Sunday, June 25, 2017

Our Lady of the Roadside

In a disturbed land cleared of life and showered with trash, sacred datura stands in regal glory. She thrives next to heat-radiating asphalt, fully illuminated by the sun and soaking up the rare runoff. At dusk, when the searing heat of daytime subsides, she opens great white trumpet-shaped flowers to welcome night visitors. The large fragrant patches of white, conspicuous even in the dim light, are produced for one reason—sex. Datura may be sacred but she’s hardly celibate.

Her lure is irresistible. Hawkmoths flit from flower to flower, feeding on copious nectar deep in the throat, bumping anthers, picking up pollen. Then at the next flower (1), a moth brushes against the receptive stigma waiting at the opening, leaving behind a few grains. Within minutes pollen tubes grow down the style, and male gametes make the long journey to fertilize ovules. With seed development, the ovary matures into a large plump spine-covered seed pod—a “thorn apple.”
Datura wrightii can grow to 3 feet in height, with flowers up to 8 inches long.
Bud and flower (beginning to wilt at 9 AM, 75ΒΊ F).
Flower in bud. When the time is right, it will unroll and turn white.
Exserted stigma waits for pollen, at the tip of a very long style; anthers visible at top of throat.
With age, sacred datura becomes the thorn apple (source).
In winter, if temperatures fall well below freezing, sacred datura disappears—or so it seems. But she’s only dormant, resting underground as large fleshy tuberous roots until spring. Then she rises again.
Sacred datura’s mighty root (source).

Sacred datura’s beautiful fragrant flowers, though clearly worthy, do not explain the widespread reverence she commands. Something much more exotic attracts worshippers—her powers of prophecy, divination and healing. Indigenous peoples throughout northwest Mexico and the southwest US have taken advantage of datura’s psychoactivity. In her early study of the California flora, Mary Elizabeth Parsons noted that “they [California Indians] administer it to their young dancing women as a powerful stimulant, and before going into battle the warriors take it to produce a martial frenzy in themselves” (Parsons 1907). In several cultures, boys were given datura as part of initiation into manhood (Fuller 2000). They fell into unconsciousness, which destroyed all memories of childhood (and sometimes the initiates as well), and then were taught the stories, traditions and rituals of their tribe.

Shamans, mystics and seekers continue to worship datura (2), even though it’s among the most dangerous of vision-inducing plants. All parts contain deadly tropane alkaloids—atropine, hyoscyamine, and hyoscine. Safe dosage is difficult to determine because individual plants vary in potency, humans differ in tolerance, and the onset of noticeable effects is slow. Cases of datura poisoning, including death, are usually due to deliberate consumption (source). Survivors can be temporarily or permanently scarred—with damaged lungs, stomach, intestines, kidneys and/or heart, and perhaps blindness and mental impairment.

Yet there will always be people willing to risk datura’s dangers for her promises—sufferers and seekers, lost souls and spiritual adventurers, thrill-seekers and those hoping to see beyond what we’ve been programmed to perceive. I suspect that as long as humans exist, sacred datura will be a “plant of the gods.”
Original source unknown.


(1) Datura wrightii is self-compatible—fertilization does not require pollen from a separate plant. But pollinators are helpful. Visiting hawkmoths have been shown to increase rates of self-pollination as well as outcrossing (Bronstein 2009).

(2) Datura wrightii is native to southwest North America, not just on roadsides but also in other open sandy habitat, e.g. desert washes. Its close relative, Datura stramonium, is the similarly-revered jimsonweed. Datura belongs to the nightshade family (Solanaceae), as do many other plants that offer us pleasure, poison, or both—chilis, petunias, potatoes, tomatoes, tobacco, belladonna, deadly nightshade and more.


Bronstein, JL, et al. 2009. Reproductive biology of Datura wrightii: the benefits of a herbivorous pollinator. Annals of Botany 103: 1435–1443. doi:10.1093/aob/mcp053

Fuller, RC. 2000. Stairways To Heaven: Drugs In American Religious History. Basic Books.

Parsons, ME. 1907. The wild flowers of California. Dover Books (1966).

Schalau, J. 2015. Backyard Gardener: Sacred Datura. Arizona Cooperative Extension, Yavapai County.

