Friday, March 20, 2015

The Crystal Unicorn

Wyoming has been good to me.  For 25 years I made a living traveling to its far corners and obscure non-destinations in search of plants.  It rewarded me with spectacular landscapes, fascinating natural history, and surprises.  But now in retirement (mostly), I usually go elsewhere – Utah, Nevada, California.  This year I resolved to spend more time in my state.

Wednesday was to be a gorgeous spring-like day, so on Tuesday I picked a place I’d never visited, did a little research, and left home early the next morning.  It would turn out to be a perfect outing.  A not-particularly-scenic part of the state revealed itself to be beautiful, fascinating, even magical.

That trip was less than two weeks ago yet already it seems like a dream.  But then maybe I was dreaming.  For you see … I came upon a unicorn!

Moonshine Hills and Grayrocks Reservoir are northeast of Wheatland, WY.  Google Earth view.
I drove northeast from Laramie to the western edge of the Great Plains – the open rolling grassland country around Wheatland.  Destinations were the Moonshine Hills and Grayrocks Reservoir.  Few people know of the Moonshine Hills.  Ownership is private; access requires permission.  Grayrocks Reservoir is more popular, mainly for fishing.
View west from dam overlook: Grayrocks Reservoir, Laramie River Power Plant, Laramie Mountains.
I had two objectives:
1.  find small outcrops of Precambrian (very old) rocks among Tertiary (fairly young) surface rocks in the Moonshine Hills;
2.  find the “gray rocks” of Grayrocks Reservoir.
The Precambrian objective was quickly met.  I had with me Laura McGrew’s 1953 thesis about the geology of the Grayrocks area.  She had clearly indicated two small Precambrian outcrops on the beautiful hand-colored map in the back pocket.  Both were visible from the county road.
From map by McGrew, 1953.  Added letters refer to photos below.
Precambrian outcrop A across creek, looking southeast from pullout.
Precambrian outcrop B, looking northeast from road.
These outcrops include schist, gneiss, quartzite and granite.  They’re thought to be part of an eroded uplift now mostly buried under younger sedimentary rocks.  It probably dates to late Cretaceous-Paleocene time, contemporary with other Laramide features such as the Hartville Hills to the north and the Laramie Mountains to the west (McGrew 1953, Blackstone 1996).

In her thesis, McGrew also described gray rocks – part of the Gering Formation of the Arikaree Group.  She recognized two facies (subunits), including a “gray, well-consolidated conglomerate.”  Blackstone (1996) noted that “Grayrocks" refers to an old rural post office named for gray cliffs nearby.  I went looking for well-consolidated gray conglomerate cliffs.
Layers of gray rocks are common around the reservoir, visible here on slope beyond dam.
At the time of McGrew’s work, the Gering was thought to be earliest Miocene.  Now it’s usually considered late Oligocene, roughly 25 million years old.  The conglomerate is the lithified remains of stream channel deposits.

