Sweetwater River at the Devil’s Gate

Devil's Gate in 2014.

In the mid-1800s, hundreds of thousands of gold-seekers, pioneers, pilgrims and other dreamers took the Oregon Trail west.  They followed the valley of the blessed Sweetwater River across what is now central Wyoming, finding grass and drinkable water in an arid land of alkaline streams.  Several miles upstream from its confluence with the North Platte, the Sweetwater cut through a granite ridge via a gap impassable to wagons, hand carts and other vehicles of the day.  Travelers christened it “Devil’s Gate” and made a short detour to the south.  Why didn’t the river go that way?  It would have been so much easier!

Western part of original Oregon Trail; red arrow points to Devil’s Gate.  Map by Ezra Meeker.

Devil's Gate in 1870, William Henry Jackson; Public Domain.

“This gap is truly wonderful, being a space not over twenty yards wide and about five hundred feet high, having very much the appearance of being chiseled out by the hand of man rather then [sic] the work of nature.”  Osborne Cross, 1849 (diary entry)

Devil’s Gate isn't as odd as it might seem.  In Wyoming, it’s not unusual for rivers to cross ridges and even entire mountain ranges.  We have lots of superimposed drainages.

superimposed drainage: A naturally evolved drainage system that became established on a preexisting surface, now eroded, and whose course is unrelated to the present underlying geological structure. (McGraw-Hill Science & Technology Dictionary 2003)

Devil's Gate is where the superimposed Sweetwater River crosses a granite ridge at the east end of the Sweetwater Rocks.  These were the high peaks of the Granite Mountains before the range collapsed (see this post).

From Google Earth; labels added.  Click on image for a better view.

Roughly five million years ago, Wyoming mountain ranges were largely buried.  Streams flowed on the surface above, oblivious to underlying topography.  Then widespread erosion set in and streams were lowered down onto whatever was below -- in some cases rock ridges or mountain ranges.  They had no “choice” but to cut through.  For more about cycles of regional burial and exhumation, see this recent post.

Block diagrams by Brainerd Mears, Jr., after originals by Samuel H. Knight (Regional Geomorphology class, University of Wyoming, 1984).

The ridge crossed by the Sweetwater River is mainly Precambrian granite, estimated to be 1.8 billion years old (Lageson and Spearing 1988).  There also are occasional dikes of darker rock, for example in the area of Devil’s Gate.

"Granite with dikes of dark intrusive rock (Devil’s Gate) ... June 1922."  WT Lee, USGS Photographic Library [arrows added].

Devil’s Gate and dikes, August 2014; click on image to view.

The interpretive sign at the end of the trail explains that “volcanic activity split the granite ... and forced molten basalt into the fissure”.  But the dikes more likely are diabase, the shallow intrusive equivalent of basalt (see Granite Mountains by the Wyoming Geological Survey).  They're Precambrian in age, truncated before deposition of Cambrian sandstones (Love 1970).  The diabase is softer than the surrounding granite, so the Sweetwater River was able to cut through the ridge via a conveniently-located dike.

Start of trail to Devil's Gate; Split Rock in distance.

Devil’s Gate is a National Historical Site, part of the California National Historical Trail.  The gap is on public land (Bureau of Land Management) and can be reached by a short trail that crosses the Sun Ranch, now owned by the LDS Church.  The Church bought the ranch to develop a site memorializing an ill-fated Mormon party that may have sought refuge nearby.  It also tried to buy adjacent public land through Federal legislation (public land normally isn't for sale).  This stirred up quite a bit of concern and protest among Wyoming citizens.  The effort failed for good when several Indian groups also expressed interest in purchasing sacred sites.  Instead, an agreement was reached which includes access across the ranch.  See this site for more information.

The hike to Devil's Gate.

Sources (in addition to links in post)

Blackstone, DL.  1988.  Traveler’s guide to the geology of Wyoming, 2nd ed.  Geological Survey of Wyoming Bulletin 67.

Lageson, DR and Spearing, DR.  1988.  Roadside geology of Wyoming.  Missoula, MT:  Mountain Press Publishing Co.

Love, JD.  1970.  Cenozoic geology of the Granite Mountains area, central Wyoming.  USGS Professional Paper 495-C.

The Most Mysterious Mountains in Wyoming

This is the final post in a series about a recent trip through the “Heartland of Laramide Tectonics” in south central Wyoming.  By definition, this a folded land.  It was deformed during the Laramide Orogeny, the period of mountain-building that created the Rocky Mountains.  But when I left Alcova Reservoir and drove south, the landscape quickly changed from folded to flat.

