Mysterious Stones of the Laramie Mountains

A story of granite, grus, and tors.

If you wander the summit of the Laramie Mountains as dusk falls, chances are you will spot mysterious creatures silhouetted against the evening sky—giants forever waiting by their castles, life-like but never moving. But if you get too close, they disappear, no matter how stealthily you approach. In their place are peculiar stacked stones and huge rounded masses of rock.

These are our tors (from the Old Welsh "twr" meaning pile or cluster). In shapes ranging from rounded blocks to dancers, turtles, beehives, and potato chips, they inhabit magical places like Vedauwoo, Blair, and the Devils Playground. But why? How and when did such fantastical beings appear?

Pioneering geologist Ferdinand Vandeveer Hayden once admonished humanity for not asking such questions. "Like the ripe fruits which so many pluck from the tree, and enjoy without a further thought, [landscapes] are accepted by mankind, and how few are thoughtful enough to inquire from whence they come!"

But not us, Professor Hayden. We are among the thoughtful few!

Monuments left by erosion, pleasing in their variety

Hayden's complaint appeared in his 1871 Preliminary report of the U.S. Geological Survey of Wyoming, specifically in the section about the Laramie Mountains, which he had crossed by train the previous year. At the crest, he came upon "scenery which is quite unique and remarkable, differing in many of its features from that at any other point along the [rail]road."

It was a landscape dominated by granite, as Hayden explained. The plateau-like summit was "literally paved with small fragments", and "massive piles, like the ruins of old castles, are scattered all over ... the difference in texture of the rock is such as to give a most pleasing variety, hardly any of these piles being alike."

Hayden then turned to the difficult question—from whence they come. "There is an interesting thought just here as to the real origin of these granitic, ruin-like piles that give the peculiar distinction to the plateau surface of the Laramie Mountains. I believe it is entirely due to erosive forces, which have operated here on a gigantic scale, and these cones and natural temples are the monuments that are left to tell the tale."

"I am convinced that the surface was at one time at least on a level with the highest of [the granite piles]." he continued. "How much more has been removed it is now impossible to tell, but I am convinced that comparatively few geologists have fairly estimated the immensity of the time required and the vastness of the amount of material removed from the surface by erosion."

Broadly-speaking, Hayden was correct. The tors of the Laramie Mountains were created by erosion. But there's much more to the story. It seems that erosion played multiple roles—shaping, revelation, and now destruction.

"Skull Rock ... has been peeled off, coat by coat, by the fingers of Time ..." wrote Hayden in 1871. (LOC).

A granite named Sherman

In early August 1907, the Laramie Republican (newspaper) announced the arrival of a "geological savant"—Professor Eliot Blackwelder, of the University of Wisconsin and the U.S. Geological Survey. He would study the surface geology of the southern Laramie Mountains, specifically the oldest rocks of the range.

For regular readers of this blog, "Eliot Blackwelder" may ring a bell. He appeared in a recent post about the Snowy Range, where he also studied ancient rocks. In fact, Precambrian rocks—the oldest on Earth—were Blackwelder's specialty.

Shortly after arriving, Blackwelder recruited an assistant—Herbert Kennedy, of the Republican's business department (he was replaced by his brother, Leon). Two days later, they left to set up their field camp near Tie Siding. They would work into late September, when Blackwelder returned to Wisconsin and Herbert Kennedy returned to his studies at the University of Wyoming.

In 1908, Blackwelder published "Pre-Cambrian rocks in southeastern Wyoming". He named the granite at the crest of the Laramie Mountains the Sherman granite, after the now-defunct Sherman railroad stop. The next year, in text accompanying geologic maps for the area, he described what he considered "the most important event of pre-Cambrian time in this district ... the intrusion of a vast mass of coarse-grained granite."

Rapakivi and grus—killer conversation starters

Like Hayden, Blackwelder was impressed by the small fragments that covered the ground to great thickness. "Although hard in its unaltered condition the Sherman granite disintegrates readily under the influence of descending surface waters and produces a coarse gravelly soil ... In excavations at Buford the granite has been found to be decayed to a depth of 40 to 50 feet ...".

This would greatly benefit the Union Pacific Railroad. The Buford quarry furnished trackbed ballast for 800+ miles of track, from Omaha to Rock Springs. From 1900 to 1914, an estimated 10,000 railcar loads were produced each year. Furthermore, Sherman ballast was cheap—a ton for just 6¢! —while average cost for the region was 49¢ per ton.

UPRR track builders approaching crest of Laramie Mountains on a bed of Sherman grus; 1867-68; Beinecke Library (cropped).

