Ice Age Floods National Geologic Trail
Ice Age Floods National Geologic Trail
The hillsides of Mount Sentinel and Mount Jumbo flanking downtown Missoula are striped with horizontal lines that look like contour marks drawn by a surveyor -- but they were drawn by waves. These are the strand lines of Glacial Lake Missoula, ancient shorelines carved by wave...
The hillsides of Mount Sentinel and Mount Jumbo flanking downtown Missoula are striped with horizontal lines that look like contour marks drawn by a surveyor -- but they were drawn by waves. These are the strand lines of Glacial Lake Missoula, ancient shorelines carved by wave action as the enormous lake repeatedly filled and drained over roughly 2,500 years. The highest strand lines sit at approximately 4,200 feet elevation, marking the maximum depth of a lake that held 500 cubic miles of water -- as much as Lake Erie and Lake Ontario combined -- and stretched over 3,000 square miles across western Montana. The multiple, parallel lines record different lake levels from successive fill-and-drain cycles: as the ice age waned, each new ice dam that reformed was likely smaller than the last, so each successive lake was shallower, leaving its strand line lower on the hillside. The lines are most visible when dusted with light snow or in low evening light, and public hiking trails on both Mount Sentinel (the 'M' trail) and Mount Jumbo (the 'L' trail) switchback through them. Stand in downtown Missoula, look at the mountains on either side of Hellgate Canyon, and you can see the bathtub ring of the largest lake in the American West.
Visible from public viewpoints around Missoula, Montana. The most accessible strandline-marking trails are the "M" trail on Mount Sentinel (steep 0.75-mile climb to the white concrete M, with strandlines visible above and below) and the Mount Jumbo Saddle Trail (off Upper Lincoln Hills Drive). Both are free; parking varies. Best viewed from a distance in low-angle morning or evening light.
The strandlines are roughly 30 horizontal benches etched into the hillsides above Missoula at elevations between ~3,200 and ~4,200 feet — wave-cut shorelines from successive high stands of Glacial Lake Missoula. The lake formed each time the Purcell Trench ice dam reformed and impounded the Clark Fork drainage; at maximum stand (~4,200 feet), the lake was over 2,000 feet deep above modern Missoula, larger than Lakes Erie and Ontario combined. Mount Sentinel (5,158 ft) and Mount Jumbo (4,768 ft) projected as islands above the lake's surface. The number and spacing of strandlines record the lake's repeated fill-and-drain cycles between roughly 18.2 and 14 ka. Pardee (1942) recognized these as glacial-lake shorelines, providing the lake-side evidence that complemented Bretz's downstream flood evidence.
Hanson et al. (2017, "New Interpretations of Old Strandlines") and Smith et al. (2018) revisited the Missoula strandlines using LiDAR and reinterpreted the number of fill-and-drain cycles, suggesting fewer (perhaps 25–30) but larger fills than the 40+ previously assumed. This work is consistent with Balbas et al. (2017) flood chronology and continues to be discussed in the Montana Natural History Center's Cabin Fever Lecture series.
The Glacial Lake Missoula Chapter is headquartered here and operates the strandline interpretive program in coordination with the Montana Natural History Center. NPS interpretive signage is in place at multiple trailheads.
Best March–November. Mid- to late afternoon side light reveals the benches best; flat midday light flattens them. The Montana Natural History Center (below) is the indoor companion stop.
Every site along the trail will receive the full Terrain360 capture treatment: ground-level 360° panoramas, drone aerial imagery, and photogrammetry-based 3D models that visitors can spin in their browser. This page reserves the slots; the imagery flows in as field capture completes.
Ground-level 360° panorama, every step along the feature, captured by Terrain360 field crews.
Drone flyovers reveal the geometry of catastrophe — ripple marks, gravel bars, and scour patterns invisible from the ground.
Photogrammetry and Gaussian-splat models let visitors rotate, measure, and inspect features in detail-page WebGL viewers.
Ice Age Floods National Geologic Trail
Imagine you are standing on the edge of glacial Lake Missoula 15,000 years ago. You can hear lapping waves cutting benches known as “strand-lines” into the shoreline. Today, you can see these huge strand-lines on hills surrounding Missoula, Montana, marking changes in lake level over time.
On Mount Sentinel, marked with an “M”, and Mount Jumbo, marked with an “L”, the strand-lines are seen as horizontal lines in the vegetation or highlighted by snow in the winter. Public hiking trails switchback through the strand-lines on Mount Sentinel and Mount Jumbo.
Ancient shorelines or strand-lines of Glacial Lake Missoula are visible as perfectly parallel horizontal benches on hillside slopes around Missoula. They are most visible with light snow cover, or in low evening light on the mountains marked with an”M” and an “L” on either side of Hellgate Canyon looking east from the downtown.
These shoreline benches cut by wave action in the lake recorded various lake levels as the ice dam blocking the Clark Fork River far upstream on the Idaho border repeatedly failed and refilled 40 times or more.
It may be that as the ice-age waned each successive ice dam that reformed was smaller and failed under less pressure from a lower lake level than the one before, leaving behind its bench as a record of the successively lower ancient lake shorelines. It’s also possible they record winter still-stands in rising lake levels over time.
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