Glaciers have worked like carving tools on the landscape of Southeast Alaska, profoundly shaping the area. They are the scenic jewels of the region, and hundreds of glaciers can be counted in the mountains and valleys. In addition, a series of ice fields and glaciers straddle the crest of the Coast Range of Alaska, running the length of the border between Alaska and British Columbia.
The largest of these is the Juneau Icefield. This vast plain of ice is 45 miles wide and 85 miles long, fed by snowfall that exceeds 100 feet a year. The chiseled peaks of buried mountains rise above the white expanse of snow-covered ice, which is a mile deep in places. Under the weight of this accumulation, the snow recrystallizes and ice crystals fuse. The ice overflowing from this mile-high plateau surges down valleys and feeds 38 large glaciers.
Glaciers are slow-moving rivers of ice, and even receding glaciers are moving forward; they're just melting faster than they advance. The Mendenhall Glacier near Juneau is 13 miles long, and the ice takes about 250 years to flow from the "headwaters" in the Juneau Icefield at 5,000 feet to its terminus in the Mendenhall Valley near sea level.
Like most glaciers in Southeast Alaska, the Mendenhall Glacier is retreating. It has retreated two and a half miles in the past 250 years. A retreating glacier exposes a scoured, rocky landscape, scraped bare by ice and rocks imbedded in the bottom of the glacier. Patches of rubble, sand and silt offer a foothold to wind-borne seeds and the spores of moss and lichens. Moss and lichens are the first plants to grow on the landscape, and produce acids that help to break down the rocks. Over the course of a few years, the seeds of fireweed and willows sprout. Plants such as lupine and alder help "fix" atmospheric nitrogen to the mineral soil, making it more fertile. Decomposing leaves contribute to the developing soil, and spruce, willow and cottonwood (which thrive in the full sunlight of these open spaces) begin to sprout. Over time, this succession of colonizing plants transforms the landscape into the temperate rain forest of Southeast Alaska.
The last global ice age was at its maximum about 18,000 years ago, at the end of the Pleistocene era. A warming trend caused glaciers to retreat for thousands of years, but about 3,000 years ago, cooler weather resulted in the advance of some glaciers in Southeast Alaska. The oral history of Native Alaskans recounts these changes, especially in Glacier Bay. These glaciers reached their most recent maximum about 250 years ago, and since that time have been retreating.
As the massive ice sheets that covered Southeast Alaska melted over the past 18,000 years, plants and wildlife began colonizing the land. A few pockets of land (such as Coronation Island on the Outer Coast) were not overrun by glaciers and served as "refugia," harboring life that later dispersed throughout the region. Many of the post-glacial pioneers such as black bears, brown bears, wolves, moose and marten also came into Southeast Alaska from unglaciated areas to the north and south. Evidence indicates that the distribution of these animals in Southeast Alaska is related to their routes of colonization.