on image to see tracks in beach sand turned to rock, and tilted to almost vertical.
Then there are the mysteries -- mountains flanked by steeply-tilted rocks on one side and a normal fault on the other, buried mountains, mountains crossed by rivers, and most puzzling, mountains far from any active plate boundary (where mountains usually arise).  How did these things come to be?  Sometimes we have a good idea, sometimes there are multiple theories based on the same incomplete evidence.  And sometimes we just don't know, which is fine.  This mystery adds to my sense of awe, and allows for informed speculation, a favorite pastime.  I love looking at Laramide landscapes and pondering what might have happened.
Water gaps were cut by the now-drowned North Platte River, as it made its way across Laramide folds (second gap visible in distance).
The gap and river before Alcova Dam was built in the 1930s.  From the Collections of American Heritage Center, University of Wyoming, Copy and Reuse Restrictions Apply.
My tour began at Alcova Reservoir, where the North Platte has cut through the Alcova uplift instead of going around it.  Much of the scenery is dominated by Permian and Triassic redbeds, in part because their color is so eye-catching.  My guide was Knittel et al.’s Field guide for the Alcova area (2004), which provides lots of information, including explanations for non-geologists.  The visuals are great -- maps, aerial photos, historical photos, photos with geologic features labeled, and panoramic shots.  There’s even a tour up Fremont Canyon from Alcova Lake, if you have a boat.  The road mileages didn’t match my car’s odometer, but that’s not so unusual and wasn’t a problem.  A virtual guide provides additional labeled photos linked to map locations.
Alcova Reservoir amid tilted redbeds.
From Alcova I drove south to Fremont Canyon.  Here the North Platte has cut through uplifted strata all the way down to basement rocks, revealing an intriguing gap in the rock record of some two billion years.
The Archean granite in Fremont Canyon above Alcova Lake is about 2.4 billion years old.
Pathfinder Dam was named for "The Pathfinder" -- JC Fremont.  His boat capsized nearby.
Next stop was Pathfinder Dam, one of BuRec’s earliest (1905-1909).  Then I crossed miles of Tertiary basin fill with Precambrian granite knobs sticking up through -- perhaps the most mysterious “mountain range” in Wyoming.  I reached the south "flank" of the "range" at the amazing Seminoe Mountains -- steep dark foreboding rocks crossed by both the North Platte River and the county road.  How improbable!  I emerged at Seminoe Reservoir, with views of spectacular Laramide flatirons.  This area is covered in A new look at the Laramide orogeny ... in southeast Wyoming, which includes several road logs (Lillegraven and Snoke 1996).
Above, the Pedro Mountains in the distance are Precambrian basement rocks that were uplifted, down-dropped, buried, and partially exhumed.
Above, approaching the north side of the Seminoe Mountains.  Yes, the county road really does cross here ... so does the North Platte River, a short distance east.  Below, looking north from the crest of the Seminoes down into the drainage of the North Platte.  The river has eroded off enough Tertiary cover to reveal parts of the underlying Pedro Mountains.
My tour of the Heartland of Laramide Tectonics ended on the south side of the Seminoe Mountains, where steeply-tilted sandstone and limestone flatirons made a nice backdrop for a portrait of a ruggedly beautiful limber pine (Pinus flexilis).

Posts about some highlights of the Heartland trip will follow.

Sources

Casper College.  Alcova geological site 2008; Virtual field trip of Alcova Lake, and Fremont Canyon, Natrona County, Wyoming.  Accessed July 2014.

Knittel, P, Van Burgh, Jr., DP, Logue, TJ, Strube, BE, and Jones, RW.  2004.  Field guide for the Alcova area, Natrona County, Wyoming.

Lillegraven, JA and Snoke, AW.  1996.  A new look at the Laramide orogeny in the Seminoe and Shirley mountains, Freezeout Hills, and Hanna Basin, south-central Wyoming.  Wyoming State Geological Survey Public Information Circular No. 36.

Snoke, AW  1993.  Geologic history of Wyoming within the tectonic framework of the North American Cordillera, in Snoke, AW, Steidtmann, JR, and Roberts, SM, eds.  Geology of Wyoming.  Wyoming State Geological Survey Memoir 5:2-56.