Monday, June 19, 2017

Lace Curtain in the Desert

Why would there be a lace curtain out in Utah’s West Desert, and why would anyone want to see it? Maybe it's in a ghost town (they're popular)—a faded old curtain hung by some hopeful soul long ago, now blowing in the wind. But no … it's something much more intriguing. The road led to a tattered rock curtain, hanging on an extinct volcano. And not just any volcano but a hydrovolcano, born in a lake. Water explains the curtain … and more.
Utah’s West Desert today.
After driving several hundred dry sparsely-vegetated miles through western Utah, it was hard to envision this place being deep underwater just 15,000 years ago. But it was; the evidence is clear. During the cooler wetter times of the last glacial advance, much of Utah’s West Desert was covered by giant Lake Bonneville; the “Great” Salt Lake is a pond in comparison.

In 1876, geologist Grove Karl Gilbert published an exquisite map of Lake Bonneville (click on image below). It was indicated with light blue crayon; today’s lakes were colored dark blue. [In spite of the great care that went into the map, the scale was incorrect. A corrected version, on the map below, was pasted on later.]
Restored Outline Of Lake Bonneville, Gilbert, 1876; David Rumsey Map Collection.
Gilbert and EE Howell worked on several geographical surveys with the US Army Corps of Engineers, led by Lt. GM Wheeler. Among other things, they documented location and elevation of many relic Bonneville shorelines, visible as terraces above valley bottoms. They determined the maximum height of the lake, and based on known topography, mapped it in detail. Their map even shows smaller islands—the tops of today’s hills and buttes.

In the Sevier arm of Lake Bonneville (home of today’s Sevier Lake/Playa), there were at least 15 islands, many of which were volcanic. The highest was today’s Pahvant Butte (arrow below). “It is the tallest of all the volcanic hills, and, standing alone upon the plain, is a conspicuous landmark. Its general form is that of a cratered cone, but the crater is open at the south, and the circling crest has an acute culmination at the north” (Gilbert 1890).
Pahvant Butte—an extinct hydrovolcano and former island in Lake Bonneville.
Pavant Butte, from the south. Gilbert 1890.
Pahvant Butte, from the south. May 2017.

At the recommendation of a friend, I visited Pahvant Butte in late May. It’s hard to miss, being clearly visible from a distance. The state of Utah provides directions and roadsigns to the butte and other volcanic features in the Black Rock Desert.

From the Clear Lake Bird Refuge Road, the road to Pahvant Butte heads north, with fine views of the south side of the cone with its crater, and the Bonneville and younger Provo shorelines below. Not long after Pahvant’s last eruption, Lake Bonneville rose and breached the divide into the Snake River drainage in southern Idaho. It dropped to what’s called the Provo level. “The Bonneville shore-line is trenchantly drawn about the sides of the butte at mid-height. The Provo shore-line appears at its base” (Gilbert 1890).
Lava flows in the area predate not just the cone but also Lake Bonneville.
When I reached the butte, I turned north and drove around it on a 2-track signed with a jeep-like symbol suggesting 4WD high-clearance. But it wasn't, at least not as far as the Curtain. My first stop was “underwater” at the base of the steep slope leading up to the old Bonneville beach. I like to ponder such things!
The next stop was for great views of the palagonitized tuff that forms the upper half of the butte. It was deposited after the volcano had grown above the lake surface. It’s thought that erupted material contained very hot water (1), which altered and consolidated the dark basaltic tephra (volcanic fragments) into yellow-brown palagonite. In contrast, material in the lower half of the butte was deposited underwater, cooled quickly, and was not altered (similar phenomena have been observed in Iceland; Oviatt and Nash 1989).
Unaltered black tuff deposited underwater; yellow-brown palagonitized tuff deposited above the lake surface.

On the north side of the butte, I spotted the Lace Curtain. The 2-track to the base of the wall was rough but passable.
Lace Curtain; yellow-brown palagonite above.
The Curtain is steep, suggesting the rock is hard and resistant to erosion. And yet it's quite eroded, in its own bizarre way. It's thought that in this area, mineral-rich groundwater (2) cemented volcanic deposits, though inconsistently. Then violent storm waves on Lake Bonneville beat against and eroded the north side of the butte, exposing the cemented rock. Waves continued to batter the rock, breaking it down. Inconsistent cementation explains the “lacy” pattern.
Whose abode is this?
My photographic reverie was interrupted by shrill kik-kik-kiksfrom two prairie falcons circling and diving overhead. One landed high on the Curtain and stayed for twenty minutes watching me, until I left. I had planned to camp on the old beach below the Curtain, and “watch” the sun set over Lake Bonneville. But I let the falcons have the place to themselves.
Concerned prairie falcon.

