High in the Medicine Bow Mountains west of Laramie, near the base of the Snowy Range, lie spectacular finely-layered rounded rock structures reminiscent of cabbage. These are stromatolites—in fact, some of the most spectacular stromatolites on Earth. They’re two billion years old, are now more than 10,000 feet above where they formed (sea level), and in some cases are true monsters (15+ ft. across!). Actually, we’re doubly lucky. Not only do we have world-famous stromatolites, there’s also a free guidebook that will lead us to the finest examples.
This is why “stromatolite” should be part of our vocabulary. The word is easy to say: “stroe MAT toe light.” And its Greek roots are appropriate: “stroma” means layer, “lithos” means rock. Stromatolites consist of thin layers of sediment cemented with calcium carbonate (the mineral that comprises seashells)—and thereby turned to rock.
It was geologist Ernst Kalkowsky who coined the term in 1908, for some unusual rock structures he discovered in the Harz Mountains of Germany. “The new term ‘Stromatolite’ is proposed for limestones with unique organization and structures … a fine, more or less even layered fabric,” he wrote. Furthermore the thin layers were arched, forming domes and mounds (usually sediments are deposited in horizontal layers). Kalkowsky suggested that stromatolites were biological—made by primitive organisms.
OUR “ALGAL (?) DOMES”
In the summer of 1917, geologist Eliot Blackwelder, then at the University of Illinois, and a student assistant, spent six weeks in the Medicine Bow Mountains studying the ancient, Precambrian rocks of the high central part. A young local geologist, S.H. Knight, son of Wyoming state geologist, Wilber C. Knight, joined them for part of the season. Blackwelder noted that though the Knights had visited the range on multiple occasions, “no detailed work seems to have been attempted and the results of such examinations do not appear to have been published.” That would change, but only after fifty years had passed.
After another field season, in 1925, Blackwelder prepared what would become a classic paper on the Precambrian geology of the Medicine Bow Mountains, published in 1926. He devoted two pages to what he considered some of the most interesting features in the range:
“There are many concentric domes, or globes, crowded together and all resting on a common base. These domes range in size from an inch or more to as much as 10 feet in radius. Their gross anatomy is generally emphasized by alternations of [dark and light layers], of which the latter weather out in relief. A close study of these features strongly suggests that they have been built by colonial organisms, such as calcareous algae.”
Blackwelder sent photographs to esteemed paleontologist Charles Walcott of the Smithsonian Institution, who concluded that the odd structures were biological, probably of algal origin. But in the absence of hard evidence, Blackwelder called them “algal (?) domes” in his paper.
His caution was understandable. In Blackwelder’s day and for decades after, it was thought that stromatolites had disappeared long ago, after organisms evolved that ate the algal builders. Therefore any explanation would have to be speculative. Though common as fossils, no modern-day stromatolites were available to illustrate how the ancient ones formed.
In 1956, oil company employees stumbled upon an odd sight on the west coast of Australia. In the warm super-salty water of a shallow bay stood many finely layered rock domes about three feet tall—“living” stromatolites! More have been found since then, usually in extreme habitats such as hypersaline lakes, atoll lagoons, and hydrothermal vents (including Yellowstone). Now it’s possible to study how stromatolites form.
First though, let’s be clear: stromatolites themselves are not alive. They are structures built by resident organisms and the accumulation of sediment. Ancient ones, like those in the Medicine Bow Mountains, are fossilized stromatolites, with little or no organic material remaining.
The most abundant inhabitants of modern-day stromatolites are bacteria, specifically cyanobacteria. Like plants, they photosynthesize—convert sunshine to energy. Cyanobacteria are quite primitive, and were among the earliest forms of life on Earth. These were the builders of the Medicine Bow stromatolites.
Stromatolite construction begins when cyanobacteria form a thin sticky mat—a biofilm—on the floor of a shallow body of water. Sand and silt periodically wash in and settle on the floor, covering the biofilm. When the sediment gets too thick for sunlight to penetrate, the cyanobacteria move up to colonize a new surface.
This happens repeatedly, producing a stack of thin alternating layers: biological, sedimentary, biological, sedimentary, etc. Cyanobacteria occupy the upper surfaces, basking in sunshine. Meanwhile, the layers are cemented with calcium carbonate, either from the water or the bacteria (or maybe both, we’re still learning). Without this cement, stromatolites wouldn’t survive a hundred years, much less two billion.
WYOMING BY THE SEA
Two billion years ago, Wyoming was located on the coast of a much smaller North America. Our stromatolites stood in warm shallow saltwater; in the absence of predators, resident cyanobacteria flourished in the sunshine. But these good times were not to last.