Sunday, July 13, 2014

the button that would keep us there

“Our mothers wore dresses -- slim at the waist, skirts flaring”

I don’t read a lot of poetry, and much that I do read is soon left behind.  But sometimes I’m lucky enough to find a poem I really like.  It seems extraordinarily true, or well-said, or beautiful, or thoughtful, and when I read it again the next day it's just as good.
We wore dresses "so white they dazzled even the grainy eye of the camera"

Such a poem
 appeared a few days ago at The Writer’s Almanac.  It was about photography -- about how we take photos hoping to fix, set, make fast certain moments in our lives.  This seems to be a reflex, almost instinctual, perhaps something as deep as fear of death or desire for immortality.
"Everyone we knew was young then."

Or maybe we're writing our stories.  If so, then we should hold tightly those memories that fit the tales we were told as children, to overshadow disappointments that intervene.

"we looked down into the viewfinder to
press the button that would keep us there,

as if we already knew that this was
as good as it was ever going to get"
 
"that was the last perfect season"

The poem is The Last Perfect Season by Joyce Sutphen.

Sunday, July 6, 2014

Cotton Trees

This is my monthly tree-following report.  June was all about cotton, specifically the cotton attached to seeds of cottonwoods -- or “cotton trees” as they used to be called, back in the old days.
Cottonwood cotton is a collection of fine hairs that help seeds fly with the wind and float down the river.  This is good for seed dispersal but can cause problems for humans.  At peak season female trees produce masses of fluff, which Russians call “pukh”.  Pukh reaches hazardous proportions in Moscow every summer; some blame Krushchev, others Lenin (read more here and here).  In any case, landscapers now know better than to plant female trees in town.

By June 22, many of the lanceleaf cottonwoods along the Laramie River were releasing seeds, but not mine.  I wasn’t surprised.  After all, she was a late bloomer back in May.
Looking up at the cottony canopy of a cottonwood tree (not mine); click on image to view.
Female catkin with seeds ready to fly.
You may remember from my mid-May report that the flooded river submerged benches and left my tree stranded on an island.  The water started dropping in mid-June.  Soon my tree was accessible, but with all the mud and debris and decaying matter, the area was a bit stinky.  And mosquitos were out in force.  June visits were short.
My cottonwood on left; Rich's bench on right (it was completely underwater a month ago).
Mud records those who passed:  birds, raccoons, dogs, people and bicycles.
I took female catkins of various ages back to the house for portraits.
Plump capsules filled with seeds and cotton.
Several capsules starting to dehisce (botany-speak for "split open").
Cottonwood seeds are tiny, dwarfed by the cotton.  They're made to travel.
Cottonwood cotton is often blamed for hay fever, but it's not allergenic.  It may snag and carry along whatever pollen is floating in the air however.

On June 29, I walked to the river early in the morning, while the mosquitos were still asleep.  I found the path littered with female catkins.  There were patches of pukh scattered about, though nothing like the fluff that plagues Moscow.
Female catkins on path, with pukh accumulations along right edge.
Wild Alyssum covered in pukh ... and it’s not even her own!
I crossed the footbridge and visited my tree -- she was casting seeds to the wind.
I watched them fly off, covered in cotton and filled with a plant’s form of hope.
I took many many many photos before I finally caught some seeds flying away.  Then I discovered that making a movie of them was a whole lot easier!

video

Tree-following is kindly hosted by Lucy Corrander of Loose and Leafy.

Saturday, June 28, 2014

Bitterroot in the Laramide Heartland

Several weeks ago I took a vacation in the Heartland of Laramide Tectonics.  It was short but wonderful.  I came home infused with Laramide stories -- of a time when Wyoming went from being a quiet sea to a land of rugged mountain ranges and deep basins, and in only 25 million years.  My geo-posts aren’t quite ready though, so instead here’s an amazing plant -- one that blooms with a flamboyancy all out-of-proportion to its habitat.

This is a typical Heartland landscape, with juniper, small ponderosa pine, sagebrush, mountain mahogany, sparse short grassland and spectacularly tilted rock strata:
It was springtime in the Heartland and wildflowers were out in profusion but keep in mind -- this is a relative concept.  The country is dry and vegetation is generally sparse; these are landscapes of muted colors.

