Madison River Landslide, Montana
One of the most interesting spots I visited this summer was the Madison River landslide area, between Hebgen Lake and Ennis, Montana. Here's a photograph of the landslide scar:
Google Map of the area:
My "Northern Rocky Mountain Geology" class (through the MSSE program) visited the area this summer. Here's the class (all science educators of one sort or another) walking up to the viewing platform:
What happened here? On August 17, 1958, a large earthquake ~10 miles to the east occurred. Known as the Hebgen Lake earthquake, it was a magnitude 7.5 on the Richter Scale, and shook much of the northern Rocky Mountain area. The earthquake's effects were most deadly where the Madison River drops down out of the mountains and into a graben to the west. There, schistose bedrock with a plane of foliation that dipped steeply into the valley was jarred loose. Sliding along the foliation's plane of weakness, and unthinkably massive amount of rock ( estimated at 70 to 80 million tons) went downhill, crushing a forest service campground and damming the Madison River. The momentum carried the rocky debris up the other side of the valley, where the Visitor Center is located today. There are some huuuuuuuge boulders there, as big as a house. 28 people lost their lives in the landslide (and related smaller-scale rockfalls further up the valley).
The Madison River began to back up behind the new dam, and it formed a "quake lake" called Quake Lake (sometimes called "Earthquake Lake," as in the Google Map above). The U.S. Army Corps of Engineers was worried that the dam would fail, draining Quake Lake rapidly and causing a catastrophic flood downstream. (They were cognizant that this had happened at the Gros Ventre landslide several decades earlier in nearby Wyoming.) So they bulldozed open a spillway, and got the lake level down to where they felt it didn't pose a huge flood risk for Ennis and other downstream communities.
But they didn't get the water back down to pre-landslide levels. Today, you can see a drowned forest along the shores of Quake Lake:
We also visited a couple of exposures of the Hebgen Fault scarp. Here's one at a campsite in the Gallatin National Forest. You can see the big dirty slope in the background: that's the actual fault scarp. Total offset here is something like 2.5 meters.
Another view of the fault scarp, looking along its trace.
Clever wayside sign, mimicing the offset in the land with offset in the sign:
Google Map of the area:
My "Northern Rocky Mountain Geology" class (through the MSSE program) visited the area this summer. Here's the class (all science educators of one sort or another) walking up to the viewing platform:
What happened here? On August 17, 1958, a large earthquake ~10 miles to the east occurred. Known as the Hebgen Lake earthquake, it was a magnitude 7.5 on the Richter Scale, and shook much of the northern Rocky Mountain area. The earthquake's effects were most deadly where the Madison River drops down out of the mountains and into a graben to the west. There, schistose bedrock with a plane of foliation that dipped steeply into the valley was jarred loose. Sliding along the foliation's plane of weakness, and unthinkably massive amount of rock ( estimated at 70 to 80 million tons) went downhill, crushing a forest service campground and damming the Madison River. The momentum carried the rocky debris up the other side of the valley, where the Visitor Center is located today. There are some huuuuuuuge boulders there, as big as a house. 28 people lost their lives in the landslide (and related smaller-scale rockfalls further up the valley).
The Madison River began to back up behind the new dam, and it formed a "quake lake" called Quake Lake (sometimes called "Earthquake Lake," as in the Google Map above). The U.S. Army Corps of Engineers was worried that the dam would fail, draining Quake Lake rapidly and causing a catastrophic flood downstream. (They were cognizant that this had happened at the Gros Ventre landslide several decades earlier in nearby Wyoming.) So they bulldozed open a spillway, and got the lake level down to where they felt it didn't pose a huge flood risk for Ennis and other downstream communities.
But they didn't get the water back down to pre-landslide levels. Today, you can see a drowned forest along the shores of Quake Lake:
We also visited a couple of exposures of the Hebgen Fault scarp. Here's one at a campsite in the Gallatin National Forest. You can see the big dirty slope in the background: that's the actual fault scarp. Total offset here is something like 2.5 meters.
Another view of the fault scarp, looking along its trace.
Clever wayside sign, mimicing the offset in the land with offset in the sign:
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