Back at home, I searched the scientific literature for information specific to the Lace Curtain. White (1996) studied only the underwater part of the Pahvant volcano; he makes no mention of the Curtain. Nor do papers about above-water deposits. Most online sources reference Case (2002):
“… intense storm waves cut a vertical cliff into the cone. The cut exposes an intricate lacy pattern caused by the partial cementing of the tuff by minerals in ground water. The cliff is known as the ‘Lace Curtain’ because of its white color and mysterious lacy pattern.” (italics added)
If you know of more information about the geology of the Lace Curtain, please add a Comment below.

Sunset over Lake Bonneville, from just south of Pahvant Butte.


(1) Experts disagree on the source of water for palagonitization. Some suggest groundwater (e.g. Case 2002), others lake water (e.g. Oviatt and Nash 1989).

(2) Cementing is sometimes attributed to Lake Bonneville water rather than groundwater. But it seems too localized to be due to lake water.

Sources (in addition to links in post)

Case, WF. 2002. Geosights: Pahvant Butte in the Black Rock Desert, Millard County, Utah. (accessed 18 June 2017).

Farrand, WH, and Singer, RB. 1991. Spectral analaysis and mapping of palagonite tuffs of Pavant Butte, Millard County, Utah. Geophysical Research Letters 18: 2237-2240.

Gilbert GK. 1890. Lake Bonneville. US Geol. Surv. Monogr. 1: 1– 438.

Oviatt, CG, and Nash, WP. 1989. Late Pleistocene basaltic ash and volcanic eruptions in the Bonneville basin, Utah. Geo. Soc. Amer. Bull. 101: 292-303.

White, JDL. 1996. Pre-emergent construction of a lacustrine basaltic volcano, Pahvant Butte, Utah (USA). Bull. Volcanol. 58: 249–262.

Tuesday, June 13, 2017

Tree of the Month: Desert “Willow”

After a short drive on a rough 2-track through creosote scrub, I parked at a dry sandy wash below the object of my desire—a young lava flow, maybe just 10,000 years old. But as soon as I got out of the car, all thoughts of geology fled. They were driven off by a huge tree, flamboyant with flowers.
Huge and flamboyant by desert standards for sure!
Based on the impressive green (not gray-green) canopy and large trumpet-shaped flowers, I concluded the tree was an escapee, an ornamental that somehow managed to get established in the wash. Such verdure and floral showiness are out of place in the desert. Later I stopped at Kelso, the old railroad depot in the middle of the Mojave, where helper engines were stationed to help trains up the steep Cima grade (more here). I immediately noticed the same trees lining the parking lot. Yes! definitely an escaped ornamental. But then a vague memory began to take shape.

The depot now houses the Visitor Center for Mojave National Preserve. In the bookstore, I read about trees in desert washes. Then it all came came back: Chilopsis linearis, the desert willow, was one of the key species of desert washes that we committed to memory forty years ago. If we saw it on our field trip to the Mojave, that memory is long gone. But I’m sure we did not see it in flower … at least not flowering in such profusion. That I would have remembered.
The desert willow is unusual in other ways. Not really "unusual" of course, but rather surprising … as I knew so little about its life. Since that day we met beside the lava flow, I’ve learned a lot.

First, desert willow is not a willow, not even close. If it were, it would be in the genus Salix in the Salicaceae (willow family). Instead, the genus is Chilopsis in the Bignoniaceae (catalpa family, mainly tropical). The common name is understandable. Desert willow is associated with stream channels (though usually dry), and the leaves look superficially like those of several familiar willows (e.g., Salix exigua, narrowleaf willow).
Chilopsis linearis is the only species in the genus, and is the only native member of the catalpa family in California (source). It’s native to south-central Texas, south to Nuevo Leon and Zacatecas, and west to southern California and Baja California.

