Thursday, September 1, 2016

Stromatolite Pilgrimage

Stromatolite Hike, my seventh post, was the one that got me hooked on blogging. It was the first one I was pleased with. It told a good story, I liked the photos, and putting it together cemented knowledge and memories firmly in my head. When I tossed it out into the blogosphere, somehow 40 readers found it ... that same day. “Amazing!” I thought. What a great way to share my adventures and enthusiasm for natural history.
“This is what I love about geology, the opportunity to look far into the past and unravel a tiny bit of the surprising mysteries around us.” -- October 7, 2011
Five years later, I’m still hooked on blogging and looking into the far distant past. In celebration, here’s another stromatolite hike—actually, more like a pilgrimage.

“For my knowledge and appreciation of the stromatolites of the Nash Formation, I thank one of the great legends of Wyoming geology, Dr. Samuel H. Knight. It was because of his “Precambrian stromatolites, bioherms and reefs in the lower half of the Nash Formation, Medicine Bow Mountains” (1968) that we were able to hike along Precambrian reefs, for Doc Knight drew a detailed map of reef locations (above, click on image to view).”
Samuel H. Knight, University geologist, a full century ago (source).
Doc Knight joined the University of Wyoming faculty in 1916. By the time he retired fifty years later, he had taught on the order of 10,000 students, and was recognized as “Mr. Geology of Wyoming.” It was only then that he found time to study the stromatolites of the Medicine Bow Mountains. He visited, mapped, and drew them all. Now when we hike among stromatolites, we are following in the footsteps of a legend.
Doc Knight in 1965, with phyllite and stromatolites (source).
The stromatolites in the Medicine Bow Mountains are not obscure fossils requiring expertise to find. In fact, they’re downright conspicuous—if you know where to look. Fortunately, the Wyoming State Geological Survey published a guidebook with a map, aerial imagery, GPS waypoints and photos, as well as in-depth discussions of stromatolite morphology, paleo-environments and origins (Boyd & Lageson 2014). Now—with a little imagination—it’s easy to enjoy the warm marine waters that lay just off Wyoming two billion years ago.
Before taking the tour, I plotted the tour stops in Google Earth.
Two billion years ago (Early Proterozoic) Wyoming was on the southeast coast of a young small North America. Immediately offshore was a calm warm shallow sea underlain by limestone or dolomite. It was home to one of the dominant life forms of the day—cyanobacteria. These primitive single-celled photosynthetic organisms are best known today for their slimy green biofilms, but two billion years ago, when there were no higher critters around to eat them, they were more abundant and widespread. They built spectacular structures out of sand, mud and slime—stromatolites.
Now stromatolites are restricted to harsh environments where predators can’t survive, for example the hypersaline waters of Hamlin Pool in Shark Bay, Western Australia (source).
Whenever sand and mud washed into the sea and buried existing mats, the cyanobacteria moved up through the debris and made new mats … over and over. The soft fragile organisms were rarely preserved, but cemented layers of sand and mud remained and were eventually fossilized—turned to rock. Now ancient stromatolites lie scattered high in the Medicine Bow Mountains, at 11,000 feet above sea level.

The stromatolites are part of the Nash Fork Formation—a 2 km thick collection of tan dolomite and stromatolitic dolomite with thick beds of argillite and phyllite (mud turned to rock and metamorphosed). Even though these rocks are almost two billion years old, and have been through several episodes of mountain building and deformation, metamorphism was low-grade so sedimentary features are preserved.
Orange lichen highlights the laminar structure of fossilized stromatolites (pen points to original base).
These stromatolites are just before and upslope from Stop #1. The Snowy Range in the distance is a beach turned on its side.

At Stop #2, stromatolites lie between beds of phyllite (metamorphosed slate) with thin layers of dolomite.
Sediments were tilted to vertical with continental collision (more info here).

Stop #2A is optional. But it’s only a quarter mile away, and it would be a real shame to miss the huge glacially-polished stromatolite that sticks out into Prospector Lake. It’s about 15 feet across, and flat enough for a lunch stop. 
This famous stromatolite has been “affectionately named Big Daddy” by Boyd and Lageson.

