Wednesday, October 22, 2014

On the Road: Geo-challenge 1

Where are we?

Plants & Rocks is on vacation.  First destination was an east-west mountain range often described as an anomaly because contemporaneous ranges mostly trend southeast-northwest.  Actually there are several others like it, though not as large.  This one may have been influenced on the north by the Cheyenne Belt -- an east-west suture zone where crust was accreted to ancient North America roughly 1.5 billion years ago.
Red sedimentary beds on left are Precambrian.  They fooled all three early geological explorers.
The Precambrian core of the range is sedimentary – conglomerate, sandstone, siltstone and shale.  Sediments were deposited in a rift valley when the continent was coming apart, close to where it had been sutured.  It didn’t tear all the way, but was deep enough to accumulate 20,000+ feet of material.
Something like seven hundred million years later, the east-west mountains rose during a great regional mountain-building event (below).
Sedimentary strata on the flanks of the uplift were steeply folded, making for great scenery.  Some roads follow strike valleys between spectacular hogbacks.
The sedimentary rocks are just as wonderful on the south side of the mountains – like the ones below.
Josie Bassett ranched here at the mouth of the box canyon until she was 89.  “Independent in both action and thought, she lived life on her own terms.”

Sunday, October 12, 2014

Walking the Devil’s Backbone

Driving home from Rocky Mountain National Park we stopped just a few miles west of Loveland, Colorado, to examine the Devil’s Backbone.  How could we not?!  It’s an eye-catching landform and there’s a trailhead at the south end, right off Highway 34.

Demonic geologic features are common in Colorado -- there are at least twenty-one (Colorado Geological Society 2009).  Curiously, all are composed of either igneous rock or Mesozoic sandstone (Mesozoic means 252-66 million years old).  Some early geologists assumed the Devil's Backbone was igneous, a dike.  It has that look -- long, linear and narrow.  But then someone investigated and found it’s Mesozoic sandstone, specifically the Dakota sandstone -- sediments deposited by rivers flowing into an interior seaway to the east roughly 100 million years ago.
Here the Dakota is a mix of fine and coarse river deposits.  Note nearly vertical orientation of beds.
Along the Front Range of the Rocky Mountains, the Dakota forms hogbacks -- ridges of steeply-tilted sedimentary strata.  In the Devil’s Backbone the sandstone is really steep, basically vertical.  It's part of the west flank of a small anticline (uplift) east of the main Rocky Mountain uplift.  The more gentle east flank of the anticline is visible from the trail.  In the next photo, it's marked by rimrock with trees on the horizon, above boulder-strewn slopes.  The Triassic red Chugwater Formation is exposed in the valley below.  The valley also contains the “crest” of the breached anticline (cut through by erosion.)
Looking from the Backbone toward the hogback on the east flank of the anticline. 
The Wild Loop trail along the Devil’s Backbone is about two miles roundtrip, and is an easy stroll.  A short spur leads to the “Keyhole” where you can stand among the Devil’s vertebrae!
A demonic vertebra.
The Keyhole.
Long's Peak (Rocky Mt NP) from inside the Keyhole; Dave's finger rests on summit (click on image to view).
For more information, download the trail brochure provided by Larimer County’s Parks and Open Spaces.  This is the southern end of a network of trails extending north along the Front Range to Fort Collins.  Consider a weekday visit.  It’s popular, and the parking lot often fills on weekends.

NOTE:  The Devil has multiple backbones -- at least eight in the USA.

