A visitor arriving
at the banks of the Kennicott River finds a peculiar piece of
landscape immediately upstream. Jumbled piles of rocks,
sand, boulders and gravel are heaped everywhere as though they
had been deposited by enormous earth-moving machines. This
is the terminus (lower end) of the Kennicott Glacier. The
rock piles are not really rock all the way through, but rather
are ice hummocks mantled by a surface layer of debris ranging in
size from fine silts and sands to boulders the size of a small
house. Here at the terminus, the finer materials typically
lie up to about a foot thick over the ice. On some of the
steep faces of the hummocks the debris may be less than an inch
thick and dark ice can sometimes be discerned underneath.
The debris-covered ice of the lower glacier is a
treacherous surface for walking. The rocks are loose and
slick ice can easily by exposed by an unwary footstep.
If this is a glacier, where is the bare ice? There is plenty of it further upstream where the Kennicott Glacier originates on the flanks of Mt. Blackburn and other surrounding peaks. At higher altitudes, some of each winter's snow accumulation survives summer melt and piles up century after century to form glacier ice. Under the influence of gravity, this ice flows down the mountainsides, where various stream coalesce to form the long valley tongue of the glacier. Moving glaciers collect rocks. Some are scoured from the glacier bed and walls, generating finer sand, silt and rock flour in the process. Some are dumped on the surface by rock slides and snow avalanches. Coalescing ice streams produce a lot of erosion at their junctures, the resulting rocks being carried along the main glacier as stripes of surface debris separated by "streets" of clear ice. These stripes are called medial moraines.
At lower altitudes, the
winter snow entirely melts away each year, exposing the glacier
ice to heat from sun, wind and rain. Some of the ice carried down
from higher altitudes thus is lost each year. The farther down
the glacier one gets, the more rock debris accumulates on the
surface.
Finally, in the lowermost reaches of
the glacier, the entire surface is covered with such debris and
the underlying ice is no longer visible. This mantle of
debris is called a surface moraine. Because the mantle is
uneven and summer heat seeps unevenly through to the ice, this
kind of a moraine forms an unstable surface which soon develops a
chaotic texture of hollows and hummocks. A visitor
approaching this glacier on foot, at the terminus or along the
margin while strolling from McCarthy to Kennicott, sees only the
end-product, a chaotic, rock- covered surface. A flightseer
can see the whole process lain out from start to finish. A
short climb up the mountainside above Kennicott gives a good
overview of the lower glacier and the evolving surface moraine.
The lower Kennicott
Glacier has been thinning for many years. This means that the
flow of ice down the valley has been inadequate to replace that
lost to melt. Within the last decade, the lowermost couple
of miles of the glacier have thinned to the point where there is
insufficient pressure at the glacier bed to sustain sliding.
This part of the glacier has come completely to rest.
The ice now simply sits there and slowly continues to melt
underneath its mantle of rocks.
First-time visitors to
this area sometimes interpret the rock piles of the surface
moraine as leftover debris dumped by the Kennicott mines,
expressing dismay at the effects of mining on the landscape.
Once they learn that this is all the work of the glacier,
the dismay vanishes. Glaciers, it seems, are indeed privileged to
revise landscapes on a scale that makes mining activities appear
puny in comparison!