Notes: Wed. 13 Nov
More on glaciers
Review: how glaciers move (last lecture). said that most glaciers are at
the pressure melt point at their bases (see discussion of basal slip from
Ice is a mineral with a well-defined atomic structure. Deformation occurs
on planes of weakness in the ice - called crystal gliding.
In order for a glacier to survive, precipitation must accumulate on some
part of the glacier. This precipitation is snow, and it accumulates best
at high latitudes and high altitudes, where temperatures are cold. The snow
accumulates at the head of the glacier, the part at higher elevation. Typically,
snow will not fall below a certain elevation in the mountains. This elevation
is a consistent line across the whole mountain range and is called a snow line. The only time there is extensive precipitation below this line is if glaciers
extend below it.
Glaciers need snow to form, but not all places with snow will form glaciers.
Snow is deposited in layers and can be deformed by wind just like sand
Ice that does not move is not a glacier.
Glacial ice is dense and bluish in color. It is brown and dirty-looking
near the base because it picks up debris as it moves.
There are layers within the glacier called foliation. Foliation is a result of deformation.
Classification of glaciers & their landforms
Glaciers can be classified by their extent. There are continental or mountain (also called valley or alpine) glaciers. Glacial depositional landforms
are composed of three general types of material: till (unsorted, unstratified; deposited by glacier), outwash (sorted, stratified sands and gravels; deposited by water), or lake clays (see end of previous lecture for desciptions).
Continental glacial landforms:
- composed of till
First, a note on till. There are three locations till forms: at the base,
on top, and at the advancing edge of the glacier.
- The till that forms at the base contains fine clays because the glacier
grinds debris as it moves forward. This is called lodgement till.
- Debris that sits on top of the glacier is dropped down after the glacier
retreats. This is named ablation till.
- Debris that is pushed forward at the edge of the glacier as it advances
is called a moraine.
- drumlin: smooth, elongated, elliptical hills created by the glacier moving over
a plain of till. They somewhat resemble inverted spoons. The drumlin is aligned with the direction of travel of the glacier. There is a steep end and a gently sloping end on the drumlin. The gently sloping side points in the direction of travel of the glacier. The steep side points to the direction the glacier came from.
- moraines: composed of till, they are ridges of debris pushed by the glacier as it advanced. Hilly
topography often exists on moraines.
- lateral: at the side of the glacier
- end: at the front of the glacier (by the terminus)
- terminal: the farthest end moraine left by the glacier before it started receding
- composed of outwash
- esker: a ridge composed of outwash that shows where a river once ran under the glacier
- kame: a poorly- to moderately-sorted, often irregularly shaped hill of outwash.
Kames form in contact with glacial ice. For instance, lakes can form on
the top of the ice sheet. In this lake, sediments can settle on the bottom.
When the glacier melts away, the sediments are dropped onto the ground,
leaving a hill of outwash. You could also have a river running from the
glacier right into a proglacial lake (see below for definition). The river
carries lots of debris and forms a delta as it goes into the lake. After
the glacier recedes and the proglacial lake is gone, the delta may be left
behind and is now a kame.
- outwash plain: the area past the terminus (end) of a glacier that is composed of sorted
outwash (meltwater deposits). Outwash plains can be tremendously big and
- associated bodies of water
- braided streams (also called anastomized): typically found in glaciated areas. Their source is the meltwater runoff
from the glacier. Because they come from the base of the glacier they carry
lots of debris and are choked with sediment. Therefore, the pattern of a
braided stream looks like channels that weave in and out of a plain of sediment.
This sediment is sorted so it is outwash. The meltwater coming off a glacier is so sediment-laden that it is not
clear. Valley glacier runoff can be brown with sediment or milky-looking
with fine suspended particles.
- kettles, kettle lakes: form typically on outwash plains. A block of ice can be left behind, buried in the outwash as the glacier
recedes. When this block of ice finally melts, it leaves a depression in
the shape of the chunk of ice that formed it (typically circular or elliptical).
If the depression ends up filled with water it is called a kettle lake. If not (i.e., if the depression drains), then it is just a kettle.
- proglacial lake: melting ice around a glacier can produce a lake on the glacier's perimeter.
At the bottom of this lake you can get fine sediments, often clays. At the edge of the lake, as with many lakes today, you can get a beach. Therefore, finding lake clays next to sands is a sign that a proglacial
lake used to be there.
See the figures in your book to see what these features look like.
Where the Ohio and Missouri Rivers join the Mississippi is the approximate
southern boundary of the last ice sheet that covered the Great Lakes and
Valley glacier landforms:
- composed of till
- lateral, end, terminal: just as in continental glaciers above
- medial: forms between the main glacier and a tributary glacier (a glacier that
has merged downstream with another glacier) or between 2 tributary glaciers.
Medial moraines do not extend all the way down to the valley floor.
- composed of bedrock
- cirque: rounded, bowl-shaped depressions at the head of the glacier, where snow
accumulates (see zone of accumulation on Nov. 11). Forms when ice scours out the rock as it begins to move downslope.
- are^te (the word arete with a ^ over the first e): sharp, knife-edged ridges separating
- horn: a sharp mountain peak where 3 or more are^tes meet. Most famous example:
the Matterhorn in the Swiss Alps.
- associated with bodies of water
- tarns: present-day lakes within a mountain valley or in a cirque
Valley shapes and erosion:
A glacier that fills a valley is similar to a river. It flows "downstream"
and can join with other valley glaciers.
You can tell if a valley was formed by a river or glacier from the shape
of the valley. A glacier is more effective than a river at eroding a valley.
A river creates a V-shaped valley (in cross-section), because it only erodes along the center of the valley
floor. A glacier creates a U-shaped valley, because it is contact with both the valley walls and valley floor and
therefore erodes more. The size of a glacial valley is proportional to how
big the glacier was.
Hanging valleys are U-shaped valleys which join the main valley but are at much higher
elevation than the main valley floor. They are formed by a tributary glacier
(which occupied the hanging valley) merging with the main glacier (which
occupied the larger, main valley).
One last feature of glacial ice itself is common to valley and continental
glaciers: crevasses, or deep cracks within the ice.
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