Imagine ice two miles thick covering your town!  If you are from the Great Lakes region, the Dakotas, Montana, Idaho, Washington, or most of Canada, that may well have been true of your region during the Pleistocene ice ages.  During the past 2-2.5 million years, perhaps as many as 10 distinct ice ages have occurred.  But each ice age didn’t last 1/10 of that time because there were warmer “interglacial” periods between each glacial advance.  Why are these ice ages important in understanding the geology along the Alaska Highway and its approaches?  Because nearly all the landforms you are seeing along your route—mountains, hills, river valleys, lakes, waterfalls, fjords, sand dunes, remaining glaciers—were modified by, or actually created by, this ice, its melt-water, or its winds.

During the Pleistocene, so much of the earth’s water accumulated in the thick “ice sheets” as snow and ice that sea level around the entire globe was as much as 450 feet lower than it is today.  This lower ocean exposed the Bering Sea “land bridge” that connected North America to Siberia and the remainder of the Eurasian continental land mass.  Also, at the times of peak glaciation, only the highest peaks in the Canadian Rockies were exposed above the ice.  This thick ice had a bulldozing effect many times greater than any glacial erosion that we see from mountain glaciers today.  So, even the hard bedrock of the Rockies shows the erosive effects all this ice.

Lobes of the ice sheets dammed up huge rivers like the Columbia, creating gigantic lakes, with names like Glacial Lake Missoula and Glacial Lake Columbia.  When these ice dams broke, as they did multiple times, the resulting floods (“jokulhlaups”—Icelandic, not Swiss!) gouged out great valleys (e.g., Grand Coulee) and stripped the soil and loose rock off the lava flows of eastern Washington (the “channeled scablands”).  In addition, the constant cold winds coming off these miles-high sheets of ice picked up sand and silt from the outwash plains at their edges, creating sand dunes, as at Carcross, YT, and in the Kobuk Valley National Park, AK, and windblown silt/rock flour-covered (“loess”) hills, like the Palouse of eastern Washington.

Features created directly by the ice include moraines and till (discussed in the previous post), drumlins, eskers, kettles, kames, tarns, arêtes, and horns.  Drumlins are rounded hills of till smoothed by the passing ice, with a steeper slope in the direction of movement.  Eskers are long, narrow, snake-shaped ridges of gravel that were formed by water flowing beneath the ice of a glacier or ice sheet. 

Kettles are small, generally circular lakes or ponds created when a chunk of gravel-covered glacial ice, left behind by a retreating glacier, melts and the surface collapses.  When I was investigating a proposed damsite in Devil’s Canyon on the Susitna River, our helicopter flew daily over hundreds of kettle ponds located between the Denali Highway and Devil’s Canyon.  These ponds contained huge numbers of grayling, but most had probably never been fished because the ponds were too small to land a float plane.  In my 2000 aerial photo below, the kettle ponds are clearly visible in the outwash plain below Ruth Glacier. 

Kettle ponds are often associated with kames, which are holes in the glacier filled with gravel by streams flowing across the surface of the ice.  When the glacier retreats, the gravel is left as a small hill. 

Also visible in the photo to the left are the areas bare of trees.  These grassy areas are not due to clear-cutting by loggers (they are all down in the Alaska Panhandle, not in Denali National Park).  They are the result of an extremely high water-table combined with one last feature left from the ice ages that no Alaska RVer will soon forget—permafrost.  Permafrost is just short-hand for “permanently frozen ground” (does anyone remember how to take short-hand?).  Soil that was frozen during the last ice age has, in Northern Canada, Alaska, and parts of the Rocky Mountains, never thawed out.  This frozen soil, on which only a thin surface layer thaws each year, creates the roller-coaster frost heaves that are the bane of RVers throughout the Yukon and Alaska.  All those little red or yellow flags that you soon come to love to see, but hate driving past, are due to glacial geology!

I don't know if it still exists in Fairbanks, but in the early1970's, I visited a "permafrost tunnel" operated by the University of Alaska and the U.S. Army Cold Regions Research Laboratory.  After we passed through an airlock designed to prevent thawing, we walked into a tunnel excavated in frozen Pleistocene gravel, sand, and silt.  Being an engineer, I felt very strange walking under boulders the size of a small car hanging in the ceiling with no visible means of support.  There were also interestingly shaped, dark-colored ice lenses in the walls and ceiling.  It was a tour I'll never forget, so not everything about permafrost is bad!

Next time: Tarns, arêtes, and horns should have been discussed in the last post because they are related to mountain glaciers, not ice sheets.  So, they will be discussed in the next post.