Alcova Reservoir is in a land of folds and faults.

The scenery along the county road to the south is very different.

Occasionally there were curious granite hills and mountains sticking up above the rolling land.  They didn’t look much like Laramide mountains, which are elongate ranges with upturned sedimentary rocks on the flanks.  These were bare rounded granite outcrops of various shapes and sizes, with no obvious geologic structure.

“massive pink granite ... crops out in bald stark knobs that stand above the light colored, flat-lying sediments” -- D. Blackstone, 1988

This kind of scenery continued for miles and miles until I arrived at the north flank of the Seminoe Mountains -- steep, dark and ominous.  It was hard to believe that the county road continued on, but it did!  Here the North Platte River had cut into the surface I’d been driving on, revealing flat, undeformed rock strata.  Aha! ... Tertiary fill!

 Informative exposure of nearly-flat pale Tertiary sediments, with North Platte River in foreground (bridge) and Seminoe Mountains behind.

“Tertiary fill” refers to sediments that were eroded off Laramide mountain ranges and deposited in adjacent basins.  When the basins were filled, deposition continued onto the flanks of the ranges themselves.  Eventually they were mostly buried, with just the highest peaks exposed (cross-section below).

[Note:  The Tertiary Period has been replaced with two subunits -- Paleogene and Neogene -- but we still use “Tertiary fill” as it’s a useful descriptor.]

Laramide uplifts were eroded and largely buried in Tertiary fill (yellow).  Block diagrams by B Mears, Jr, from originals by SH Knight (Regional Geomorphology class, University of Wyoming, 1984).

Then came the Great Exhumation (Snoke 1993) -- a time of widespread erosion.  Much of the Tertiary fill was removed, uncovering the old landscapes.  Why this happened isn't clear.  Perhaps regional uplift increased erosive power of streams, or maybe climate change brought more rain and more erosion.  Whatever the cause, exhumation was followed by yet another depositional phase (burial), and then another period of erosion (exhumation) to produce today’s great Laramide scenery.

For reasons still debated, the old landscapes were exhumed ... 

... and then buried again ... 

 ... and then re-exhumed to produce today's Wyoming landscapes.

Turns out I had just driven across some of the best preserved Tertiary fill in Wyoming.  But why wasn’t it removed during the Great Exhumations?  And is there something still buried down there?  Here are some clues as to what that might be:

-- Tertiary fill laps directly onto clusters of Precambrian granite outcrops.

-- Boulders of the same Precambrian granite have been found on summits of mountains a short distance south, at higher elevations.

-- Normal faults bound the area on the north and south (click on map to view).

Neogene normal faults in Wyoming (Flanagan & Montagne 1993), with Granite Mountains labeled in red.

Buried in the Tertiary fill are the remains of a Laramide uplift!  These are the Granite Mountains, the most mysterious mountains in Wyoming.  They were once a typical Laramide uplift, but then the core collapsed and the crest ended up below the flanks.  The down-dropped block is bounded on the north and south by normal faults, forming a graben.  Displacement is estimated to be 2000 feet.  Because the block subsided significantly, much of the Tertiary fill has been preserved, with only the highest peaks of the old range exposed.  So this is a remnant of Wyoming landscapes prior to the Great Exhumations (block diagrams 1. and 3. above).

A bit of the Really Old West:  high peaks of the Granite Mountains rise above Tertiary fill.  These are the Pedro Mountains -- the east end of the old range.

Cross-section through the Sweetwater graben (Mears et al. 1986); click on image to view.

Only the core of the Granite Mountains collapsed; the flanks remained in place.  These are now represented by the Rattlesnake Hills on the north side of the graben, and the Seminoe, Ferris, Green and Crooks mountains to the south.  On the summits of Green and Crooks mountains are granite boulders eroded off the high peaks before they were down-dropped.  In other words, the boulders now lie higher than their source.

South side of Seminoe Mountains in distance, with steeply-dipping sedimentary strata.  They were part of the south flank of the Granite Mountains before the core collapsed.

Dark foreboding metamorphic rocks on the north side of the Seminoes, exposed by normal faulting.

The normal faults of the Granite Mountains are fairly recent.  Flanagan and Montagne (1993) dated them to about 11 million years ago, at least 30 million years after Laramide uplift ceased.  The cause is debated.  Normal faults generally are associated with crustal extension, yet there’s no clear link between normal faulting in Wyoming and the major episodes of Tertiary extension in western North America -- Basin and Range province and Rio Grande Rift.  So “anomalous” is what it’s called for now (Snoke 1993).