The Gering Formation is well-exposed around the reservoir but ownership is mostly private.  Fortunately a small Habitat Management Area lies just off the county road about three miles southwest of the dam (HMAs are public hunting areas created through agreements with landowners).  There was no season underway and no one else around, so I parked at the trailhead and walked up Cottonwood Creek.
Not far from the trailhead.
Grassland with sagebrush; Rocky Mountain junipers grow in drainages and on canyon rims.
The creek was just a trickle and there were no cottonwoods, but the country was still beautiful.  It was great to stroll through juniper woodland on a spring-like day.
Juniper woodland in Cottonwood Creek drainage.
the unicorn is commonly described as an extremely wild woodland creature
Along the creeklet were boulders that matched the well-cemented (and therefore resistant to erosion) conglomerate described by McGrew:
“conglomerate made up predominantly of unsorted, rounded to sub-rounded fragments of pre-Cambrian granite and quartzite and limestone and dolomite from the Hartville formation.  The fragments range in size from pebbles 1 inch or less in diameter to boulders 2 feet or more in diameter imbedded in a gray, fine-grained, limy, well-consolidated quartz matrix.”
I liked the varied composition of the conglomerate ... made it photogenic.
The conglomerate boulders looked like old blocks of concrete ... with super heterogenous aggregate. 
Layer of gray conglomerate in cliff above creek.
The drainage narrowed into a canyon with more boulders and more water.  The creek gurgled over rocks and fell into small pools.  There were occasional cottonwoods.  The trail disappeared, but it was easy enough to keep going upstream.
Then conditions began to deteriorate.  There were more and bigger boulders, cockleburs, mud, fallen trees, and eventually snow and ice.  Was it worth continuing?
The gentle and pensive maiden has the power to tame the unicorn.
I caught a glimpse of ice on cliffs and realized this was the head of a box canyon.  I picked my way through ice and snow to a sitting spot.  It seemed like a fairy-tale place:  a small waterfall rushed over the cliff, and ice decorated the canyon walls and floor.
Small waterfall upper right.
The alcove faced north, so the ice hung on even though it was a spring-like day.
In some places water seeped from the rock to form ice sculptures (note healthy moss).
I shot lots of photos and a movie.  That’s when I saw it:
video
An exquisite crystal unicorn rested against the rock wall.  It stood perfectly still, blending in with the ice around it.
I sat in wonder and admiration, but then remembered:
the unicorn, for the love it bears to fair maidens, forgets its ferocity and wildness; and laying aside all fear it will go up to a seated damsel and go to sleep in her lap.
So I got up and left quietly, tip-toeing through the cockleburs, boulders and mud.


Sources

Blackstone, DL.  1996.  Structural geology of the Laramie Mountains, southeastern Wyoming and northwestern Colorado.  WY State Geological Survey Report of Investigations No. 51.  PDF

McGrew, LW.  1953.  The geology of the Grayrocks area, Platte and Goshen counties, Wyoming.  MA Thesis, Department of Geology, University of Wyoming.

7 comments:

  1. What a special glimpse, Hollis! And so fugitive, given the changing weather...
    I have a very amateur question about conglomerate rocks because I know almost zero about geology. While digging for my garden, I find many rocks as well as building material, e.g. concrete chunks. I've wondered about the chunks that look like concrete with rocks mixed in. Are these artifacts of the construction process or just pieces of a natural conglomerate? How can I tell? Hope you don't mind my asking!

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    1. As we like to say when we're not sure: "that's an interesting question!" I know that aggregate is combined with cement and sand to make concrete. Coarse aggregate includes easily visible rock fragments, and is common (it's the type I'm familiar with). I suspect that's what you have if you have other construction rubble. Google tells me aggregate fragments vary in size but usually are less than 1.5 inch in diameter. For a given batch of concrete, I believe aggregate is pretty consistent in size and type compared with what you might find in conglomerate. The gray rocks I found had fragments ranging from fine gravel to rocks over 2 feet in diameter, and many colors and types. But I suppose there could be conglomerate with fragments that are fairly uniform in size and type.

      So ... kind of an answer :-) Thanks for visiting!

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  2. At first, I couldn't see the unicorn. I thought perhaps I was supposed to notice it in the movie. But then scrolling down after the movie, it was plain to see! Fun post. We see similar formations along the cliff walls facing the Mississippi River near La Crosse, Wis., and La Crescent, Minn. Our daughter goes to school near there, and often this time of year the water flows and then freezes/thaws/freezes fast and creates interesting formations. The conglomerate rock you show is, indeed, photogenic. Thanks for explaining the rock formations and highlights from your outing.

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    1. I didn't see it when I was taking the movie. But then after spotting the unicorn there, when I watched the movie at home I saw it! Neat that you have ice sculptures too - they help with winter :)

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  3. Isn't it great that wherever you go you can find surprisingly beautiful things in Wyoming? Fun post!

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    1. Thanks, Ken. yeah -- it's wonderful! We're lucky :)

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  4. I put together a hiking piece on Cottonwood Draw for the Cheyenne Tribune Eagle. While researching it I came across your blog and have really enjoyed it. You have such and educated and interesting eye for the things your see and write about. I've linked your post to my piece at www.AwayFromTheGrind.com. If you'd rather I didn't let me know.
    Hope to meet you someday.
    Roger Ludwig

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