The Juneau Icefield provides an excellent opportunity to observe and study the effects and features of glaciation. This series of photos gives an idea of the wide variety of geomorphic features to be seen on the Icefield.

Glaciers aren't always made entirely of ice. Here, a rock glacier cascades down Atlin Mountain in northern British Columbia, Canada. The periglacial environment, in conjunction with the highly friable nature of the rock in the source area (the steep headwall of the high-level cirque) contributes substantial amounts of material to this rock glacier. It's hard to find a better example of a rock glacier than this one.

Here's another view of the Atlin rock glacier. While surface movement of this glacier has not been measured, it is easy to infer that the movement is negligible, as evidenced by the vegetation that has become established on the right-hand edge of the glacier.

This shows the old, dense ice at the terminus of the Mendenhall Glacier. This clearly illustrates the ability of ice to absorb most wavelengths of light, while reflecting the blue wavelengths.

This photo of formerly glaciated terrain on the periphery of the Juneau Icefield shows a textbook example of a tarn lake filling the depression of a cirque basin.

All the glaciers along the inland, northeastern margin of the Juneau Icefield are undergoing rapid retreat, such as this small alpine glacier. Notice the extensive deglaciated area and the small moraines. This unnamed glacier near Atlin Lake is no longer active as evidenced by the total absence of crevasses. Notice too the well-defined unconformity on the surface of the ice. This indicates a time period of extended ablation followed by a period of increased accumulation.

This particular feature is the largest cryoconite that I've seen on the Icefield. Cryoconites form when a rock or other dark material comes to rest on the snow or ice surface. The absorption of solar radiation by the rock in the center melts the surrounding snow and ice to form a cone shaped depression such as this 3 meter wide by 2 meter deep example. Although extensive ablation has taken place to form this cryoconite, evidence of the firn stratigraphy can still be clearly seen in the walls.

Seen from the summit of a peak called Taku B, the
Demorest Glacier is shown here flowing around a
groups of peaks. The main glacier flows toward the
right, while a distributary branch flows into the Taku
River valley on the left. The group of peaks in the center
display classic evidence of glacial erosion.




Glaciers are very effective agents of erosion. These sharp
spires were formed by the action of glacial erosion and
frost shattering. Based on recent seismic depth studies, the
glacier in the foreground is approximately 5,400 feet thick.
Directly between the glacier edge and the Tusk, a nearly
ice-free valley lies 3,000 feet below.




Here's a dramatic example of the power and effect of
frost shattering. Minute amounts of water infiltrate small
incipient joint systems in rocks. Over time, the force
exerted by the expansion of the freezing water wedges
the rock apart. Scale is indicated by the metal rod,
which is 1 meter in length.




Structurally, geologists and glaciologists consider ice
to be a rock. Just as sedimentary rocks are laid down
in numerous layers, so too is ice. This is a striking example
of the sedimentary nature of a glacier. This photo shows
the surface of an inactive glacier in which the primary
stratification has been minimally deformed by the
movement of the glacier. Each layer seen here
represents one year of accumulation.




Glaciologists aren't the only ones to study glaciers.
Hydrologists study the formation and propagation
of meandering stream channels on the surface of
glaciers in order to understand the mechanics of stream
erosion in soil and bedrock. Whereas typical stream
erosion and the formation of meandering rivers may take
decades or centuries in soil and bedrock, the formation
of such features in ice occurs in a matter of a few days to
several weeks. This photo shows the erosional effects of a
small stream on the surface of a glacier.




The characteristics of the surface of a glacier are highly
dependant upon the weather. Suncups, such as these,
form during periods of clear weather. Cloudy, rainy
conditions tend to smooth the surface, eliminating the
suncups. The prominent small-scale angled ridges shown
in this photo were formed primarily by solar radiation
during cold, clear conditions in the presence of a
strong downglacier wind.




Crevasse zones tend to occur along the margins of
glaciers. This is due to the slower velocity of ice at the
margins with respect to the central portion. The crevasses
shown here are along the margin of the Taku Glacier, which
flows from the right to the left.




The sharp peaks shown here are called horns and
are created by the action of erosion and oversteepening
of the headwall of a glacier cirque. The knife-edge ridges
are called arêtes.