Because it breaks down so readily, geologists classify the Sherman granite as "rapakivi" ("mud rock" in Finnish). If wetted, mica in the granite expands, breaking the rock to create gravel, mainly crystals of feldspar and quartz. Geologists call this kind of decomposed granite "grus" (pr. gruce or sometimes gruss).

Blackwelder all but ignored the other remarkable feature of Sherman granite—the tors. He provided one photo and a single sentence. "Where thus deeply weathered, the outcrops of granite are smoothly rounded and free from visible ledges."

Sherman granite on crest of Laramie Mountains; looking north across "wide rolling plain of the mountain plateau". From Blackwelder's 1909 report (USGS); photographer unknown.

The problem of tors

Tors have long been mysterious and difficult to explain. One of the biggest obstacles is the many kinds, which likely differ in origin. Fortunately, the tors of the Laramie Mountains appear to be very similar to those of England, which have preoccupied humans for millennia. Perhaps their accumulated knowledge will provide some insight into the origins of our tors.

Haytor, perhaps the grandest of Dartmoor tors (southwest England; courtesy Smalljim).

The earliest accounts invoked mysticism and magic. Often tors were said to be living creatures turned to stone during some foolish encounter with a witch or the devil. A magical explanation for our tors would be wonderful, since the geology is complicated and still debated! But it appears there are none available, so we will rely on science.

English antiquarian and naturalist William Borlese of Cornwall may have been the first scientist to study tors. In a 1758 account, he concluded they were human creations, like the obvious constructions at Stonehenge. This was a popular theory for a time, but then geologists intervened.

Early on, geologists suggested that tors were once sea stacks—isolated stone towers just offshore. But sea stacks are angular and jagged, whereas the tors of England (and the Laramie Mountains) are distinctly rounded.

Sea stacks, Victoria, Australia; photo by Sam Abell.

Need to look in surprising places

In the second half of the 19th century, several English geologists made a conceptual leap. They spotted similar forms, not above ground, but in granite quarries! They realized that tors look very much like the quarryman's rounded "corestones" but with the "growan" (grus) removed. These geologists concluded that tors develop underground, where they wait to be revealed, whether by quarrymen or erosion.

In spite of the remarkable similarity of tors and corestones, this theory was largely ignored until 1955. That year, an English specialist in landforms, geomorphologist David Linton, published a paper explaining a process that could sculpt tors underground—"profound rock rotting." Groundwater flowing through fractured granite can break down narrow zones of rock, creating rounded blocks and filling the cracks with grus.

Profound rock rotting seems a likely explanation for our tors, made as they are from rapakivi granite!

Stages in the evolution of tors, by subsurface rock rotting (Linton 1955).

From birth to emergence

By now, readers surely are at the edge of their seats, eager to learn how our tors made it from their subterranean birthplace to the surface. But first we must backtrack 1.43 billion years. That is the estimated age of the Sherman granite, the time when it crystalized from molten magma, the actual birth of our tors. 

They may have remained in their infant form for hundreds of millions of years. Geologists still debate when shaping of the tors took place. The granite may have fractured when it solidified. Or maybe it happened later when erosion at the surface reduced pressure on the granite underground. Or maybe both! Nor is it clear when groundwater reached the fractured granite and began breaking it down. And there may have been several episodes.

We have a better understanding of how and when the tors were finally revealed at the surface. Their emergence is related to the Laramide Orogeny—the great mountain building episode that started 80 million years ago, lasted almost 40 million years, and created mountain ranges from Mexico to Canada—the Rocky Mountains.

It was during creation of the Rockies that the Laramie Mountains were uplifted. Like all mountains, as soon as they began to rise, erosion set in. This was erosion on a grand scale, just as Hayden suspected. After about 35 million years, enough overlying rock had been removed to expose the ancient Sherman granite. Grus in the joints was washed away, leaving behind our remarkable tors.

Visit our aging tors

Erosion hasn't stopped. It continues to slowly expose more tors, but it also destroys the ones we know and love. Wind, rain, and freezing remove a bit more rock every year. Occasionally a block falls! But from our perspective, ephemeral beings that we are, this destruction is incredibly slow. We still have time to enjoy our tors, so let's go!

From Interstate 80 about 15 miles east of Laramie, take the Vedauwoo Road east. Drive a quarter mile beyond the entrance to Vedauwoo Glen (fee area) to a grus-covered parking area on the left (free, with bathrooms). Follow the grus-covered trail past an interpretive sign and Pedestal Rock. Look for low mounds of granite—are these young tors emerging from below? or fallen blocks buried in grus? Curve left uphill and through a gate. Wander to the right across a large slab of Sherman granite. Note crystals of pink and white feldspar, sparkly quartz, and black biotite mica between the patches of lichen. Take in terrific views of tors, pleasing in their variety, just as Hayden said. Explore cracks and gullies to see decomposing rapakivi granite.

