A Glacial Primer

Regardless of the route you use to get to Alaska   and to circle through the central part of that state, you cannot miss seeing glaciers.  There are glaciers in Banff and Jasper NP in Alberta; near Stewart-Hyder in BC; around Juneau, Haines, and Skagway for those traveling on the AMH ferries, along the Glenn Highway; near Valdez, Portage, and Seward; in Glacier Bay, Kenai Fjords, and College Fjord; and on the slopes of Mt. McKinley.  There are many types of glaciers, but the named glaciers along your routes are generally either "mountain glaciers" or "tidewater glaciers." 

Both mountain glaciers and tidewater glaciers may be tongues of larger, mountaintop glaciers called "icefields."  The large icefield that feeds the famed Athabasca Glacier in Jasper National Park is called the Columbia Icefield.  The icefield that feeds the Exit mountain glacier at Seward and the tidewater glaciers of Kenai Fjords National Park is called the Harding Icefield, named for the 1920's U.S. President who died of a heart attack on the way home from his only trip to Alaska.

Mountain glaciers consist of many parts, most of them originally named by the Swiss and some of which are shown in my 2000 photo taken from the air of Ruth Glacier.  Ruth Glacier is located on the southern flank of Mt. McKinley in Denali National Park.  A "cirque" is a bowl-shaped snow accumulation area that can be a source of snow and ice for a mountain glacier.  Because all glaciers act as very efficient rock excavators, they break rocks off valley walls as they move downslope.  This rock is called a "lateral (ie., side) moraine."  The rocks being bulldozed off the bottom of the glacial valley, pushed ahead of the toe, and left there when it begins to retreat are called a "terminal moraine."  And, when two glaciers flowing down adjacent valleys meet and join, their adjacent lateral morains also join forming a "medial moraine."  A "hanging glacier" is a steep wall of ice flowing out of a cirque or hanging valley, just like Yosemite Falls flows out of a hanging valley located above the U-shaped, glaciated Yosemite Valley.

If large quantities of rock fall from the cliffs of an oversteepened, U-shaped glacial valley onto the surface of a glacier and the glacier begins to retreat, the glacier can become completely covered by the rock debris. This is sometimes called an "ice-cored moraine" because it looks like a moraine even though it is actually still a moving glacier. The lower parts of Dome Glacier in Jasper National Park and Ruth Glacier in Denali National Park are ice-cored moraines.  My 2000 photo of Ruth Glacier on the right clearly shows both lateral moraines, the medial moraine, the ice-cored moraine, and an amazingly cloudless Mt. McKinley.

Remember, a mountain glacier is just a river of ice instead of a river of water and moves downslope by gravity flow just like a river of water moves downstream.   Therefore, "right" and "left" on a glacier are just as they are on a river--the right bank/lateral moraine is to your right if you are looking downstream or toward the "toe" (as opposed to "head") of a glacier.

Finally, the random mix of silt ("rock flour"), sand, gravel, pebbles, cobbles, and boulders dropped by a glacier when it melts ("retreats") is called "till."  And the better-sorted rock flour, sand, and gravel moved downstream by streams of glacial meltwater gushing from the toe of the glacier are called "glacial outwash," as shown in my photo of the meandering meltwater streams and glacial outwash plain below the toe of Ruth Glacier.

Next time: A little about some slightly larger glaciers--ones that covered most of Canada and much of the U.S., reached depths that left only the highest peaks of the Canadian Rockies exposed above the ice, and disappeared only 14,000 years ago.