The Sweetwater River flows though the graben from west to east, through the old high peaks of the Granite Mountains.  This was the route of the Oregon and Mormon Trails, traveled by many thousands of pioneers headed west to better lives.  About 50,000 passed in 1847 alone (Blackstone 1988).  The granite outcrops became landmarks -- Devils Gate, Split Rock, Independence Rock.

"Independence Rock" by WH Jackson.  Split Rock is in distance.  Source.

“Camp of the US Geological Survey" in the Sweetwater graben; by WH Jackson, 1870.  Source.

"The Emigrant's Grave" along the Sweetwater River; by WH Jackson, 1870.  Source.

Other posts about the “Heartland of Laramide Tectonics” include an overview of the Folded Land, a look at the Great Unconformity, a tour of Permo-Triassic redbeds, and a search for Jurassic pterosaur tracks.

Sources

Blackstone, DL, Jr.  1988.  Traveler’s guide to the geology of Wyoming, 2nd ed.  Geological Survey of Wyoming Bulletin 67.  Laramie, WY.

Flanagan, KM and Montagne, J.  1993.  Neogene stratigraphy and tectonics of Wyoming, in Snoke, AW, Steidtmann, JR, and Roberts, SM, eds.  Geology of Wyoming.  Wyoming State Geological Survey Memoir 5:572-607.  Laramie, WY.

Mears, B, Jr, Eckerle, WP, Gilmer, DR, Gubbels, TL, Huckleberry, GA, Marriott, HJ, Schmidt, KJ and Yose, LA.  1986.  A geologic tour of Wyoming from Laramie to Lander, Jackson and Rock Springs.  Geological Survey of Wyoming Public Information Cirucular No. 27.  Laramie, WY.

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.  Laramie, WY.

Wyoming State Geological Survey.  Granite Mountains.  Accessed August 2014.  http://www.wsgs.uwyo.edu/research/stratigraphy/GraniteMts/Default.aspx

Colorado Butterfly Plant and the US Air Force

Last week’s plant quiz featured the Colorado butterfly plant, growing in Wyoming.

Every summer for 28 years, field botanists have walked through riparian habitat on FE Warren Air Force Base counting Colorado butterfly plants.  It’s not always easy.  August days are hot, and there are thick stands of coyote willows and Canada thistle to fight through.  Butterfly plants are about as tall as the grass, so they’re hard to spot when it’s thick.  They’re a shifty bunch too, changing location from year to year.  Particularly difficult is the monotony of it all.  By afternoon, it can be tough to stay focused.

Butterfly plants trying to hide in the grass, but the white spots give them away. 

Colorado butterfly plant was first collected in 1894, near Fort Collins, Colorado, hence the “Colorado” part of its name.  “Butterfly” comes from the shape of the flowers.  They are bi-laterally symmetrical -- the two pairs of petals are mirror images -- and the petal pairs are reminiscent of butterfly wings.

Top to bottom:  buds, flowers, young fruit.  B. Heidel, WY Natural Diversity Database (WYNDD).

Taxonomic aside:  The butterfly plant was first called Gaura neomexicana spp. coloradensis, a name that lasted until 2007 when the genus was combined into Oenothera.  In 2013, the Colorado butterfly plant was recognized as a full species, and is now Oenothera coloradensis (source).  This change is so recent that you won’t find it in the usual online plant resources (e.g. ITIS and USDA PLANTS).

Colorado butterfly plant, from the Wyoming State Species Abstract.

You may have guessed why these butterfly plants are counted each year -- they are rare and threatened.  The species grows only in north-central Colorado, western Nebraska, and southeast Wyoming (where most of the populations occur, contrary to the plant’s name).  In 2000, it was listed as a threatened species under the Endangered Species Act (ESA) by the US Fish and Wildlife Service.  A listed-threatened species is  “likely to become in danger of extinction within the foreseeable future throughout all or a significant portion of its range. Federal law prohibits the removal or destruction of threatened plants on any federal land or as a result of federal actions.”

For plant species, ESA protection is applicable only on federal land.  Contrary to widespread fear, it does not cover populations on private land unless the owner happens to be receiving federal funds of some kind.  The only place where Colorado butterfly plant grows on federal land is FE Warren Air Force Base near Cheyenne, Wyoming.  Thus the Base has been become the focus for protection and research.