The best time to visit is during the golden hour before sunset—the magic hour when the light is warm and soft. You will be tempted to stay longer. But if dusk falls, do take care, lest you too be turned to stone!

Vedauwoo tors during the golden hour, not long before dusk.

This post is based on an article I wrote recently for the Laramie Boomerang's "History" column. In case you're wondering, I don't believe in magic ... under normal circumstances.

Of Woolsacks, Witches, Cheesewrings & Tors

Baa baa black sheep,

Have you any wool?

Yes sir, yes sir,

Three bags full.

One for my master,

One for the king,

One for the Geologist

Who likes such things!

Between Laramie and Cheyenne, Interstate 80 crosses the crest of the Laramie Mountains. But there are no rugged high peaks, no sparkling alpine lakes, no steep narrow canyons. Instead, the highway traverses a broad plain populated with peculiar rocks. They rise abruptly—like simple alters, or ancient castle walls, or stone creatures frozen on their way to stone temples. Travelers curious enough to stop will find they’re made of blocks: block walls, stacks of blocks, blocks scattered across the ground. The blocks themselves are distinctive. The rounded edges indicate they were born deep underground, where they were shaped by woolsack weathering. Only much later did they emerge into this world.

“granites rise in thick picturesque ridges, 50 to 100 feet high, like ruined walls, lending a peculiar as well as picturesque appearance to the landscape” wrote geologist FV Hayden of his visit to the Laramie Mountains in 1870. Photo by WH Jackson (USGS).

Why "woolsacks"? … these rocks look more like wool bales. In fact, woolsack means bale in the case of rocks—specifically the bale of wool on which King Edward III (1327-1377) commanded his Lord Chancellor to sit while in council, in recognition of the importance of the wool trade. It became known as The Woolsack. Six centuries later it’s still in use, by the Lord Speaker in the House of Lords.

Wool bales, 1900 (source).

The Woolsack, 1897 (source).

Woolsack weathering isn’t unusual, but the resulting forms are so fanciful that they grab our attention and spark our imaginations. Often they have evocative names and other-worldly explanations. We designate parks around them, for protection and public access.

The Cheesewring of Bodmin Moor, southwest England; John MacCulloch (1814).

The Cheesewring was named for its resemblance to slabs of cheese on a press. But legend says otherwise—it's a stack of rocks created in a contest between a Giant and a Saint. In spite of the great weight of the stones, the diminutive Saint built the taller stack (about 15 feet high), thereby avoiding death. The Giant was so impressed that he immediately converted to Christianity.

Vedauwoo Glen, in the Laramie Mountains, is home to Earthborn Spirits. It's managed by the US Forest Service.

The Great Staple Tor in Dartmoor National Park, a textbook case of woolsack weathering.

A stack or pile of woolsack rocks is often called a tor, possibly derived from the Old English torr—related to Scottish Gaelic tórr for a bulging hill—or possibly from the Celtic word twr meaning tower. Dartmoor National Park in southern England is famous for its legendary tors. There are at least 160, most with colorful names and stories.

Vixen Tor; John MacCulloch (1814).

Vixen Tor was home to the wicked witch Vixana. Whenever a traveler foolishly passed nearby, she called up a thick mist—so thick that the traveler lost his way, stumbled into a bog, and met an excruciating end, sucked screaming to his death. Vixana would dispel the mist just in time to enjoy the final moments of his desperate and hideous struggle. Finally Vixana herself was killed—by a handsome young moorman, of course!

The tors of Dartmoor are legendary not just for tales and spirits, but also for pioneering studies by early geologists. In 1754, antiquarian, geologist and naturalist William Borlase concluded that Druids carved the tors, based on the prevalence of blocks. Druids were said to worship cubes, symbolic of the god Mercury—even though almost nothing was known about Druid culture then, as now.

In 1814, Scottish geologist John MacCulloch read a paper before the Geological Society of London titled On the granite tors of Cornwall, in which he discounted Borlase’s theory. He didn’t mince words:

“… learned antiquarians have tortured their inventions and have constructed religious systems for the purpose of explaining these very simple and intelligible natural appearances, by the rites of a mysterious and Druidical worship. … It is unnecessary to suppose that the chisel of Druidism has been employed to reduce it [the Cheesewring] to an image of Saturn. Natural causes are sufficient to account for its appearance.”

The tors’ rounded blocks convinced MacCulloch that they had been shaped by air and water, not by human hands:

“The changes which they undergo in their places of rest, by their more rapid disintegration at the angles than at the sides, are sufficient to prove that this spheroidal shape may be produced by chemical action of air and water, without the necessity of any mechanical violence. However difficult it may be to give a very satisfactory account of this peculiarity, the fact is undoubted.”

