Bluff Point Landslide, Kachemak Bay

Bluff Point Landslide, Kachemak Bay

by | Aug 5, 2019

Bluff Point is a conspicuous headland on the Kenai Peninsula at the north entrance to Kachemak Bay, about 5 miles (8 km) northwest of Homer, Alaska. The headland was named in 1880 by William Healey Dall of the U.S. Coast and Geodetic Survey.

The Bluff Point headwall is the result of an ancient landslide that occurred about 2,250 years ago, probably triggered by a large subduction zone earthquake. The landslide is the largest slope failure along the southwestern shore of the Kenai Peninsula. The irregular 300 to 600 feet (100-215 m) high headwall extends for 3.4 miles (5.5 km) in a northwest-southeast direction roughly parallel to the coastline. Depth soundings indicate that the body of the landslide, measured perpendicular to the coastline, maybe 1 to 1.2 miles (1.7 to 2.0 km) long. The composition of the deposit indicates that the landslide resulted from failure of the moderately to weakly consolidated mudstone and sandstone of the Kenai Formation. The initial failure probably occurred when the late Wisconsinan glacier that once filled Kachemak Bay receded leaving the slope unsupported. Subsequent slumping may have been caused by progressive wave erosion at the base of the high sea cliff and seismic shaking, although no additional slumps were identified following the March 27, 1964, Alaska earthquake.

In July 2009, between Bluff Point and Diamond Creek, the high bluff face collapsed onto the upper end of an old slide block, rotating the block and pushing up an unusual mound about 15 feet (4.5 m) high into the intertidal zone for a distance of 1200 feet (366 m) along the beach. Rotated shale layers in the mound center were nearly vertical. Over the following winter, the mound was completely removed by wave erosion. Read more here and here. Explore more of the Bluff Point landslide here:

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