The most obvious bloomers were the rough mule’s ears (Scabrethia scabra), with flower heads several inches across.
Rough mule's ears, with yucca, sagebrush, blackbrush and other highly drought-tolerant plants.
Real mule’s ears (Wyethia) has leaves that really do look like a mule’s ears.  But this close relative gets the same name even though its leaves don’t match.  However, they’re definitely rough to the touch (scabrous).
Most of the wildflowers were not nearly as showy as mule's ears.  But in the harsh dry setting, even plants like wild buckwheat and desert sandwort were impressive.
Wild buckwheat (Eriogonum) in center with desert sandwort (Eremogone) on right, on a dry rocky south-facing slope.  The buckwheat is about four inches tall.
A warm dry wind blew most of the time so I rarely tried to photograph flowers.  But there was one morning when I couldn’t resist.  I was walking along a ridgecrest to view lithified remains of the tidal flats that were so common in Wyoming 200+ million years ago ...
Arrow points to brick-red Chugwater Formation -- sediments deposited in Triassic tidal flats.
... when suddenly I was stopped in my tracks by a flash of bright pink.
There are at least four bright pink flowers in this photo -- click on image to find them.
It was bitterroot.  It always does that!  It catches me by surprise with its unexpected showiness and odd growth form:  no leaves and large radiant flowers barely above the ground.  They looked like party decorations that the wind had blown in and scattered along the ridge.
Bitterroot in typical dry rocky habitat, with desert sandwort.
Bitterroot spends most of the year as a fleshy root that is surprisingly large (to 12 inches) given the size of the plant we see.  The unassuming leaves appear in spring, photosynthesize for awhile to replenish root stores, and then wither as the first flowers open.  The flowers are spectacular -- bright, well over an inch across, with many petals.  Color ranges from pale pink to deep rose to occasionally white.  All the ones I saw were deep pink.
Bitterroot with leaves and first flowers.  Lady Bird Johnson Wildflower Center, photo by Fran Cox.
Flower with several buds below.  Upper right, prickly pear cactus almost in bloom.
Bitterroot bud in what strikes me as a really tough place to grow!
Then I found some curious coppery delicate flower-like items lying on the ground.
They were the remains of bitterroot flowers.  After the petals wither, the sepals dry and fall as a unit that is light and papery and easily blown about by the wind.  Inside the sepals is a capsule; it splits open to scatter small black seeds while tumbling across the ground.
Black seeds in opened capsule, surrounded by dried sepals. 
The sepals together form the calyx; source (modified). 
Here a dried calyx has been trapped by several bitterroot buds.
The scientific name for bitterroot is Lewisia rediviva.  For many of us in the western USA, “Lewisia” brings to mind (correctly) Meriwether Lewis -- the Lewis of Lewis and Clark who explored the immense Louisiana Purchase acquired from France in 1803.  The expedition left St. Louis in May of 1804, reached the west coast in November of 1805, over-wintered, and arrived back in St. Louis in September of 1806.  On the way home, they stopped at Traveler’s Rest, Montana in early July, where Lewis collected several specimens including six plants of what would become Lewisia rediviva.  This was one of 134 specimens sent to botanist Frederick Pursh, who determined it was a new genus and named it in honor of Lewis.  Several roots had been planted, and they produced leaves even though they had been out of the ground and without water for several years.  This inspired the specific epithet rediviva, meaning “brought back to life” (source).
Meriwether Lewis's bitterroot specimen, collected in 1806.  Source.
The origin of the common name is not clear.  When Lewis tasted the roots, given to him by Indians, he found them “naucious to my pallate” (source).  How could this be?!  They were highly valued by many tribes, both as a staple and a trading commodity.  However some tribes did call the roots “bitter” or “black medicine” (source).  Perhaps that’s the source of the name.
The amazing and beautiful bitterroot, so well-adapted to its harsh environment.  This is one of the wonderful plant illustrations of Mary E. Eaton.

For more information on bitterroot biology and history, as well as tips for cultivation, see Lewisia rediviva, Bitterroot (PDF) from the Montana Native Plant Society.