Glaciers form at high, cold altitudes where snowfall accumulates faster than it melts. The weight of increasing layers of snow compacts the older layers into ice. The ice mass begins to slide downslope, and for this reason, glaciers are considered to be rivers of ice.
Glaciers are the most powerful natural force of erosion on the planet. They cut, carve, and obliterate solid rock as they flow downslope. Glaciers leave behind very distinctive landscapes, so it is easy to tell where glaciers have previously existed.
A glacier consists of two parts: first, an area of gain (accumulation) where new snow is added to the glacier each year; second, an area of loss (ablation) where mass is lost each year to melting and calving. These two areas are divided by a snowline. As the glacier flows downhill it grows thinner and narrower. Ultimately it reaches a point where the ice front advances no further, because the ice there melts as rapidly as ice flows down from above.
GLACIALLY CARVED VALLEYS
Like water, glaciers follow the path of least resistance and reshape the land as they flow through. The process of glaciation follows these three general steps:
Uplift causes running water to cut a V-shaped valley.
Glaciers fill the river valley, widening and straightening the channel, into a U-shaped valley.
Glaciers melt, revealing the U-shaped valley. If the sea later fills in the valley, it is called a fjord.
FEATURES OF A GLACIAL LANDSCAPE
Some of the glacial features that can be seen from Skagway or just a short hike out of town include:
Cirque - A bowl-shaped basin at the head of a glaciated valley produced by glacial scouring and erosion.
Crevasse - Open fractures or breaks in glacial ice. As ice flows, the base of a glacier moves more slowly than the surface, producing deep cracks and crevices in the surface of a glacier.
Arete - A narrow ridge separating two glaciated valleys.
Horn - A pyramid-like peak formed where several cirques meet.
Moraine - Rock and other sediment carried and deposited by a glacier. Moraines can be found in front of, alongside, and within a glacier.
GLACIATION NEAR SKAGWAY
Glaciers have advanced and retreated through the Skagway area for millennia due to a series of cold intervals (Ice Ages), which cause glaciers to grow and advance, alternating with intervening warm periods, which cause glaciers to shrink and retreat.
Each subsequent period of glaciation obliterates the geologic record of the previous once. Evidence of only the two most recent periods of glaciation are visible today in the Skagway Valley.
Some 49,000 years ago glaciers filled the Skagway Valley up to the level of the craggy peaks, burdening the land with flowing ice over 5,000 feet thick. The ice ground down through the river's course and carved a steep-sided, broad U-shaped valley. In passing it polished any projections in the valley, leaving massive, rounded rock shoulders behind.
Between 13,000 and 24,000 years ago a smaller ice age occurred in this area. During this time glaciers carved a smaller U-shaped valley into the existing valley floor.
In very recent times during a period called the Little Ice Age, approximately 500 to 250 years ago, small glaciers formed at high altitudes. Harding Glacier, visible from downtown Skagway, is a remnant from this time.
The highest peaks near Skagway are referred to as nunataks. A nunatak is an isolated hill or peak which projected through the surface of a glacier and was completely surrounded by ice or snow. As glaciers flowed over the landscape, they ground the land beneath them smooth. The peaks above the ice escaped this grinding, leaving behind nunataks that maintain their original sharp and jagged appearance. Nunataks mark the highest extent of glaciation, which can be seen from the boardwalks of Skagway.
The Skagway Valley also includes a number of hanging valleys. These were formed when a large glacier, cutting a very deep valley, crossed the path of a smaller glacier. Since the smaller glacier was not powerful enough to grind down into the mountains as deeply as the larger glacier, its valley floor stood much higher in elevation. While the glaciers were active, the mouth of the smaller one hung over the larger. An icefall, the glacial equivalent of a waterfall, emptied into the deeper valley below. After the glaciers melted, a small U-shaped valley at a high altitude remains and "hangs" over the landscape.
Not just the motion, but also the weight of glacial ice left its mark here. A river of ice 5,000 feet thick is quite heavy. Relative to the rest of the Earth, the crust is very thin. The weight of the glaciers depressed the crust into the mantle. Now relieved of that load, the crust is springing back in a process called glacial rebound. Due to uplift and rebound, Skagway rises about 0.76 inches each year. That's about six feet since the gold rush.
Photographs and figures were provided by National Park Service rangers B. Aulick, P. Cooper, M. Hachtmann, S. Hinderman, and S.Millard; the National Archives, This Dynamic Earth by Jacqueline Kious and Robert Tilling, Earth's Interior and Plate Tectonics by Calvin and Rosanna Hamilton, the Alaska State Library Alaska Purchase Centennial Collection, Melanie Moreno and the United States Geological Survey.