Locate Mount Sentinel on the topographic map below.
The University of Montana is in Missoula, Montana, at the foot of Mount Sentinel. It is stop one on our virtual tour.
Mount Sentinel is a grassy hill located above the University of Montana’s campus. Observe the series of horizontal lines across the face of this hill. What could have caused these lines? Do you have any ideas? Notice that the lines are parallel and the spacing between the lines is not uniform. A vertical gulley lined with trees cuts through these lines. On the left side of the photo below, ledges are visible. How many can you count?
Similar lines can be seen on many of the adjacent slopes in the Missoula region, such as Mount Jumbo. Can you find Mount Jumbo on the topographic map?
The horizontal lines are shorelines left behind by ancient Glacial Lake Missoula. Waves and lake ice battered the shoreline leaving the horizontal marks. The fact that there is more than one lake shorelines is an indicator of a changing lake level over time. These shoreline markings can be seen for miles on the surrounding hills.
“Old shorelines are unique in the landscapes of the world in being perfectly horizontal and perfectly parallel”
T.C. Chamberlain, 1886
Glacial Lake Missouola held 500 cubic miles of water and covered 2,900 square miles and its highest shoreline was at an elevation of 4,150 ft. It was so deep that if it existed today, the city of Missoula would be under 950 feet of water! Because of the large size of the lake, it never completely froze over. As a glacial lake, it would have contained a great deal of slit and would have been a lovely turquoise blue.
How it fits into the big picture…
The shoreline markings are the “footprints” left behind on the surrounding hills. The lake shorelines are part of the evidence that a series of catastrophic events that took place between 10,000 and 12,000 years ago. During the most recent Ice Age the Cordilleran Ice Sheet crept down from Canada and covered northern Montana and Idaho. The Purcel Lobe of the ice sheet flowed across the Clark Fork River, creating an ice dam. The glacial melt water backed up behind forming Glacial Lake Missoula. The lake filled with water from the Clark Fork River and glacial melt water from the Cordilleran Ice Sheet.
At intervals of approximately 60 years the ice was buoyed up and the lake water under cut the ice dam, destroying it and allowing a tremendous volume of water to cascade down the canyons of what is now northern Idaho Montana and eastern Washington, Geologists have theorized that the water velocity may have traveled at speeds in excess of 60 mph. Over the next 2,000 years this event would take place at least 40 times as evidenced by the lake deposits and the down flow “scab lands,” which were scoured by the humongous volume of melt water.