Alaska
Like many glaciers in Alaska, Bear Glacier is retreating rapidly as a consequence of climate change. Here the glacier and the lagoon are pictured in 2002 (left) and 2007 (right).
An impressive sight from any angle but especially from high overhead, Bear Glacier is the eastern gateway to Kenai Fjords National Park in southern Alaska. From the perspective seen here, the river of ice has flowed downhill for thirteen miles to its terminus in the Bear Glacier Lagoon, a bright blue lake of glacial meltwater separated from the open water of Resurrection Bay in the Gulf of Alaska by a terminal moraine, or ridge of rock deposited by the glacier. A dozen miles upslope to the west lies Bear Glacier’s parent, the Harding Icefield, a 300-square-mile mountainous expanse of ice that’s one of the largest ice fields in North America.
Glaciers form when snow accumulates at high elevations year after year, compressing into a thick sheet of ice. As this ice gets thicker and heavier, it begins moving downslope under its own weight, grinding and carving the rock at its base on its journey to the ocean. These slow-moving rivers of ice are powerful erosive agents, capable of chiseling entire mountain ranges into broad U-shaped valleys and grinding even the hardest rocks into fine powder.
Moraines are accumulations of debris formed by this glacial movement. From above, it’s easy to see that Bear Glacier is adorned with a number of moraines. Lateral moraines are stripes across the glacier’s surface, formed from eroded dirt and rocks that gather at the edge. When tributary glaciers join the main flow, as they do upstream in the Harding Icefield, these stripes merge together into medial moraines toward the middle of the merged ice flows. A terminal moraine is a mass of rocks carried to the end of the glacier and left as the toe of the glacier melts and retreats upslope. Terminal moraines mark the historic end of a glacier, before it began melting back to its current location. Bear Glacier’s terminal moraine dates back several hundred years, when the glacier made its way down to Resurrection Bay. During times of increased melting, the waters of the lagoon spill over into the bay, in a glacial lake outburst flood. As overflow sloshes out, the lagoon’s water level drops and chunks of ice break off from the toe of the glacier, forming car- and house-sized icebergs that can be seen floating in the lagoon. The lagoon’s bright green-blue color results from light bouncing off glacial flour—very finely ground sediments in the water.
Bear Glacier displays many features of classic glacial anatomy along its dozen-mile journey from the Harding Icefield to the sea, including lateral, medial, and terminal moraines and cross-hatched and perpendicular crevasses.
Just upstream from the lagoon, the body of Bear Glacier is broken into deep crevasses that crack open as the glacier moves. Crevasses typically open in lines parallel to the downhill direction of glacial movement, but sometimes cracks will also appear running perpendicular to the direction of travel, making a cross-hatch pattern. These deeply crevassed areas most often occur atop rocky outcrops buried deep under the ice.
Keep an eye out for a long tongue of a glacier that starts in a massive white ice field and ends just short of the open bay in a bright blue lagoon. You might fly over Bear Glacier en route to Anchorage or Homer, Alaska.
