Glacier Features
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.

 

 

The Atlin Mountain rock glacier in Atlin, B.C.

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.

 

Another view of the Atlin Mountain rock glacier

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.

 

The calving face of the Mendenhall 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.

 

Tarn lake filling a glacial cirque

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.

 

A dramatic example of a dying glacier

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 This unnamed glacier near Atlin Lake is no longer active as evidenced by the total absence of crevasses.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.

 

A large cryoconite hole

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.

 

The Demorest Glacier and glacially-eroded peaks

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.

 

Glacial erosional features

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.

 

This rock illustrates the incredible force exerted by water as it freezes

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.

 

Undeformed primary stratification of a glacier

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.

 

A meandering supra-glacial stream channel

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.

 

Suncups 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.

 

Crevasses on the Taku Glacier

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.

 

Horns and aretes

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.

 

Icefall and wave bulges

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.

 

Wave ogives on the Vaughan Lewis Glacier

Here's another view of the icefall-generated wave ogives 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.

 

A supraglacial lake

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.

 

Marginal lakes at the head of the Lemon Glacier

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.

 

The surface effects of a drained sub-glacial lake

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.

 



A water filled crevasse

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.

 

A frozen moulin

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.

 

Trimlines provide graphic evidence of the downwasting of a glacier

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.

 

Trimline showing glacier recession

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.

 

The terminal moraine of the Cathedral Glacier

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.

 

An interesting type of rock fracture

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.

 

Foliation at the base of a glacier

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.

 

The stuff of moraines

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.

 

Just one of the massive rocks from a large rockfall event

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.

 

Path of the rock as it slid down the glacier

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.

 

Debris from a major rockfall on the Taku Glacier

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.

 

A large ice avalanche

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.

 

Lateral moraine advancing into the forest

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.

 

Trees toppled by the advancing Taku Glacier

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.

 

An example of what ice can do

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.

 

A supraglacial stream

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.

 

Trimline and the downwasting Llewellyn Glacier

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.

 

Healed crevasses and lateral fault

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.

 

Another rock for the moraine

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!

 

The terminus of the Gilkey Glacier

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.