Icefalls are impressive features of many icefields. They
are usually present near the margins of an icefield and
act as spigots for the drainage of ice from the main mass
of the icefield. This particular icefall is notable for the
particularly striking wave bulges, or wave ogives, which
are formed at its base. These structures are created by a
combination of compression at the base of the icefall and
by the rapidly flowing ice through the icefall. Unlike band
ogives which have no vertical relief, these ogives, or bulges,
have an amplitude of about 25 meters. This particular
icefall and its ogives have been featured in numerous
glaciological texts and publications.




Here's another view of the icefall-generated wave
bulges pictured in the previous photo. It is a view
looking down the longitudinal axis of the glacier
and was taken from the top a nunatak. During the
summer ablation season lakes form in the troughs
between the crests. In most years the water is able to
cut through the crests, forming stream channels
which connect the lakes, as shown here




One of the interesting characteristics of temperate glaciers, such
as the Lemon Glacier shown here, is that ice and water can coexist.
Thus water can flow over, within, and under the glacier. Here, two
lakes have formed on the surface of the glacier at the headwall
moraine. These lakes (Lake Lynn on the left and Lake Linda on the
right) form annually and usually drain in mid-July via a series of
sub-glacial tunnels. The water level varies daily depending on the
local weather conditions, as evidenced by the strand lines around
the perimeter of the lakes. Lake Lynn covers an area of 3.3 hectares
(8.2 acres) and Lake Linda has an area of 2.2 hectares (5.4 acres).
Lake Linda appears larger here, but this is due to the optical distortion
induced by the wide angle lens used to take the photograph.




Lakes can exist not only on the surface of temperate
glaciers, but also underneath them. In this photo, a
subglacial lake in this cirque basin has drained through
a system of sub-glacial tunnels. Without the water to support
the ice above, the surface has collapsed. The radial
crevasses around the periphery clearly indicate the 
extent of the sub-glacial lake.



.
During the summer ablation season, water is everywhere,
particularly in those areas below the ELA -- that area
where the snow has ablated away, exposing the ice. Water
is on, in, and under the ice. Here's a photo of one of the
wave bulge lakes that are seen in the previous two photos.
The maximum depth of this lake is about 6 meters.




Surface water can also fill crevasses, as shown here,
although a subsurface passage could develop at any
moment, thus releasing the water into the glacier's
internal plumbing system. I once witnessed in this
area a crevasse rapidly filling with water from below
with such force that a fountain of water shot up about
a meter. After several minutes, the fountain rapidly
subsided and stopped. This was immediately followed
by the rapid draining of the crevasse, complete with the
characteristic "toilet flush" sound effect! And all this
happened within about 6 minutes.




Streams on the surface of a glacier eventually find their
way into the internal plumbing system via crevasses or
other weaknesses. Moulins are vertical "pipes" through
which the water flows into the interior of a glacier. Because
the glacier is constantly undergoing changes in the strain
regime as it flows downhill, the streams flowing into the
moulins often get diverted, leaving the moulin without
a steady supply of water to keep it open. Under the right
conditions, water trapped in these orphaned moulins can
freeze, as is shown in this photo. The ice axe is 80 cm long.




Here's an example of a trimline produced by the downwasting
of the Llewellyn Glacier. This photo shows the lower Llewellyn
Glacier as it flows around Red Mountain on the left. The glacier
branches out into three distributary lobes at this location.
Notice the prominent trimline on the lower flanks of Red
Mountain. This indicates the magnitude of downwasting
(~ 50 meters) since the time when the Llewellyn Glacier
reached its Little Ice Age maximum around 1917.




Here's another excellent example of a trimline produced as
a result of glacier recession. Lichens cover the rock in the
dark colored area, making them appear dark. The lichen free
lighter colored rocks were recently exposed by the downwasting
of the Cathedral Glacier, shown in the foreground. Bands on
the glacier surface show the primary stratification.




Here's the terminal moraine of the Cathedral Glacier
near Atlin, British Columbia. The glacier occupied
this position during the late 19th century and has
since retreated dramatically. The trimline shown in
the previous photo is clearly seen here on the flanks of
Splinter Peak.