At first glance, desert washes look just as dry as the surrounding uplands. But they’re not. After the occasional heavy rain, torrents of water rush down the wash, and quite a bit gets stored underground. This explains the heavier growth of wash-specific shrubs and trees on the banks. But desert willow often grows in the middle of washes, where it must bear the full brunt of flash floods. It can do this because it’s so well-anchored with its strong deep root system. And even in the searing heat of a Mojave summer, its roots provide enough groundwater to support a full compliment of leaves until the first hard frost.
Desert willow in mid-wash, with young lava flow and somewhat older cinder cone behind.
Desert willow is tough in other ways. Atchley and colleagues (1999) noted that water transport in trees and gas exchange in leaves (e.g., uptake of carbon dioxide for photosynthesis) change very little during times of low water availability:
“Desert willow can maintain near normal water potentials and gas-exchange rates during periods of low water availability (Odening et al., 1974; de Sozya et al., in review). This obligate riparian species appears to be highly adapted to minimizing its physiological responses to the often extreme environmental changes common to arroyo habitats.” (italics added)
Flowers typically are most abundant in May and June, with stragglers hanging on through the summer. If there’s enough late summer rain, another burst of flowers may follow. The seed pods are conspicuous, being up to ten inches long (35 cm). They stay on the tree, and split to produce abundant tiny winged seeds.
Seed pod from last year, still on tree.
Desert willow seeds (source); photo by Russ Kleinman. Scale is metric.

Turns out my first impression wasn’t totally wrong. Desert willow is used as an ornamental. In fact, many cultivars have been developed. It’s also popular for soil stabilization projects in the America Southwest.
Cultivar 'Rio Salado' at the Springs Preserve garden in Las Vegas (courtesy Stan Shebs).

This is my contribution to June’s tree-following gathering, kindly hosted by The Squirrelbasket. Obviously I’m not following a given tree. I have just about given up on the one I chose for 2017: Sabalites powellii, an extinct palm. If things had gone as planned, I would be visiting its ancient habitat today!! And you would read about it next month. But the Fickle Finger of Fate intervened, specifically the Wyoming weather. After finishing field work, I drove to the nearest town and checked the 3-day forecast for Fossil Butte National Monument: heavy rain, rain and snow with a low of 34ΒΊ F (1ΒΊ C), then more heavy rain. So I drove home. Maybe in September …

Sources (in addition to links in post)

Atchley, MC, et al. 1999. Arroyo water storage and soil nutrients and their effects on gas-exchange of shrub species in the northern Chihuahuan Desert. J. Arid Environments 43: 21–33. PDF

USDA Forest Service. Fire Effects Information System (FEIS). Chilopsis linearis. (accd 12 June 2017).
If you aren’t familiar with the FEIS database, you should be! It contains thorough compilations of plant information (though not for gardening). You’re a lucky person if your species of interest is included.

Monday, June 5, 2017

Missing View of El Huerfano (The Orphan)

On a chilly afternoon in late November, photographer Simon Nunes Carvalho stood in “the most romantic and beautiful country I ever beheld.” The high snow-covered Rocky Mountains formed the horizon to the west for as far as he could see. Nearby was Huerfano Butte (the Orphan), “which rises perpendicularly to the height of four or five hundred feet, from a perfectly level valley … like a mammoth sugar loaf.” Carvalho made several views of the butte and surrounding area, recording the images not as digital files nor even on film, but on delicate silver-plated copper sheets. Daguerreotypy was his specialty.
An artist's rendition of Huerfano Butte, from a daguerreotype by SN Carvalho (Fremont 1887).
Just two months earlier, Carvalho had been in New York City demonstrating his new protective varnish for daguerreotype images. There he met Colonel John Charles Fremont, who just happened to be looking for a daguerreotypist. Fremont invited him to join an exploratory expedition into the wilds of the American West, and Carvalho quickly agreed:
“I impulsively, without even a consultation with my family, passed my word to join an exploring party, under command of Col. Fremont, over a hitherto untrodden country, in an elevated region, with the full expectation of being exposed to all the inclemencies of an arctic winter.” (All Carvalho quotes are from the 2004 edition of his book.)
The goal of the expedition, Fremont’s fifth, was to find a railroad route across the central Rocky Mountains, an alternative promoted by his father-in-law, Senator Thomas Hart Benton. Benton and Fremont were strong abolitionists determined to deny southern pro-slavery states the benefits of a railroad. If an expedition could cross the central Rockies in winter by horseback and foot, then no one would doubt the feasibility of the route. This would be Fremont’s second attempt. The first, in the winter of 1848, was a horrible failure, with ten men dying from cold and starvation. Yet Fremont had no qualms about trying again.

