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Atlin Lake is no longer active as evidenced by the total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 arÍtes.
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 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 ice flowing rapidly through the icefall. Unlike band ogives which have no vertical relief, these ogives have an amplitude of about
15 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
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.
During the summer ablation season, water is everywhere,
particularly in areas below the ELA -- the 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 ogive lakes
that are seen in the previous two photos. The maximum depth of
this lake is about 6 meters.
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.
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
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
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
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
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
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
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
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.