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By Rebecca K. Hunt-Foster, paleontologist at Dinosaur National Monument, for the National Park Service's Park Paleontology newsletter, fall 2018.
A Siyeh Formation bioherm along the Going-to-the-Sun Road below the Loop. NPS photo/Rebecca Hunt-Foster.
The rugged peaks of Glacier National Park draw visitors from around the world. The rocks that form them record a time about a billion and a half years ago when a vast, shallow sea covered the area. It looked more like today's Bahamas, with clear water holding some of the most primitive life on Earth: cyanobacteria, or blue-green algae, preserved in great abundance.
What stromatolites are
Stromatolites are fossil mounds of sediment and cyanobacteria. They form when sediment is trapped and the algae grow over and through it, building alternating layers of algal mat and sediment. These often grow into mounds and columns, and clusters of them make a kind of reef called a bioherm. They grew in shallow, clear, quiet water, and with no grazing animals around, they were a large, dominant form of life.
Some rocks in the park hold ripped-up chunks of fossil algal mat, probably torn loose by powerful storms and dropped back on the seafloor. Over time the fossils were buried, turned to rock, and lay hidden for hundreds of millions of years until they were thrust up some 70–80 million years ago as the Rocky Mountains formed. Recent glaciers, which carved the park's landscape, helped expose them. A careful eye can spot stromatolites along much of the western Going-to-the-Sun Road, at Logan Pass, at the foot of Grinnell Glacier and elsewhere.
Formation by formation
| Formation | Age or setting | Stromatolites |
|---|---|---|
| Altyn Limestone (upper) | 1.35–1.45 billion years old | among the park's oldest, near Apikuni Falls, first described by Carroll and Mildred Fenton in 1931. Baicalia, branching columns from near-shore subtidal areas, and Collenia columnaris (also called C. frequens), long, strongly tilted columns from quiet water farther offshore, both in tightly packed masses |
| Appekunny and Grinnell | bright red and green claystones (argillites) seen in almost every stream and lake | rare and small: one known occurrence in the Appekunny, three in the Grinnell, whose mounds are much smaller than elsewhere. Sediment washing off the land likely clouded the water, and cyanobacteria need clear water for photosynthesis |
| Siyeh (also called Helena) | 1.1-billion-year-old limestone, the park's best-exposed unit | abundant mound, conical and dome-shaped stromatolites and rip-up clasts, often in bioherms ("zones" or "cycles") traceable for miles. One cycle, in six units, holds 70% of the stromatolite volume in the formation |
| Snowslip | argillites, siltstones and sandstone from tidal flats covered at high tide or bared at low tide | "pseudocolumnar" and mound-shaped forms at five locations, often in alternating green and pink layers |
| Shepard | dolomite, siltstone, argillite and quartzite at high elevations | mound and columnar forms, notably mounds on Reynolds Mountain |
A Snowslip Formation stromatolite along Highway 2, with the alternating red and green layers typical of the formation. NPS photo/Rebecca Hunt-Foster.
Where to see the Siyeh bioherms: along the Going-to-the-Sun Road by the Garden Wall and up the west flank of Going-to-the-Sun Mountain, in the Saint Mary Valley, Two Medicine and Many Glacier areas, at the foot of Grinnell Glacier, on the Highline Trail to Granite Park Chalet, under and around the Logan Pass Visitor Center, and near Hole-in-the-Wall.
A Siyeh Formation bioherm on the Highline Trail. NPS photo/Rebecca Hunt-Foster.
A Snowslip Formation stromatolite high in the Lunch Creek area. NPS photo/Rebecca Hunt-Foster.
Why they matter
Glacier's variety of stromatolites is one of the richest collections of Precambrian life in the United States. The fossils tell the park's deep history, show how drastically the environment has changed, and are among the oldest fossils preserved in any National Park Service unit.
Siyeh Formation bioherm, Highline Trail. NPS photo/Rebecca Hunt-Foster.
Sources
- Rebecca K. Hunt-Foster, "The Stromatolites of Glacier National Park," Park Paleontology News 10(2), fall 2018, National Park Service: https://www.nps.gov/articles/park-paleo-fall-2018-stromatolites.htm
In these publicationsGlacier National Park
Licence: CC0 1.0 (public domain) · Adapted from www.nps.gov
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