About 1.75 billion years ago, the coast was subjected to continental collision—goodbye ocean!—accompanied by mountain-building, which tilted the horizontal stromatolite-bearing rock layers to near vertical. Then 60 million years ago, there was more deformation when the Medicine Bow Mountains were pushed up during creation of the Rocky Mountains. This was followed by tens of millions of years of erosion culminating in scraping, planing, and polishing by glacial ice until just recently (about 11,000 years ago).
Now these relics of a subtropical ocean lie far from any coast, over ten thousand feet above sea level, and covered in snow for half the year—a graphic example of the dramatic change possible with the immensity of geologic time.
Actually, given all that has happened, it’s quite remarkable that the stromatolites of the Medicine Bows have survived. That they remain largely intact and recognizable is a stroke of good fortune for us— we can stroll among ancient stromatolites, ponder the passage of two billion years, and imagine a very different world. But to find this world, we need a guide.
NOW YOU CAN SEE FOR YOURSELF
In the mid 1960s, fifty years after he worked with Eliot Blackwelder in the Medicine Bow Mountains, Samuel H. “Doc” Knight—by then a highly-respected geologist, beloved teacher, and professor emeritus at the University of Wyoming (UW)—finally found time to study the local stromatolites:
“The writer has visited many of the stromatolite occurrences with student groups during
the past forty years. Only recently, upon his retirement, did the opportunity present
itself to undertake a detailed study of the occurrence and character of these stromatolites.”
Knight spent six months in the field, over the course of three seasons, assisted by his grandson David Keefer. They mapped, measured, described, drew, and photographed 150 stromatolite-bearing outcrops. Knight’s pioneering paper, Precambrian stromatolites, bioherms and reefs in the lower half of the Nash Formation (1968), drew international attention. As a result, the UW Department of Geology and Geophysics and the UW Geological Museum fielded numerous inquiries: “Where are the stromatolites in the Medicine Bow Mountains? How do we get there?” After forty years of this, it was clear that an updated publication was needed.
Professors Donald W. Boyd of UW (who was introduced to the Medicine Bow stromatolites by Doc Knight, in 1956), and David R. Lageson of Montana State University devoted themselves to the task. They relocated, photographed, and took GPS waypoints for many of the stromatolites described by Knight, and developed a walking tour that includes some of the best examples. Their Self-guided walking tour of Paleoproterozoic stromatolites in the Medicine Bow Mountains—“an illustrated field guide to some of the world’s best stromatolite outcrops”—was published in 2014 by the Wyoming State Geological Survey. It’s available free online (https://www.wsgs.wyo.gov/products/wsgs-2014-pic-45.pdf).
The guide includes information about stromatolites in general, as well as those of the Medicine Bows. This is followed by the walking tour, and directions to other sites. While much of the text is technical, the introductory material, map, photos, and GPS coordinates will be useful to anyone who wants to stroll among our stromatolites, and contemplate the immense changes in the Earth that they represent.
The most accessible stromatolites lie adjacent to the road into the Sugarloaf Recreation Area, just a half-mile from the Snowy Range Road (Wyoming 130). The cabbage-like structures are obvious (left photo, with Professor Boyd). It’s easiest to park at the Sugarloaf Picnic Area (pass required), and then stroll a quarter mile back to the site. For details, see “STOP #10” in the guidebook.
Those who are more adventurous will want to do the walking tour, which includes the famous Big Daddy at Prospector Lake, and the wonderful Valley of Stromatolites (right photo). The tour traverses an old road, overgrown in places, and includes several cross-country options. Distance is roughly 1.5-2.0 miles, depending on options. Follow the guidebook’s detailed directions carefully, and refer to the excellent photos. For reassurance, bring a GPS-capable device. The tour starts off Forest Road 332 (dirt), which is rough and rocky in places. Snow can be late in melting. For conditions, check with the Forest Service office in Laramie.
By Hollis Marriott
Caption: Arrow points to famous 15-ft long compound stromatolite in the Valley of Stromatolites. Original orientation inferred from arching layers—“top” marks youngest growth surface. Blackwelder sent a photo of this “large dome-shaped structure” to Walcott at the Smithsonian, and included it in his 1926 paper. Photo courtesy Hollis Marriott.
Caption: Guidebook author Don Boyd kneels by stromatolites along Sugarloaf Recreation Area Road. Having been tilted 90º, they’re now seen in cross-section, as “cabbages.” Arrow shows original orientation—youngest growth surface on the right. Photo courtesy Wyoming State Geological Survey.