Woolsack weathering—“more rapid disintegration at the angles than at the sides”

MacCulloch was only partly right. The rounded shapes were produced by chemical action of water but not air. That would be impossible, for woolsacks and tors form underground.

Woolsacks at Vedauwoo, Laramie Mountains. For origins, see diagram below.

Our local woolsacks are made of Sherman granite, which began as magma intruded into the crust about 1.4 billion years ago. It never reached the surface, but cooled underground into a huge mass of granite, shrinking and cracking in the process. Fractures often formed 90º angles—the beginnings of blocks. Groundwater percolated through the cracks, and chemicals broke down the rock, rounding the edges (spheroidal weathering).

Then roughly 70-40 million years ago, during an episode of mountain-building (Laramide Orogeny), the Laramie Mountains rose, erosion set in, and the Sherman granite was gradually exposed. Weathered debris washed out of the fractures, and a multitude of wondrous rock forms emerged.

Birth and emergence of the woolsacks in the previous photo (click on image for details).

This land of science and the supernatural, where knowledge coexists with legend and whimsy, is just 15 miles from town. We're lucky to be able to wander among the real and the otherworldly as our mood sees fit.

Debris from weathered granite, called regolith or grus.

The Potato Chip.

Prow of the Nautilus.

You name this one.

Watch out, Emmie ... it's ready to leap!

Unnamed stone creature, stone temple in the distance.

Woolsacks at sunset.

The witching hour. Click on image ... if you dare!

Sources (in addition to links in post)

Borlase, W (Jackson, W). 1754. Observations on the Antiquities, Historical and Monumental, of the County of Cornwall. Available here.

MacCulloch, J. 1814. On the granite tors of Cornwall. Trans. Geo. Soc. London Ser. 1 Vol. 2:66-78.

Vedauwoo in Black and White

Much about Vedauwoo is mysterious, including its name.  Vee-dah-vú is said to be an Indian word meaning earthborne spirit, but no one knows which tribe called it that, if any.

Curious granite mushroom.

Vedauwoo is inhabited by tors -- odd rock sculptures that defy understanding.  In the early days of my blogging, I naively chose Vedauwoo tors as a topic.  It took me quite a long time to put something together, and in fact I ended up with three posts: Creatures of Stone, Creatures of Stone II -- genesis, and Creatures of Stone III -- the rest of the story?

Here’s a quick summary of the best explanation currently:  1) granite fractured while cooling underground; 2) ground water moved through the fractures weathering the rock and widening the cracks; 3) the granite was exposed during and after uplift of the Laramie Mountains, when overlying sedimentary cover was removed;  4) weathering continues, with blocks periodically falling.

Two-staged tor formation; click on image to view.

The tors are made of Proterozoic Sherman granite.  It was emplaced 1.4 billion years ago, but it’s not clear how nor why.  It’s close to and appears to be associated with the Cheyenne Belt, a continental suture created when the Yavapi province, a collection of island arcs, collided with the Archean Wyoming Craton (below).  The suture may have provided a zone of weakness for mantle-derived material (the granite) to be emplaced near the surface (Frost et al. 1999).

Vedauwoo is just south of the Cheyenne Belt, where the Yavapi province was accreted to the Wyoming craton (map courtesy Written in Stone).

Then there’s the mystery and intrigue of black-and-white photography -- I love it!  But I’m still experimenting and learning what works.  For black-and-white I like relatively simple compositions, such as patterns and collections of bold shapes, especially with strong contrast in light.  Odd arrangements and perspectives also attract me.  The tors of Vedauwoo are great in all these ways, but it takes thought, effort and especially trial-and-error to shoot them to good effect.

Bold shapes,

strong contrast in light,

unusual perspectives,

and odd arrangements -- all are common at Vedauwoo.

There’s general agreement that one should think in black-and-white, shoot in color, and then convert to black-and-white in post-processing.  That’s what I’ve done so far.  My post-processing is simple, all in iPhoto.  I reduce saturation (color) to zero, and often adjust light and shade with the histogram.  Sometimes I increase definition.  This works pretty well, but one of these days I’m going to try a high-contrast setting on my camera when I shoot in color for black-and-white.

Quaking aspen against Sherman granite.

More aspen -- with leaves.

Small aspen and a tiny tor dance in the wind.

Where does this mysterious trail lead? east of the Moon? west of the Sun?

For more on black-and-white photography, see Centennial in Black and White.

Literature Cited

Frost, C.D. et al.  1999.  Petrogenesis of the 1·43 Ga Sherman batholith, SE Wyoming, USA: a reduced, rapakivi-type anorogenic granite.  J. Petrology 40:1771-1802.