Geological Mementos

Memento (n.): an object kept as a reminder of an event or person.

My knick-knacks are dominated by rocks. But they're not just rocks. For in addition to sand, silt, quartz, feldspar, anorthosite and serpentinite, they contain memories.

Some are mementos of events: magma almost but not quite reaching the surface; raging rivers carrying debris from disappearing mountains; a rift tearing the continent, fortunately stopping in time to keep the country whole. (I'm exaggerating—it ceased hundreds of millions of years before the colonies left England.) Others remind me of people: early explorers of the American West; pioneering geologists; 19th-century naturalists; today’s geo-bloggers.

Tired of unappealing weather and cabin fever, I decided to make portraits of these rocks so I could enjoy the memories they trigger. (Note re scale: Lady Liberty's portrait is an inch across, 2.5 cm.)

“Hillers trachyte is a pale gray paste with large white crystals of feldspar and crystals large and small of hornblende.” Grove Karl Gilbert.

In 1869, while traveling down the Colorado River, John Wesley Powell spotted an unnamed cluster of peaks in the distance, which he christened the Henry Mountains. He presumed they were volcanic. Volcanic mechanisms were a hot topic at the time, so he sent the great pioneering geologist, Grove Karl Gilbert, to investigate. Gilbert discovered the mountains were not volcanic. In fact, they didn’t match any type of uplift then known. He concluded the Henrys were created by shallow intrusions of magma. He would describe and name this new type of uplift, calling it a laccolith.

In 2012, I camped where Gilbert had camped, at the base of Mount Hillers. I hiked up the slope to look at sedimentary strata tilted nearly vertical by intruded magma. A piece of “Hillers trachyte” went home with me as a memento.

South side of Mount Hillers.

Large crystals mean magma cooled underground rather than on the surface, as would be the case for a volcano.

• • •

Igneous and metamorphic stones pose on a sandstone boulder.

Three years ago, while walking up a draw in the sandstone country of eastern Utah, I was surprised to find rounded polished stones made of igneous and metamorphic rock. What are they doing here?! I was even more astounded when I read that they came from the ancient Uncompahgre uplift, which hasn't been around for at least 80 million years. These stones are ghosts of mountains past.

Enchanted by their story, I returned last year and followed the Cutler Formation south almost to Arizona. There, far from the source, the Cutler is made of sand and silt instead of cobbles. On the way home, I stopped near Gateway, Colorado, where the Cutler includes cobble/boulder conglomerates, indicating the source had been close by. I was standing on the "slopes" of the ancient Uncompahgres! I took home several ghostly mementos.

Coarse conglomerate deposited on the slopes of the Uncompahgre uplift.

• • •

The deceptive Uinta sandstone.

A billion years ago, the supercontinent Rodinia, which included North America, was coming apart. A tear ran east from the vicinity of today’s Salt Lake City. It eventually failed, but streams continued to fill the rift valley with sediments to a depth of some four miles! By the 1870s, it had become a sandstone that fooled the great geologists—Hayden, King, Emmons and Powell. None had expected to find sandstone that old (Precambrian).

It fooled me too. The cores of our mountain ranges usually are made of granite and metamorphic rocks. I thought maybe the mileage in the guidebook was wrong.

This is the Precambrian core of the Uinta Mountains?!

Layers of sediments deposited in a rift that once threatened to split the continent.

• • •

Sparky contemplates his humble role in the universe.

On my drives from Laramie to the West Coast, I’ve stopped a half-dozen times at the Lunar Crater area in central Nevada. I like to camp in the wide open country there, free of crowds and regulations. Usually I hike, and take photos of the amazing landscapes. But one time I set out in search of mantle material in a lava field (the mantle is the molten layer beneath the Earth’s crust, miles below the surface).

Volcanic magma sometimes carries up pieces of the mantle. I was looking specifically for olivine—greenish and a bit translucent. I think I found some. (I used Geology Underfoot in Central Nevada as a guide).

Green olivine, with pale plagioclase and dark pyroxene behind.

The Black Rock lava flow.

\

• • •

Strolling down a river, atop a ridge.

My most recently-acquired geological memento is the lithified remains of a river. Camels, rhinos and ruminating hogs used to come here to drink, 19 million years ago. Now a trail follows the river bed—on top of a ridge! In other words the valley bottom now stands high, thanks to topographic inversion.

The “little gray potatoes” are calcified silt nodules.

Silt nodules and sand, with bits of quartz from the Rocky Mountains. Not until I took the macro shot did I notice the tiny lichen "cups" (upper left; click on image to view).

• • •

And finally ... a rock wall?

The summit?

No.

This round green rock—serpentinized harzburgite—is the most popular of my geological mementos. Friends pick it up, surprised at its heft. More than one has noted “it looks like a brain!” It's a memento of two different events. One is the creation of the California Coast Ranges—a crazy geological story which no one fully understands (or even close?). Somewhere in the process, mantle rock was squeezed up to the surface, becoming serpentinized en route.

The Big Sur Coast, land of dreams and mystery. Photo by R. Koeppel.

The elements took their toll—the green rock was weathered and broken. A fragment traveled down an ephemeral river, almost reaching the Pacific Ocean. But I intercepted it. That’s the other memorable event.

I dug the boulder out of the sandy bed of the Santa Maria River, shortly before meeting a man pushing a bicycle. “Buenos días” he said, looking at the rock. “Piedra grande verde” I replied. He smiled and continued on.

• • •   • • •

Notes on Gear

Aside from camera and tripod, the setup I used for rock portraits was simple and cheap. The light box is home-made, following Amanda’s instructions. The sun shining though a window provided light. I put the box on a music stand, with the desk almost flat. It was easy to raise, lower and tilt … except for the heavy serpentinized harzburgite brain which collapsed the stand to its lowest position. I had to adjust the tripod instead.

The dogs found this rock portrait stuff quite boring.

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