Rocks don't always split in half when struck by another
one. The spherical fracture on this rock clearly shows the
cone of percussion that results from an impact.




The ice at the base of a glacier is subject to extreme
forces. Here, you can see the folding of the basal ice
as the glacier moves down a steep slope and encounters
a relatively level bench. Debris entrained in the basal ice
clearly delineates the zone of folding.




Cirque headwalls are continually being eroded and
oversteepened. This ultimately results in sometimes
massive rockfalls from the walls. This particular rock
is just one portion of a large rockfall from the steep
headwall of a cirque occupied by the Taku Glacier. This
photo was taken about 12 hours after the event, as
evidenced by the snow that is still melting from the surface
of the rock. The dark area on the snow surface in the
background marks the path of the main debris field.




Another view of the rock shown in the above photo. The
time scale of geologic change is such that witnessing events
such as this massive rockfall is not an everyday occurrence.
This slide occurred in the early morning hours of July 28, 1993,
originating from the vertical northwest face of Taku D. This
massive block of granite was the largest single rock to survive
the fall. This rock now serves as a handy reference in tracking
annual snowfall and mass balance.




This is a view of the same rock as in the previous photo.
Here, the view is from near the base of the wall from
which it fell. The path of the rock's slide is clearly marked
on the glacier surface.




This is the main portion of the rockfall that is shown in
the previous two photos. The large single rock from those
photos is out of view to the right. Although not discernible
in this photo, the dust cloud from this rockfall settled on the
glacier out to a distance of about one kilometer. This dust
layer clearly marks the summer, 1993 surface.





The Juneau Icefield is an extremely dynamic place. Not
only are there rock falls, as in the above photos, but ice
is also continuously avalanching. This large ice avalanche
occurred in late June, 2000 and clearly illustrates the sometimes
catastrophic nature of change in glacial environments.




Unlike all the other glaciers which drain the Juneau Icefield,
the Taku Glacier continues to advance. This is graphically
evidenced by this lateral moraine which is overriding
the forest along the margin of the glacier. The contrast
is striking -- lush, mature spruce and hemlock forest on
the right, and only a few feet away there's nothing but
rock and ice.




This is what happens when a glacier advances into a forest.
This is on the eastern margin of the Taku Glacier. As you
can see, the trees didn't stand a chance against the power
of the glacier. The glacier toppled and overrode these trees
in the winter of 1992-93. Summer melting of the ice has
exposed the debris. The ice is shown in the lower left.




Here's evidence of the power of ice. This former tree
trunk, about 50 cm in diameter, has been totally crushed
by the advance of the Taku Glacier. Only splinters
remain.




During the summer months, surface melting occurs
on the Icefield. This produces large amounts of melt
water. Some percolates down through the snow and
firn and refreezes. Some water remains free however,
forming streams on the surface of the ice. This
particular stream is on the lower Llewellyn Glacier.
A medial moraine is to the left of the stream.




While the Taku Glacier is advancing, the Llewellyn
Glacier is rapidly receding. The obvious trimline
shown here indicates the degree of downwasting that
has taken place in the last 80 years.




The stresses within a glacier are constantly changing as
it moves through different zones of compression and
extension. Crevasses open and close, and just as faults
occur in rock, they also form in glaciers. This photo
shows several old crevasses which have closed due to
compression. The highly deformed flow foliation
also shows a right lateral fault.




This photo, taken in the Gilkey Trench, illustrates the
extremely dynamic nature of this sector of the Juneau
Icefield. Combined with constant ice avalanches in the
nearby Vaughan Lewis Icefall and rockfall from the
surrounding oversteepened valley walls, this area is
continually on the move. We were out on a survey one
day when we saw this rock break loose and add itself to
the moraine. This is a small rock -- several other rocks
in this moraine are 50 feet in diameter!




 Like the Mendenhall Glacier, the Gilkey Glacier
terminates in a lake. But unlike the small icebergs
found in Mendenhall Lake, the ice in Gilkey Lake
bears more resemblance to tabular Antarctic
icebergs. The largest berg shown here is approximately
600 meters across. This pattern of breakup indicates
that the terminus of the Gilkey is floating, making the
advance or retreat of the terminus extremely sensitive to
climatic changes.

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