Montana Earth Science Picture of the Week

Cabbage Heads

Going to the Sun . . .
This photo was taken along the “Going-to-the-Sun Road” in Glacier National Park about 2 miles from the Continental Divide (as the crow flies). Although this rock formation is approximately 5,000 feet above sea level, it contains fossils of algae colonies that lived in a shallow sea .8 – 1.6 billion years ago. The algae fossilized here probably formed in an environment similar to what exists in the Florida Keys today. The fossil forms of these algae, called “stromatolites”, have shapes and internal structures similar to the blue green algae that live in present-day seas. The outcrop shown in the photo contains excellent examples of algae colonies that resemble heads of cabbage (see close-up below).

Gases and Rocks . . .
Algae such as these would have taken carbon dioxide from seawater and released oxygen as a waste product of photosynthesis. The algae were a major factor in producing an oxygen-rich atmosphere, and their removal of carbon dioxide caused the formation of large quantities of calcium carbonate. This contributed to the formation of great thicknesses of carbonate rocks in the park.

Blame it on Plate Tectonics . . .
So how did these sea-dwelling organisms end up on a mountaintop in Montana? . . . Blame it on “plate tectonics.” About 100 million years ago, massive segments of Earth’s crust (crustal plates) moved eastward from the area of the Pacific Ocean, pushing into the western edge of North America. This caused the rocks containing the “cabbage heads” to rise from sea level, forming huge mountains such as those found in this part of Montana. A similar process is happening today in the Himalayas where fossils of ancient sea creatures can be found among the world’s highest mountains.

Source: Geology Along Going-to-the-Sun Road, Glacier National Park, Montana, by Omer B. Raup, Robert L, Earhart, James W. Whipple, and Paula E. Carrara: prepared by the USGS in cooperation with the National Park Service, Published by Glacier National Park Historical Association in 1983

Terms: algae, carbonate, photosynthesis

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	*Plate Tectonics (USGS site)
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By Rod Benson
Earth Science Teacher at Helena High School

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