The route would be documented with photographs. Though expensive, gear-intensive, time-consuming, and difficult in general, they had immense power  photos were truth! Unlike sketches and written descriptions, they weren't subject to interpretation and artistic license (then).

Fremont considered two technologies. Daguerreotypy, then at its peak, produced a positive image essentially in one step, but it was a tricky process, and the resulting image was fragile. In contrast, the newer cheaper wet-plate technology produced a negative image from which paper prints could be made as needed. There were clear advantages to the new process. Most notably, multiple copies could be printed from a single negative. But wet-plate photography required a darkroom, which would necessitate adding at least one more mule or even an entire wagon to the expedition. So Fremont opted for daguerreotypy.
Daguerreotype of Solomon Nunes Carvalho, ca. 1850 (Library of Congress).

The expedition departed St. Louis in early September, 1853. Near the end of November, they reached Huerfano Butte at the base of the Rocky Mountains, north of today’s Walsenburg, Colorado. “Col. Fremont expressed a desire to have several views of it from different distances” so Carvalho and four others stayed to make views (landscape photos), while the expedition continued west. By this time, the procedure was routine.

First they unloaded equipment and chemicals, carried on three or four mules balanced with other gear (1). This was not a trivial task; much of the time needed to make daguerreotypes was spent loading and unloading mules. When everything was ready, Carvalho carefully and thoroughly cleaned a silver-plated copper sheet, and buffed it to a mirror-like sheen. He sensitized it over iodine in a closed (dark) box, put it in a light-tight holder, and transferred it to the camera. The protective cover and then the lens cover were removed to expose the silver coating to light, creating a latent image. Carvalho had to choose an exposure time based on light level. Even a sunny day could require as much as a minute, especially in the low light of November.

The exposed plate was covered again, and moved to the box for developing. It was held over hot mercury until the image appeared. If it was satisfactory, Carvalho fixed it with an appropriate solution, such as sodium thiosulfate or salt (source). Perhaps he then applied a coat of his protective varnish.
Click on image to view details (source).

After the views had been made, they carefully repacked everything and loaded the mules. Finally they left, hurrying west to catch the expedition. But it was late November and the day was short. Soon it was too dark to continue. They camped among pines on the slope of a nearby mountain, building a huge fire for “the weather was intensely cold and disagreeable.” They had buffalo robes for bedding, but nothing to eat or drink. They went to bed hungry and thirsty—a taste of what was to come.

Carvalho would spend nearly two months making daguerreotypes in the Rockies, in situations far more demanding than the Huerfano Valley. Two years later, he would write that he “succeeded beyond my utmost expectations.” Yet by February, success was no longer certain. For weeks the men had had little to eat, sometimes going several days without food. The horses were starving. Whenever one died, it was quickly butchered, cooked and eaten. Eventually none were left, yet many of the men could barely walk. Fremont announced that all unnecessary gear would be cached so that they could ride the pack mules. Then a man died. Because the ground was frozen, burial was impossible. They could only continue on, surely wondering who would be the next to go.

Amazingly, good fortune soon intervened. That same day, the expedition stumbled upon a road and followed it into Parowan (Utah). Residents were shocked, noting that all members were in a state of starvation. Carvalho and topographer Egloffstien were the worst—sick, weak and terribly malnourished. When the rest of the expedition continued west, the two men stayed in Parowan to recover. Carvalho then traveled to Salt Lake City, where he painted portraits and took notes on Mormon life. He went next to California, living a short time in Los Angeles before returning home.

In 1856, Carvalho published a detailed account of his experiences: Incidents of Travel and Adventure in the Far West with Colonel Fremont’s Last Expedition. He was especially proud of his daguerreotypes, made “on the summits of the highest peaks of the Rocky Moiuntains … often standing to my waist in snow, buffing, coating and mercurializing plates in the open air.” How strange then that the fate of those images is unknown! None were published, though several drawings based on them appeared in Fremont’s memoirs. Carvalho’s book included no images at all.