Tuesday, October 7, 2014

Tree-following: that first question

Do you remember that first question ... back in February when the cottonwoods were leafless, the ground in snow, and the river under ice?
“Is this a single individual, connected underground?”
Time passed, more questions arose:  What kind of cottonwood? (lanceleaf)  How tall? (58.4 feet)  Male or female? (female).  But the first question was never answered.
Cottonwood at dawn.  Is this a single tree, or six?
Most cottonwoods send up suckers -- shoots from buds on the roots.  Old stumps and even fallen branches sometimes produce shoots that grow to maturity.  So it’s reasonable to suspect that this clump is a single individual.  Most cottonwoods along the Laramie River are clumped like this.
Lanceleaf cottonwoods along the Laramie River in early morning light.  Note bird on wire (more later).
Then a clue appeared.  The east half of the cottonwood tree I'm following is now yellow, the west half still green.  Perhaps this isn’t a single tree after all.  We investigated.
My cottonwood tree(s) -- now green and yellow.
Its canopy.
Glen at base of tree(s).
We crept into the little glen among the trunks.  The sound of pitter-patter footsteps and buzz-like whispers swelled and then quickly subsided -- probably river elves fleeing their sanctuary.  To the east towered three stems (maybe-trees) with yellow leaves.  Two to the south still had green leaves as did the younger one to the northwest, which split just above the ground.
Three stems with yellow leaves (green ones belong to stems on right out-of-sight).
Two stems to the south still sport green leaves, though they're fading.
This younger stem became two at some point.
So how many trees?  Might we say at least two -- the yellow and the green?  Then a vague distant memory from botany-student days surfaced.  Being immobile, plants may resort to phenotypic plasticity and produce different forms from the same inherited DNA.  Besides, I like the whole clump and want to follow it.  Are you wondering how one follows an immobile organism?  Apparently you don’t know of the tree-following frenzy hosted by Lucy Corrander.  Visit this month’s gathering to learn more.
A bird with a distinctive silhouette.
Back to the bird on the wire.  A belted kingfisher has been fishing from this wire across the river just upstream from the footbridge for at least 20 years.  As the EPA says, “No information was found in the literature on life expectancy for this species.”  So every year I wonder:  Is this a single bird, or many?
Sometimes our “intrusions” benefit wildlife -- belted kingfishers love telephone wires near streams and ponds.


Tuesday, September 30, 2014

Urban Tumbleweeds -- free & hardly lonely

Tumbleweeds are integral to our beloved American West -- a land of wide open spaces where tumbleweeds, cowboys and other free spirits can go wherever the wind takes them.  They symbolize the revered rugged individualist -- loyal only to his own personal code.
Lonely but free I'll be found ... drifting along with the tumbling tumbleweeds.
The Drifting Cowboy (Tumbleweed Series), 1925.  Source.
But truth be known -- tumbleweeds are foreigners.  They snuck in from Eurasia, probably in the 1800s in “contaminated” feed.  And contrary to Western myth, they do just fine within the confines of town.  So when Lucy Corrander offered to host a virtual Street Plants gathering, urban tumbleweeds came to mind.
Tumbleweeds grow as the situation allows -- small in small spots, bigger where there's more space.
The “tumble” in their name comes from what they do.  These are traveling plants.  After they die and dry out each fall (being annuals), they break loose and tumble along, blown by the wind.  Seeds fall out along the way, or wherever the tumbleweeds land -- lodged against fences, under cars, or in trees.
The “weed” part of their name means they’re unwelcome.  That's due largely to their fecundity, which is astonishing (more below).  The results can be overwhelming ... as when unwelcome tumbleweeds invaded Clovis, New Mexico.  Neither man nor machine could handle the unruly plants.

Based on yearly inventories of my hedge, trees and fences, three species of tumbleweed inhabit our neighborhood.  Most common by far is kochia or summer cypress (Kochia scoparia).  Mature plants range from a single short stem to robust branching individuals up to six feet tall.  In a good year, a large plant may produce 50,000 seeds; the average is 14,600 (source).  And that’s a relatively small tumbleweed family! (more below).
Tiny kochia waif takes back the street.
Robust summer cypress, all decked out in fall colors.
Kochia hedge, drying in preparation for travel.
Kochia showing broken-off stem.  There’s a separation or abscission zone near the base of each plant.

Russian thistle (Salsola kali, S. tragus or Kali tragus) is the “tumbleweed” of Western legend, though in our neighborhood it’s not even half as common as kochia.  Mature plants may be up to five feet in diameter, and are oval to round -- made to roll.
Photo by Forest & Kim Starr, via Wikipedia.
Like kochia, Russian thistle scatters seeds as it tumbles along.  But the winged seeds can travel with the wind too, independent of their parents.  The reproductive output of this tumbleweed is massive -- a "typical" plant sows 250,000 seeds (source).
See them tumbling down ... pledging their love to the ground
Green Russian thistle, with drying summer cypress to right and behind.
Maturing Russian thistle has a beauty to it  -- pink and green and a bit fleshy.
The small leaves of Russian thistle are sharp-tipped when young, get even sharper with age (above), and become hard and penetrating when dry.  Lambs get mouth sores if they eat dried tumbleweeds, and cowboys wear leather gloves to clean them from fences.
Young Russian thistle with flowers in leaf axils.  Photo by 

Tumble mustard (Erysimum altissimum) is the least common and least familiar of our tumbleweeds, but look around and you’ll always find some.  In fact, my yard inventories show it to be almost as common as Russian thistle.  Plants are similar to Russian thistle in size but are even more fecund -- a single plant is said to produce up to 1.5 million seeds -- unbelievable!  Tumble mustard has several adaptations not found in the other tumbleweeds. Their pods shatter slowly, so seeds are scattered farther afield as the plant tumbles.  Kochia and Russian thistle seeds don’t survive more than a year, but tumble mustard can generate sizable seedbanks lasting 40 years or more (source).
Last year a tumble mustard dropped seeds by these PVC pipes.  Now an offspring is ready to take off.
There’s a nice geometry to tumble mustards, with their wide branching.  Thin green structures are seedpods.

