Montana Earth Science Picture of the Week

Fossils Revealed As Glacier Melts Away

Photo Courtesy of Callan Bentley, Assistant Professor of Geology
Northern Virginia Community College

Geologist Callan Bentley lies on the surface of the Grinnell Glacier cirque within several hundred feet of the Continental Divide in this 2007 photo. The strange circular shapes are ancient sea creatures called stromatolites, which until recently, were covered with ice. (photo)

Cabbage Heads . . .
The photo to the right was taken just few miles away from the top photo, on the opposite side of the divide along the Going-to-the-Sun Road in Glacier Park. Although this location is approximately 5,000 feet above sea level, it contains fossils of algae colonies. that lived in a shallow sea .8 to 1.6 billion years ago. The algae 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). CLICK HERE to see a photo of modern stromatolites in Shark Bay, Australia.

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: carbonate (rock)


*Plate Tectonics (USGS site)
Callan Bentley's Web Site (cool travel photos)
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By Rod Benson
Earth Science Teacher at Helena High School

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