It appears the daguerreotypes were not abandoned in the Rockies, not written off as unnecessary gear and chucked into the snow. Eventually they were transported east, perhaps with Fremont. It is said that he hired photographer Matthew Brady to make copies (wet-plate photos). Indeed, a daguerreotype of a Plains Indian village in Kansas Territory, surely Carvalho’s, is part of the Brady collection at the Library of Congress. It matches two written accounts from the expedition, by Carvalho and by muleteer James Milligan.
Daguerreotype by Carvalho of Plains Indian village in Kansas Territory (Library of Congress).
Photo of Carvalho's daguerreotype, enhanced to show details (LOC, via this source).
The rest of Carvalho’s daguerreotypes, as well as Brady’s copies, may have been destroyed in a warehouse fire, as is often mentioned. Yet the question of their fate remains open (2). In his introduction to Carvalho’s book, Kahn is noncommittal, stating only that “some believe they were stored in a warehouse and were burned in a fire” (italics added).

Perhaps good fortune will again intervene on Carvalho's behalf. And how wonderful it would be if his daguerreotypes were found scattered among private collections, or in a dusty box tucked away in some attic! Those old views could add so much to our vicarious participation in Carvalho’s amazing adventure.
“[the Huerfano Valley] is by far the most romantic and beautiful country I ever beheld. Nature seems to have, with a bountiful hand, lavished on this delightful valley all the ingredients necessary for the habitation of man … [but] will the progress of civilization ever extend so far in the interior?” – Solomon Nunes Carvalho
View east across Interstate Highway 25 at Exit 59, Butte Road (Colorado DOT).

About the Geology

Carvalho called the Orphan an “immense pile of granite rock” … which it’s not. But Carvalho was no geologist, nor was anyone on the expedition. In 1869, Ferdinand Vandeveer Hayden encountered it during a geological survey of Colorado and New Mexico. In a report later that year, he wrote:
“Huerfano Butte rises up in the midst of the plain in the valley of Huerfano Creek. The rocks are basaltic, some portions a true syenite. … It seems to me that this is a dike, thrust up before the super incumbent beds were swept away, and that the igneous material never reached the surface in a melted state. The butte is about two hundred feet high, the rocks being of a dark steel-gray color. There is no evidence that the underlying strata have been disturbed by this butte.”
From Hayden's 1869 report.
Current descriptions of Huerfano Butte vary. A recent online source calls the rock alkali-basalt. Durant (1989) refers to it as “a syenitic plug cut by a gabbroic dyke.” Penn’s 1995 description is probably the most accurate. He describes the butte as biotite olivine alkali-gabbro cut by two dikes of different compositions—biotite monzonite and alkali lamprophyre.

Whatever the rock type(s), the Orphan most likely is a hypabyssal plug—i.e., made from magma that cooled and solidified beneath the surface, though not at great depth. Erosion later removed softer sedimentary rocks to reveal a resistant plug about a hundred feet in height (not four or five hundred as Carvalho reported; he tended to exaggerate, probably because he was so excited about his adventure). It’s part of a cluster of volcanic and shallow intrusive features dating from about 25-20 million years ago, possibly associated with the opening of the Rio Grande Rift to the west and south. For more details and discussion, see Huerfano Butte, Colorado: The Orphan.


(1) In an interview, the producer of a documentary about Carvalho described the field daguerreotypy differently, even though Carvalho’s book was his main source: “It was extremely labor intensive to create these things… Carvalho had wagons and wagons, I think there were three or four or five, filled with gear.”

(2) When asked “What happened to all the other photographic plates?” the documentary producer replied, “I don’t want to give away too much. We feel like our research does provide an answer.”

Sources (in addition to links in post)

Many thanks to Mike of CSMS Geology Post for suggesting I add El Huerfano to my itinerary, and for information about its geology and history. Check out his post about Huerfano Butte.

Carvalho, SN. 2004 (originally published 1858). Incidents of travel and adventure in the Far West with Colonel Fremont’s last expedition; introduction by AF Kahn. University of Nebraska Press.

Durant, DG. 1989. Evolutionary aspects of Huerfano Butte. Honours BS Thesis, McMaster University. PDF

Fremont, JC. 1887. Memoirs of my life. Chicago and New York, Belford, Clarke & company.

Hayden, FV. 1869. Preliminary field report of the United States Geological Survey of Colorado and New Mexico. Washington: Government Printing Office.

Library of Congress. The daguerreotype medium (in Collection Overview). (accessed 4 June 2017).

Penn, BS. 1995, What’s the Scoop on Huerfano Butte? Abstract of paper presented at Rocky Mountain AGU meeting. (accessed 4 June 2017).