Natural Arches of Kachemak Bay, Alaska
August 13, 2013
Photographer: John Ehman; John's Web site
Summary Author: John Ehman
Natural arches (or bridges) form when erosive forces exploit weaknesses in rocky outcrops. Many examples may be found on dry land, caused by wind or by stream water. However, the sea presents an especially strong force, and the effect is magnified when rock formations protrude at a right angle to the ocean current along so-called discordant coastlines. Arches are a small sign of the larger processes of erosion that create coves. Kachemak Bay in southeastern Alaska is a good example, with a coastline containing many coves and a number of arches hewn by waves and by currents from extreme tidal cycles that regularly rise and fall by as much as 28 ft (9 m). The rock here is a mixture of volcanic, sedimentary and metamorphic types. The difference in hardness between these types results in weaknesses vulnerable to erosion.
Pictured above are two arches within about a mile of one another: on Gull Island (top) and at the entrance to Halibut Cove (bottom), near Homer, Alaska. Both are roughly perpendicular to the bay current. The same forces that make such sea arches common also make them relatively short lived (e.g., the Halibut Cove image shows fallen rocks), compared with those created by slower erosive processes that occur inland. Photos were taken at midday on July 10, 2013.
Photo details: Olympus E-PL1 camera with a Panasonic G Vario 14-45mm lens; f/3.5-5.6. The Gull Island photo was taken at 31mm; f/10; 1/250 sec. exposure. The Halibut Cove photo was taken at 45mm; f/10; 1/500 sec. exposure.