Warren Air Force Base is home to the 90th Missile Wing and the Colorado butterfly plant.

Colorado butterfly plant prefers moist sites in drainage bottoms.  Though the Base is small (630 acres, 255 ha), it has three drainages with suitable habitat, and supports one of the three largest populations known -- about 6000 to 8000 plants in flower in a given year (but see more below).

Butterfly plant with coyote willow, along small creek.

Even before the butterfly plant was listed under the ESA, the Air Force took steps toward protection.  Monitoring of population trends began in 1984.  As is commonly done, the population was sampled, rather counted in its entirety.  Permanent plots were established, each consisting of a bar driven into the ground at a chosen corner of the plot.  A metal tag stamped with the plot identification number was attached.  In subsequent years, field staff were to return to each location, set up a plot of the same size from the chosen corner, and count plants.  But then the very next year there were heavy rains, flooding and scouring of the drainage bottoms.  And there was a change of staff.

In 1986, I was hired as botanist for the state biological inventory, and one of my first projects was monitoring Colorado butterfly plants on Warren Air Force Base.  In spite of the flooding and scouring, I relocated many of the plots and duly counted butterfly plants.  Results showed a big decline.  Some plots had no plants whatsoever, in contrast with initial counts.  Yet looking around I saw lots of butterfly plants outside the plots.  Were we really capturing population trends?

Turns out not.  Colorado butterfly plant is a short-lived perennial that thrives with periodic disturbance.  It does best in fairly open habitat, without a lot of competition, and occupied habitat changes as the site changes (we say the plants move around but we really do know better!).  Sampling has to be intensive to deal with such "behavior".  So we switched to a straight census of all butterfly plants.  Given the limited area involved, this has proven to be quite doable.

Buds and flowers on left (flowers 1-1.5 cm across), fruit on right.

Colorado butterfly plant fruit.  Each capsule contains four seeds.  B. Heidel, WYNDD.

Many monitoring programs are started, but only a small percentage continue long-term.  Twenty-eight years of census data are exceptional.  So what’s up with butterfly plants on the Base?

Population size varies widely from year to year, and changes in distribution are common.  When the annual census program began in 1986, we recorded plant numbers for sections of drainages, and could therefore see major shifts in distribution.  Since 2002, the crews have used GPS to record and map patches of plants, recording changes more precisely.

Much of the open habitat created by the 1985 flood is now gone, taken over by coyote willow and other woody vegetation.  But the overall population hasn’t declined significantly, yet.  While Crow Creek numbers are way down, those on Diamond Creek and a tributary are up.  Even so, we know the butterfly plant is dependent on periodic disturbance for long-term survival, so we’re hoping for another big flood! (shhh ... don’t tell)

Monitoring will reveal the unexpected.  We can learn a lot about the biology and ecology of a target species just by being out there with it.  In 2007, field staff found almost no butterfly plants until they looked close and saw the dead and dying.  Remaining leaves were covered with shot holes indicative of flea beetles; stems and branches showed damage from their larvae.  However, it took only a few years for the population to recover.  In 2008, plant numbers were lower still, even though there was no herbivory, but by 2009, the population was clearly on the rebound (Heidel et al. 2011).

Excited field botanist strides hopefully toward red patch in left center of photo.

This summer I had the pleasure of revisiting Colorado butterfly plants on Warren Air Force Base with Bonnie Heidel, botanist for the Wyoming Natural Diversity Database.  During the drive to Cheyenne, she told me about the flea beetle scare, and worried out loud about what we might (not) find.  But as soon as we got out of the car I saw them in the distance -- not individual plants but the distinctive reddish patches (stems) with white spots seeming to float amid the grass.

Up close, we found the plants were both abundant and healthy.  And since that visit, the annual census results are in -- Colorado butterfly plant is doing quite well in 2014 :-)

We found butterfly plant haven at our first stop!

