How shall we speak of a glacier?
Call it white and that's forgetting the blue.
Define a glacier as ice and you're overlooking that it's also rock
and it's dripping rushing water.
Speak in terms of the past, of a long ago ice age and you're neglecting the present
Glacier Bay National Park and Preserve northwest of Juneau Alaska is a place to
experience both present day ice and landscape created by previous glaciation.
Peaks here rise 15,000 feet above the sea. Their fangs a white sawtooth meeting
of land and sky. Glaciers born of these Heights reached to Tidewater and calve
off icebergs.
Climb the slopes beside a glacier and you're stepping where ice repeatedly has
advanced then retreated. Reach a high vantage point and the scene reveals the
glacier's true scale, The ice forms a huge frozen river flowing to the sea fed from
far back in the mountains. the cliff calving off icebergs seems towering as we
float before it and it does rise more than 100 feet above the water, but from
the perspective of a high slope you see the vast ice that nourishes the
tremendous cliff. Fly up Reid glacier to the Brady icefield and your perception of
park glaciers changes yet again. Here is ice nourished by today's clouds sweeping
in from the Pacific and dropping their moisture.
In the Fairweather range winter's snowfall is greater than summers melt.
Fed by flakes falling from clouds and avalanches sliding off surrounding
slopes, snow packs to ice. Its own pressure causes crystals to deform.
Plastic flow begins. The whole glacier creeps downslope in response to gravity.
Viewed moment to moment, this movement is not easily seen.
Time itself seems frozen.
Like a conveyor belt ever moving but barely changing position the ice may
reach sea level. About 50 glaciers along today's Alaskan coast enter the sea.
A dozen of these are within the park. Currently some are lengthening, others shrinking
The process is unending the change constant.
where Glacier Bay whales now swim
there once was ice.
That ice melted, forests flourished,
then the ice came again.
Tlingit indian people were forced out of Glacier Bay
by advancing ice.
Captain George Vancouver exploring for England couldn't get in.
Where cruise ships today enter park waters, Vancouver noted in his log
"a compact sheet of ice as far as the eye could distinguish." He sailed on.
Today that ice is gone and waterways thread beneath mountain peaks in place of glacier tongues.
Forests recloathed the lower reaches.
Bareness still dominates
upper fjords and some glaciers have withdrawn.
Other glaciers in the park however are inching forward.
They start among high peaks where their white
blankets receive continual renewal.
No Alaskan tidewater glaciers are floating.
Their ice rests on the sea floor.
Advancing or retreating, all glaciers
shape the land.
They pluck and rasp and grind the rock they slide over,
then they deposit what they have quarried.
To mentally picture the dynamics of the ice
visualize first the mountains and the snowstorms
the snow deepens and compacts ice.
Pulled by gravity and prompted by its own internal pressures,
it begins to move.
The bottom of the glacier rides over whatever lies in its path
Where bedrock slopes deeply,
ice accommodates to the contour by splitting at the surface.
This forms crevasses.
Some gape a hundred feet deep.
When sufficient snow feeds a glacier
nothing can stop its forward motion.
If jutting bedrock lies in its path
ice will flow over and around
and we may see a knob smoothly rounded by overriding ice
or a peak jagged where it rose above the ice.
See the polish of rock walls and
you're viewing the slow work of ice pushing against its bed.
Notice scratches where the sandpapering and filing of gravel and rocks frozen into
the glacier and dragged against bedrock
Where do rock tools originate?
Some are plucked free as the ice inches forward
They are the rasps that widen and deepen the valley troughs sculpted by glaciers
Rock rolls and avalanches onto the ice surface and becomes a part of the glacier.
Some rock rides externally as visible ribbons of rubble called moraines.
Some rides unseen, part of the internal glacier.
There may be so much rock
that the ice turns black and the calving face is striped
Rock is one ingredient of a glacier in addition to ice flowing water is another.
The water comes from melt trickles and rain runoff
lubricated by even a film of
such water at its base, the glacier slides over its rough bed.
water alsocarries off and deposits material ground from bedrock by the glacier
Silt so fine it's like flour floats in suspension and turns the sea milky
Where glacier melt mixes with salt water.
Fresh water is not as dense as salt water
It rises to the sea surface and floats, blending it's glacial silt with the clear
seawater and creating color patterns carried by the currents.
Outwash deposits edge into the sea.
Over time outwash debris pouring from a glacier may fill
even huge valleys to great depths.
Today's surface gravels may support
dryas and willow and alder and spruce
Streams cutting down to former surfaces
sometimes reveal remnants from forests of long ago.
Ice advancing into salt
water mounds up rubble along its margin.
Additional silt sand gravel and rocks
from tiny to huge fall from the lower end of a glacier's conveyor belt.
Such debris forms a terminal moraine, a ridge beneath the ice snout
Glaciers with enough debris to partially fill in ahead of themselves
may advance into exceedingly deep water.
their moraines protect the ice fronts and reduce erosion.
Without such a shield a glacier could
not supply ice to its snout fast enough.
More ice would melt and break off than
can be replenished from above
the glacier would draw back to shallow water
This happened to the Lamplugh and Reid Glaciers.
At low tide it's obvious that their snouts rest on shore.
There they stabilized
and now they may again be
building underwater moraine and starting to re-advance.
that Johns Hopkins and
Grand Pacific glaciers clearly are advancing.
They carry huge amounts of rock.
Raw material for a barrier between ice front and sea.
Furthermore the upper ends of all these advancing glaciers lie high enough to be
well nourished by snowfall
Tidewater glaciers thus are different than those
entirely on land
they respond to climate and also to their own terminal moraines
Climate controls broad trends but for glaciers reaching the bay the dynamics
of advance and retreat are fine-tuned by water depth.
That crucial depth in turn
depends on the snout's relation with its moraine.
The shallower the water at the snout,
the fewer the icebergs calve off.
Those that fall from the crevasse stand
pinnacle a cliff towering above sea level of course are
affected by water depth but how much ice breaks off below the surface depends
directly on how much of the glacier is exposed to the waters warm and erosive
action
Any tidewater glacier that loses the protection of it's moraine is doomed.
Present climate can't build the upper glacier fast enough to balance the ice
calved from a snout in deep water.
Ice is buoyant. Underwater bergs rise silently creating undulating waves of
ice-covered water.
If a Tidewater glacier is under-nourished by snowfall
or in some other way loses grip on its terminal moraine.
retreat begins
with the barrier between ice and sea gone
the glacier virtually unzips
During the last two centuries,
ice in the main Glacier Bay waterway has melted back
60 miles from where captain Vancouver saw it.
In Muir Inlet more than 20 miles
of what was glacier is now fjord.
Few present-day glaciers in the park actively
thrust forward into the fjords choking waterways.
Most have retreated to the
heads of their inlets.
Will today's white scene prevail tomorrow?
No
Glaciation is a dynamic process.
snow crystals shower from the sky and
compress into ice.
Glaciers inch downslope sculpting the land
calving into the sea.
They come and go
and come again.
The glacier's story is cyclic,
ongoing,
never-ending.