13 October 2015
The Deep Lake landslide in Montana
Posted by Dave Petley
The Deep Lake landslide
The KPAX website has an interesting story this morning about the Deep Lake landslide in the Beartooth Mountains of Montana, USA:
The feature is an immense heap of rock, the result of a gigantic rockslide that formed a natural dam and created Deep Lake, one of the largest bodies of water in the Beartooths…Deep Lake is remote, but it is hardly unknown and it’s no secret that it was formed by a rockslide. Apparently, however, no one before Blevins—in consultation with Paul Burley, a geologist and civil engineer—attempted to estimate the total volume of the Deep Lake Rockslide…The slide is 1,100 feet (335 metres) deep at its apex and contains nearly 500 million cubic yards (383 million cubic metres), or about 1 billion tons, of material. By comparison, the Hebgen Lake Slide along the Madison River, which was triggered by an earthquake in 1959 and created Quake Lake, is estimated at just 43 million cubic yards. The Gros Ventre Slide, 30 miles east of Jackson, Wyo., is estimated at 50 million cubic yards.
“It does show that it’s huge,” Blevins said of their research into the Deep Lake Slide. “It’s just mind-blowing. We concluded that it’s probably the biggest in North America.” The second-largest slide in North America, Blevins said, is the Hope Slide in British Columbia. It comes in at 61.5 million cubic yards. The Deep Lake Slide, which is on the southern end of the Beartooth Plateau in Park County, Wyo., is thought to have occurred 10,000 to 12,000 years ago, at the end of the period of glaciation known as the Pinedale era. The slide dammed up Littlerock Creek, which gradually pooled to form Deep Lake, which is 1.6 miles (2.6 km) long and estimated to be about 600 feet deep (183 m). There is no surface drainage; excess water percolates through the slide, emerging downhill as large springs.
This is the Google Earth image of the Deep Lake site:
The landslide is quite interesting from a morphological perspective – it is perhaps surprisingly large for terrain of this type for example. This appears to be the main source of the landslide and the deposit, again from Google Earth:
But note that other areas, on both sides of the valley, have undoubtedly contributed volume to this landslide deposit.
The article argues that this landslide deserves more detailed study. Whilst the information about the largest landslides in North America is not correct – I looked at this issue in a post about the Bingham Canyon landslide in Utah two years ago – I can only agree that this landslide is worthy of a proper investigation.
Another interesting landslide in south-central Montana that has received almost no attention is the Black Canyon Basin landslide complex in the Bighorn Mountains. This slide, composed largely of Ordovician Bighorn Dolomite, may rival the Deep Lake slide in volume. The slide flowed down Black Canyon Creek, which flows into Bighorn Lake. The description on the Peyote Point preliminary geologic map (USGS Misc Field Studies Map MF-1884) states that this landslide is up to 100 meters thick. I did a quick area estimate on this slide and came up with 3700 acres. If the slide has an average thickness of 20 meters, the volume would be about 300 million cubic meters, an admittedly back-of-the-envelope estimation.
The Black Canyon Basin landslide is on the Crow Indian Reservation at 45.15°N 107.93°W
By the way, the Deep Lake landslide is in Wyoming, not Montana. The article has it correct, but the headline does not.