Fortification Bluff is a stratified sedimentary cliff over 3 miles (4.8 km) long that rises nearly vertically to 1,200 feet (366 m), on the south face of Step Mountain, about 16 miles (26 km) northwest of Augustine Island, in Kamishak Bay, Alaska. The bluffs were named in 1914 by the U.S. Coast and Geodetic Survey for the palisade of rock and imposing fortress-like appearance.
Kamishak Bay is 30 miles (48 km) wide and on the southwest shore of Cook Inlet. The name is a transliteration from the Native name by early Russians and first published by Gavril Sarychev in 1826 as “Kamyshatskaya Bay”. Navigation charts in 1794 showed the name “Bourdieus Bay” when Captain George Vancouver explored the area, but since the name is not mentioned in the narrative of the voyage, it may have originated with George Dixon. Dixon and Vancouver were both on Captain Cook’s third voyage in 1778 on the HMS Resolution. Dixon realized the commercial possibilities along the northwest coast of America and later became a partner in King George’s Sound Company to develop fur trade in British Columbia and Alaska. In September 1785, Dixon and fellow trader Nathaniel Portlock departed England with two ships on a three-year voyage. In the summers of 1786 and 1787, Dixon explored the shores of present-day British Columbia and southern Alaska including the Dixon Entrance of Cook Inlet.
The rock of Fortification Bluff is mostly sandstone and siltstone of the Jurassic Naknek Formation. Alaska is a collage of imported rocks called terranes. Terranes are pieces of the earth’s crust that were transported on different tectonic plates and accreted together. Most of Alaska consists of pieces of plates that arrived from elsewhere. For example, the Alaska Peninsula is underlain by the Peninsular terrane, which is thought to have originated far south in the Pacific Ocean. The Bruin Bay fault uplifted the rocks of this terrane, which were subsequently eroded and deposited into an adjacent basin, and then eventually lithified forming the Naknek Formation. Read more here and here. Explore more of Fortification Bluffs here: