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A small glacial lobe creeps down a mountainside on Gibbs Island, Antarctica. The
summit is only 500 meters above sea level and the glacier is only a couple of hundred
meters long. The sea is visible in extreme lower right. |
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An ice front on a continental glacier. The front in the background is actively flowing
into the sea, calving off chunks of ice occasionally. The ice front in
foreground with the
climber is a stagnant, ablating ice front. |
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A typical continental glacier. Note the dome-like profile and the half-hidden cirque
in the right distance. Mountain glaciers commonly merge into and are submerged by
continental glaciers. |
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This active ice front flows to the sea. Note the trapped sand and rock in the glacier.
This is the material that shapes glacial landscapes. |
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A small glacier on Gibbs Island, Antarctica. Note the crevasses created by brittle
fracture. Note that they appear to be covered with snow. In reality the snow is only a
cover, which may or may not support a climber's weight. Note also the ridge of loose
material along the edge of the glacier. Rubble falling onto the glacier is dragged along,
creating a lateral moraine. |
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Standing in Oregon, looking into California from a small cirque. The curvature of the
valley and the deposits in the foreground indicate a former glacial valley, but the valley
downslope is a typical V-shaped fluvial valley. The glacier extended about to the shadow
line. |
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An active cirque on the Antarctic Peninsula. The ridge is only about 500 meters above
sea level and a couple of kilometers from the sea. The splintery landscape above the
glacier is due to snow avalanches and intense frost action and is calle a nivation
landscape (from the Latin word for snow). |
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The head of a glacier on Mount Shasta, California. The glacier pulls away from the
headwall of the cirque, plucking rocks away and creating the distinctive shape of the
cirque. The glacier also creates a crevasse called a bergschrund adjacent to the
headwall. |
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A bergschrund at the head of a glacier on Mount Shasta, California. |
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A large cirque in the Patagonian Andes on the Argentina-Chile border. A glacier still
fills the cirque. The ridge around the cirque is composed of spiky peaks called horns
and knife-edged ridges called aretes. |
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Aerial view of cirques and glacial valleys in the Colorado Rockies. |
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A cirque at the base of Mount Whitney, California. Small lakes like this are common in
cirques. The cliff behind rises almost 3,000 feet to the summit of Mount Whitney. |
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A small cirque in the Klamath Mountains of Oregon, barely 4,000 feet in elevation.
Glaciers could form this low because of the extremely heavy winter snowfall. Note the lobe
in the valley, probably due to solifluction, or flow of saturated soil over
permafrost. It hasn't moved in some time because there is a large tree growing on it. |
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Marble Mountains, northern California. The summits are at 8,000 feet, the photo locale
is at 5,000 feet, and the valley floor is at 2,000 feet. Cirques formed near the summits
but the glaciers did not extend far down valley. |
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The premier glacial valley in the U.S. is Yosemite, in California. The valley floor is
flat because of deposits laid down in a lake after the glaciers melted. |
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A glacial valley in Scotland, the pass of Glen Coe. Ice filled the basin in the
background and flowed outward down valleys in all directions. |
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Another glacial valley in Scotland. Note the large rock in the valley, a whaleback
or roche moutonee. In the left distance is Ben Nevis, highest peak (only about 4500
feet) in the British Isles. Note that all the summits define a flat plain, a possible
relic of an ancient land surface. |
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A spectacular glacial landscape in the southern Sierra Nevada, California. The uplands
are covered with cirques and glacial troughs but the valleys show only evidence of fluvial
erosion. |
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What the scene above might have looked like during the Pleistocene. |
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The Finger Lakes of central New York are flooded valleys scoured by the glaciers and
dammed by moraines. |
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This sculpted outcrop in Ontario is typical of glacial sculpting and abrasion.
Increasingly, high-pressure melt water beneath glaciers is being shown to play a role in
subglacial erosion as well. |
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This exposure shows layered water-laid deposits and till, a coarse mixture of
everything from silt to boulders. It has all the features we might expect from any cut in
glacial deposits anywhere, but it's 2.3 billion years old. This is the Gowganda Formation
of Ontario, one of the earliest preserved glacial deposits on Earth. |
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A close-up of the previous picture. The boulder is sitting in fine-grained layered
sediment, with deformed layers underneath. This is a dropstone. It was probably
frozen into a block of ice, drifted over a glacial pond or lake, then plummeted to the
bottom. |
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The gouges in the otherwise glacially polished surface are called crescentic gouges.
They appear to form when large rocks are pushed down with great force into the underlying
surface, creating a conical fracture. Melt water gets under the lip of the fracture,
freezes and expands, and part of the fracture is exposed.
The bow of the crescent points in the direction of ice movement (here,
right to left).
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These rows of small crescent-shaped cracks are often called chatter
marks. In contrast to crescentic gouges, the horns of the
crescents point in the direction of ice movement (here, bottom to top).
Note the striations as well.
How do we know the relation between ice movement and the fractures?
Many glaciers world-wide are retreating. We need only examine rocks that
were deep under glacial ice in recorded history. |
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Sometimes you can find evidence of multiple directions of ice movement.
The pencil and pen lie on two sets of chatter marks both indicating ice
movement from lower right to upper left. |
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A small whaleback or roche moutonee (French, sheep-rock, because fields
full of them look like sheep grazing). Ice rides over the upstream side and smoothes off
the outcrop, but pulls blocks away from the downstream side, creating a blocky, steep
front. Thus the ice flow here was from right to left. |
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Glaciers frequently leave isolated stones (erratics) behind when they melt.
This one in central Washington is especially impressive. |
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The city of Madison, Wisconsin lies between several glacial kettle lakes. The lakes
formed when glacial ice remained behind in a former river valley and was buried by outwash
deposits. |
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Water diverted by ice is termed ice-marginal drainage. In the
foreground is the edge of a moraine in northeasternmost South Dakota. In
the distance are several dark strips which mark valleys cut by streams
flowing along the ice margin. The most distant one is the Missouri River,
which is essentially an ice-marginal drainage along the lower half of its
course. |
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Subtle braided textures can be seen on these fields in Ohio, marking the locations of
former braided glacial meltwater streams. |
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The Potholes Trail down the south side of the Baraboo Bluffs was clearly eroded by
fast-moving sediment-laden water, but there is no stream there now.The potholes are
believed to have formed when melt water from the glaciers flowed down the bluffs here. |