In the photo below, all three tumbleweeds line up side-by-side, with many more behind. Soon they'll break loose and roam ... drifting east with the wind.
Left to right:  tumble mustard, kochia and Russian thistle.
I'm a roaming cowboy riding all day long ... tumbleweeds around me sing their lonely song.  Nights underneath the prairie moon ... I ride along and sing this tune:
video
 Tumbling Tumbleweeds, written by Bob Nolan, 1930s.

Saturday, September 27, 2014

Shear Beauty along the Highway

Crenulated beds (chevron folds) with alpine phlox.
Wyoming Highway 130 crosses the Medicine Bow Mountains west of Laramie.  North of the highway immediately beyond the turnoff to Brooklyn Lake are low dull-brown slopes that I’ve ignored for 30+ years.  Foolish me.
On the "dull" brown slopes are glacially-polished slate outcrops with fine multi-colored folds. The outcrops aren't extensive, but the patterns and colors are so intriguing that I stayed almost an hour looking at rocks and plants.
This is the French slate, exposed in the Mullen Creek - Nash Fork shear zone -- part of the Cheyenne Belt, a dramatic geologic structure that crosses southern Wyoming.  Rocks south of the Belt are Proterozoic (really old, 1.72 to 1.75 billion years); those to the north are Archean (really really old, two billion years or more).  Roughly 1.7 billion years ago, oceanic arc terranes collided with Wyoming along what was then the southern coast of North America, creating “the best agreed upon suture zone in the Precambrian of the western United States” (Whitmeyer and Karlstrom 2011).  The French slate was caught up in the mayhem and beautifully folded.
The green Yavapai province, a collection of oceanic arc terranes, was sutured to the south coast of North America along the Cheyenne Belt in what is now southern Wyoming (click on image to view; arrow added).  Modified from Whitmeyer and Karlstrom (2011).
The deformed slate seemed quite durable.  I couldn’t find any small fragments to take home, and cracks and soil pockets for plants were few.
Requisite ruler for scale (6 in, 12 cm).
Fernleaf daisy in a narrow shallow fracture.
Mountain candytuff in a small soil pocket.
Lichen grow on the rock surface, over fine-scale folds.
Grouse whortleberry and common juniper (lower right) surround a small outcrop of deformed slate.

The French slate is thought to have started as fine sediments deposited in deep ocean waters off the southern coast of North America (Lanthier 1979).  Then along came the colliding oceanic arc terranes, and the ocean was squeezed out of existence.  We say "collision" but it was hardly a single dramatic event.  Whitmeyer and Karlstrom estimate it took place 1.76 to 1.72 billion years ago -- a period of 40 million years.

It's so hard to grasp geologic time at these scales!  The quartzite boulder below is much easier.  It's an erratic left by a glacier 12-16 thousand years ago -- just recently.  The glacier also polished the folded French slate, a nice touch.
Glacial erratic made of Medicine Peak quartzite; it was carried down from the high Snowy Range to the west.

How to get there
From Wyoming Highway 130 roughly 7.5 miles northwest of Centennial, turn north (right) on the road to Brooklyn Lake.  Almost immediately turn left into the Nash Fork Campground if it’s open (recently it hasn't been) or park nearby.  From the bulletin board/restroom area near the entrance, walk 20 to 30 yards west to the brown outcrops just above the highway.  The folds are obvious.
From Google Maps.  Arrow shows location of outcrops.


Sources

Hausel, WD.  1993.  Guide to the geology, mining districts and ghost towns of the Medicine Bow Mountains and Snowy Range Scenic Byway.  Geol Surv WY Public Info Circ 32.

Lanthier, R.  1979.  Stratigraphy and structure of the lower part of the Precambrian Libby Creek Group, central Medicine Bow Mountains, Wyoming.  Contrib to Geology, Univ Wyoming 17:135-147.

Whitmeyer, SJ and Karlstrom, KE.  2011.  Tectonic model for the Proterozoic growth of North America.  Geosphere 3:220-259.