Sources

Heidel, B,  Fertig, W, Blomquist, F and Abbott, T.  2008.  Wyoming’s Threatened and Endangered Species: Colorado Butterfly Plant. Wyoming Bureau of Land Management, in collaboration with Wyoming Natural Diversity Database.  http://www.blm.gov/pgdata/etc/medialib/blm/wy/programs/pcp/species/docs.Par.58921.File.dat/ColoButterfly.pdf

Heidel, B, Tronstad, L and Handley, J. 2011. Flea beetle (Altica spp.) herbivory on a threatened plant, FE Warren Air Force Base, Wyoming. Natural Areas Journal 31:283-287.  http://www.uwyo.edu/wyndd/_files/docs/Reports/WYNDDReports/A11HEI03WYUS.pdf

Heidel, B and Handley, J.  2014.  26-year population trends of Colorado butterfly plant (Oenothera coloradensis; Onagraceae), a short-lived riparian species on F.E. Warren Air Force Base.  Prepared for F.E. Warren Air Force Base by the Wyoming Natural Diversity Database (University of Wyoming), Laramie, WY.  http://www.uwyo.edu/wyndd/_files/docs/reports/wynddreports/u14hei08wyus.pdf

Wyoming Natural Diversity Database.  2012 (last update).  State species abstract, Gaura neomexicana ssp. coloradensis.  http://www.uwyo.edu/wyndd/_files/docs/reports/speciesabstracts/gaura_neomexicana_coloradensis.pdf

Tree-following: Mid-summer Miscellany

Let me (re)introduce you to my tree -- a lanceleaf cottonwood, Populus x acuminata.

If this is your first encounter with tree-following, first let me explain.  There’s a group of dedicated tree enthusiasts around the world (mostly in the UK) who watch and report each month on their chosen trees.  Our adventures are encouraged by Lucy Corrander of Loose and Leafy, who kindly provides a monthly tree-following page as a clearing house.  My tree is a lanceleaf cottonwood that grows on the bank of the Laramie River in southeast Wyoming, USA.  It's been fun and fascinating to watch it bloom, leaf out, and most recently, cast its seeds to the wind.  Earlier reports can be found here.

Now it’s mid-summer and my tree seems to be stuck in a holding pattern, unchanged. Perhaps the leaves have faded some, but if so, it's very subtle.  I can't be sure.

I had hoped to find seedlings and write about reproduction -- that would be the logical next episode.  I looked in a lot of places, but if there were any cottonwood seedlings out there, I didn’t recognize them.  I'll keep looking and get back to you next month.

Though there’s no obvious tree news, and no seedlings to report, it’s still a wonderful time to visit the Laramie River.  Yesterday I started out searching for cottonwood seedlings but was soon distracted by other things.

There were boys fishing -- of course!

There was the giant beaver lodge -- accessible now that the river’s not flooded.

This lodge is at least six years old, maybe even ten.  I’ve lost track.

In spring, when the river was high, we watched a beaver pull up young willows and add them to this side of the lodge.  Now the entrance is on dry ground (left side of lodge).

But the entrance on the river side is still swimmable (center of photo).

There were many wildflowers blooming.  Here's a few:

Showy milkweed, Asclepias speciosa.

This is one of the species of milkweed with elaborate flowers consisting of hoods and horns as well as petals.  Flowers are about 2 cm across.

White prairie aster, Symphyotrichum falcatum.

Delicate mats of thymeleaf spurge (Euphorbia serpyllifolia) are overlooked by most walkers.  The tiny white flowers are only about a millimeter across.  If you click on the image, you might spot tiny green lobed fruit as well.

A patch of gumweed (Grindelia squarrosa) glowing in the sunshine.  It’s generally looked down upon for its pungent odor and weediness, but I find the flower heads bright and cheery.

The gumweed patch was abuzz with insects.  I tried to photograph them, but they darted about so quickly that I captured only a few in focus.  This one has impressive pollen packs on its legs.

Canada thistle, Cirsium arvense, is another unpopular plant.  In fact, it’s designated noxious.  But there’s beauty here too -- in the whirly-bird seed hairs that are about to fly off with their payload.

There were many signs along the path.  Some were problematic.

One of several interpretive signs in a rest shelter.  Note the problem?

This is the wrong cottonwood.

I don't think plains cottonwoods (Populus deltoides) grow along this part of the Laramie River.  If there are any, they must be rare; all the trees I’ve seen are lanceleaf cottonwoods.  The habitat seems wrong too -- plains cottonwoods are trees of lower elevations and we’re at 7000 feet above sea level here.  However, they are common in town (planted).

On the way home, I checked the benches along the river.  Those signs are quite right.

Left to right:  my cottonwood, Laramie River, Rich's bench in shade, Kayla’s bench in sun.

I knew neither Rich nor Kayla but I have good feelings about them both, having read the signs by their benches many times.

Here’s another favorite:

I agree.

Nelson's spirit resides here, in the shade of lanceleaf cottonwoods.