Big Daddy is just one of the many large stromatolites found locally—“true giants in the world of stromatolites!” according to the guidebook authors. For more monsters, continue on to Stop #3, the Valley of Stromatolites.
Entering the Valley of Stromatolites (looking south).
Doc Knight’s map of the Valley of Stromatolites. He used a 4-foot protractor to accurately measure the arching layers (see Knight 1968 for details).
Some of the stromatolites were obviously deformed before they became fossilized. “… we became convinced that significant aspects of the present morphology of Nash Fork stromatolites are products of post-depositional processes.” This is the main thesis presented by Boyd and Lageson; see their guidebook for details.
Note elongated stromatolite in center right of photo. Perhaps it was compressed before being turned to rock (original top is to right).
Many stromatolites exhibit what Doc Knight called a “digitate growth pattern”—layers of small pillars between laminae.
Close-up of glacially-polished digitate stromatolite, with alpine avens (Geum rossii).
The photogenic monster below is probably the most famous of our stromatolites. It “has been figured in many publications” … understandably!
Original top is on the right.
View of opposite side.

The most accessible stop is #10—a low outcrop of small linked stromatolites next to the Lewis Lake Road. Pullouts were added on either side so geologists wouldn’t block traffic.
Looking upslope from road; original top is to the right.
But why do domal structures give way to flat layers? Always so much to ponder!

The 2014 guidebook is immensely helpful for finding stromatolites, and the introductory material is clear and interesting. The rest of the text is often deep and technical, especially the discussions of post-depositional processes and stromatolite morphology. But you don’t need this information to enjoy the tour. Just follow the map and directions and GPS points, and imagine yourself wading through warm water among castles of cemented sand, silt and slime … “look far into the past and unravel a tiny bit of the surprising mysteries around us.”
Water color painting of a stromatolite, by SH Knight. Doc Knight was an artist and a poet, as well as a pioneering geologist and beloved teacher.

Sources (in addition to links in post)

Boyd, DW, and Lageson, DR. 2014. Self-guided walking tour of Paleoproterozoic stromatolites in the Medicine Bow Mountains, Wyoming: Wyoming State Geological Survey Public Information Circular No. 45, 26 p. (free download)

Currie, M. 1999. The wonderful world of stromatolites. Hooper Virtual Natural History Museum of Carleton College [lots of information!].

Knight, SH. 1968. Precambrian stromatolites, bioherms and reefs in the lower half of the Nash Formation, Medicine Bow Mountains, Wyoming. Contributions to Geology, University of Wyoming 7:73–116.


  1. great pics Hollis! found myself reminiscing of my fields trips to central India and south Indian Proterozoic terrains..

  2. Back in Syracuse, New York (live in Boston suburb nowadays), we have lots of stromatolites entombed in early Paleozoic dolostones. In regards to your comment about their varying morphology, I suspect that the domal and tabular forms might be related to the onshore energy system that affected their architecture and not a specific cyanobacterial class. Another possibility might be a relationship to the degree and type of clastic infux. My observations are that domal forms for example are not in a locality with tabular forms and vice versa. I wonder if any studies are out there relating morphology to tidal levels and proximity to shore.

    1. Thanks for the interesting comment, Jack! Boyd and Lageson go into a lot of detail in their Medicine Bow stromatolite tour. We have a range of forms--from tabular to domal. They suspect some of the "domes" are the result of later deformation. This was not addressed in Knight's early work. The Nash Fork formation is 2 km thick, so I imagine there was time for all kinds of environmental variation (and the stromatolitic dolomite is interbedded with dolomite and phyllite).

      I also learned that microbes besides cyanobacteria may have been involved as well (but still thought to be prokaryotes). As usual ... the plot thickens as one looks closely ;-)

  3. There is always a lot to learn from your posts! Must try to know more about rocks :)

    1. Thanks for reading, Gabriela ... and for the comment! I appreciate the encouragement.