1. Glaciers are bodies of ice that form from the accumulation of snow and
advance because of gravity. Snow accumulating on the ground (firn) is a sedimentary
rock that coverts to glacial ice, a metamorphic rock, by compaction (pressure)
and recrystallization (heat).
2. Glaciers exist as either valley glaciers, occupying old stream valleys
at high elevations, or continental glaciers, ice sheets covering wide continental
areas at lower elevations.
3. The zone of accumulation lies above the equilibrium line and has perennial
snow cover, while ice is lost in the zone of ablation, below the equilibrium
line, by evaporation, melting or calving at the terminus. Advancing glaciers
have a positive ice budget (accumulation > ablation), while retreating
glaciers have a negative ice budget (accumulation < ablation).
4. Valley glaciers move by basal sliding against the bedrock in response
to gravity. The lower portion of glaciers exhibits plastic flow from basal
sliding and deformation, while the upper portion of a glacier is rigid, exhibiting
crevasses because it is brittle. Ice sheets, such as those of Antarctica,
exhibit plastic flow almost exclusively and no basal sliding.
5. Rock fragments transported at the base of a glacier are effective erosional
agents. Erosion by valley glaciers produces most of the scenery in mountainous
regions. Striated and groove bedrock is a common erosional feature of both
alpine and continental glaciation. Valley glaciers produce U-shaped valleys,
truncated spurs, hanging valleys and rock-basin lakes as they modify old stream
valleys. Cirques, horns and arêtes are formed by valley glaciers in
combination with weathering and erosion.
6. Glacial deposits are either unsorted, unlayered, angular rock debris
called till, or sorted, layered material called outwash that is deposited
by meltwater. Moraines are ridges of till formed by valley and continental
glaciers while the terminus is stationary. Both can form end, terminal, recessional,
and ground moraines. Valley glaciers also form lateral and medial moraines
because of rockfalls along their edges and convergence into trunk glaciers.
Continental glaciers deposit drumlins, spoon-shaped hills of till with a gentle
dip in the direction of ice movement. Outwash forms in front of a glacier
from meltwater. Eskers are deposited by continental glaciers from meltwater
flowing under the ice.
7. Other depositional features associated with continental glaciers are
kettles, rock flour and loess, varved lake deposits, and ice-dammed lakes.
8. Not all glaciation is associated with the Pleistocene. The oldest glaciations
occurred about 2.3 billion years ago. Late Paleozoic glaciations are evidence
for continental drift, and even the most recent ice ages began 2 or 3 million
years ago. Mean temperature was only 5 degrees C lower than current temperatures at
the height of Pleistocene glaciations.
9. Much of the current landscape is the result of glacial effects. Scoured
bedrock in Canada and thick tills in the central United States were directly
caused by continental glaciation. The Great Lakes were eroded by continental
glaciers as well. Indirect effects of glaciation are pluvial lakes, such as
the Great Salt Lake, formed by abundant rainfall associated with the Ice Ages.
As glaciers advanced during the Ice Ages, sea level fell, causing fiords along
the coastlines of Alaska, Norway and elsewhere. Uplifted and tilted shorelines
around the Great Lakes indicate crustal rebound from removal of the weight
of continental ice sheets.
