Physical Geology Slides-Glaciers
Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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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
||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
||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.
||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.
||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.
||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
||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).
||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.
||A bergschrund at the head of a glacier on Mount
||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.
||Aerial view of cirques and glacial valleys in the
||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
||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
||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
||A valley glacier flowed down from the left to make
this moraine near Leadville, Colorado. Several stages of
retreat can be seen. At its height, the glacier merged
with another one flowing down the valley in the
||Glacial margins don't get any clearer than this one
near Chelan, Washington. The area left of the line of
large boulders was not glaciated.
||Moraines are where you find them, even in Peru. The
latitude is 15 degrees south of the equator. The valley
floor is at 13,000 feet. The glacier is at 17,000 feet
and the summits hidden in the clouds exceed 20,000 feet.
||A small drumlin south of Rosendale, Wisconsin.
||Drumlins southeast of Fond du Lac, Wisconsin.
||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.
||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.
||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.
||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.
||What the scene above might have looked like during
||The Finger Lakes of central New York are flooded
valleys scoured by the glaciers and dammed by moraines.
Small-Scale Glacial Features
||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.
||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
||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.
||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.
bow of the crescent points in the direction of ice
movement (here, right to left).
||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.
||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.
||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.
||Glaciers frequently leave isolated stones (erratics)
behind when they melt. This one in central Washington is
Ice and Meltwater
||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.
||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.
||Subtle braided textures can be seen on these fields
in Ohio, marking the locations of former braided glacial
||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.
||The subtle ring-shaped features occur in a never-glaciated
part of South Dakota. They may be relics of pingoes,
mounds that form now in the Arctic when ice trapped
||In Northeastern Wisconsin, the main glacial retreat
was followed by a quick readvance that deposited a thin
layer of distinctive red till. This pit west of Oshkosh
shows red till on top of cross-bedded outwash sands.
||This wide, smoothly curving valley in South Dakota
has only a tiny stream flowing in it. A stream too small
to carve its valley is said to be underfit and
usually indicates that the valley once carried much more
water. The smooth curves and uniform width suggest large
volumes of high-speed water when the valley was carrying
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Created 13 July 1998, Last Update 12 January 2001
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