The following statements summarize and describe many of the key terms and concepts presented
in the chapter.
- A glacier is a thick mass of ice originating on land from the
compaction and recrystallization of snow, and it shows evidence of past or present flow.
Today, valley or alpine glaciers are found in mountain areas
where they usually follow valleys that were originally occupied by streams. Ice
sheets exist on a much larger scale, covering most of Greenland and Antarctica.
- Near the surface of a glacier, in the zone of fracture, ice is
brittle. However, below about 50 meters, pressure is great, causing ice to
flow like a plastic material. A second important mechanism of
glacial movement consists of the whole ice mass slipping along the ground.
- Glaciers form in areas where more snow falls in winter than melts during summer. Snow
accumulation and ice formation occur in the zone of accumulation. Beyond this
area is the zone of wastage, where there is a net loss to the glacier. The
glacial budget is the balance, or lack of balance, between accumulation at the
upper end of the glacier, and loss at the lower end.
- Glaciers erode land by plucking (lifting pieces of bedrock out of
place) and abrasion (grinding and scraping of a rock surface). Erosional features produced
by valley glaciers include glacial troughs, hanging valleys, cirques, arêtes,
horns, and fiords.
- Any sediment of glacial origin is called drift. The two distinct types
of glacial drift are (1) till, which is unsorted sediment deposited directly
by the ice; and (2) stratified drift, which is relatively well-sorted sediment
laid down by glacial meltwater.
- The most widespread features created by glacial deposition are layers or ridges of
till, called moraines. Associated with valley glaciers are lateral
moraines, formed along the sides of the valley, and medial moraines,
formed between two valley glaciers that have joined. End moraines, which mark
the former position of the front of a glacier, and ground moraine, an
undulating layer of till deposited as the ice front retreats, are common to both valley
glaciers and ice sheets.
- Perhaps the most convincing evidence for the occurrence of several glacial advances
during the Ice Age is the widespread existence of multiple layers of
drift and an uninterrupted record of climate cycles preserved in seafloor
sediments. In addition to massive erosional and depositional work, other effects of
Ice Age glaciers included the migration of organisms, changes in stream courses,
adjustment of the crust by rebounding after the removal of the immense load of ice,
and climate changes caused by the existence of the glaciers themselves. In the
sea, the most far-reaching effect of the Ice Age was the worldwide change in sea
level that accompanied each advance and retreat of the ice sheets.
- Any theory that attempts to explain the causes of glacial ages must answer the two
basic questions: (1) What causes the onset of glacial conditions? and (2) What caused the
alternating glacial and interglacial stages that have been documented for the Pleistocene
epoch? Two of the many hypotheses for the cause of glacial ages involve (1) plate tectonics
and (2) variations in Earth's orbit. Other factors that are related to climate change during
glacial ages include changes in atmospheric composition, variations in the amount of
sunlight reflected by Earth's surface, and changes in ocean circulation.
- Practically all desert streams are dry most of the time and are said to be
ephemeral. Nevertheless, running water is responsible for most of the
erosional work in a desert. Although wind erosion is more significant in dry areas
than elsewhere, the main role of wind in a desert is in the transportation and deposition of
sediment.
- Many of the landscapes of the Basin and Range region of the western and southwestern
United States are the result of streams eroding uplifted mountain blocks and depositing the
sediment in interior basins. Alluvial fans, playas, and playa
lakes are features often associated with these landscapes.
- For wind erosion to be effective, dryness and scant vegetation are essential.
Deflation, the lifting and removal of loose material, often produces shallow
depressions called blowouts and can also lower the surface by removing sand and silt,
leaving behind a stony veneer called desert pavement. Abrasion,
the sandblasting effect of wind, is often given too much credit for producing desert
features. However, abrasion does cut and polish rock near the surface.
- Wind deposits are of two distinct types: (1) extensive blankets of
silt, called loess, carried by wind in suspension, and
(2) mounds and ridges of sand, called dunes, which are formed
from sediment that is carried as part of the wind's bed load.
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