A glacier is a large, perennial mass of ice that forms on land and moves under
the influence of gravity. A glacier can form wherever more snow accumulates
than is lost. Ice sheets and valley glaciers are the two most important types of
glaciers. Glaciers move downward from where the most snow accumulates
toward where the most ice is wasted.
A glacier moves both by basal sliding and by internal flow. The upper portion of
a glacier tends to remain rigid and is carried along by the ice moving beneath
Glaciers advance and recede in response to changes in climate. A receding
glacier has a negative budget and an advancing one has a positive budget. A
glacier's budget for the year can be determined by noting the relative position
of the equilibrium line.
Snow recrystallizes into firn, which eventually becomes converted to glacier ice.
Glacier ice is lost (or ablated) by melting, by breaking off as icebergs, and by
direct evaporation of the ice into the air.
A glacier erodes by plucking and the grinding action of the rock it carries. The
grinding produces rock flour and faceted and polished rock fragments. Bedrock
over which a glacier moves is generally polished, striated, and grooved.
A mountain area showing the erosional effects of alpine glaciation possesses
relatively straight valleys with U-shaped cross-profiles. A glacial valley often
has a cirque at its head and descends as a series of rock steps. Small tarn lakes
are commonly found along the steps and in cirques.
A hanging valley indicates that a smaller tributary joined the main glacier. A
horn is a peak between several cirques. Arêtes usually separate adjacent glacial
A glacier deposits unsorted rock debris or till, which contrasts sharply with the
sorted and layered deposits of glacial outwash. Till forms till plains, drumlins,
and various types of moraines.
Fine silt and clay may settle as varves in a lake in front of a glacier, each pair
of layers representing a year's accumulation.
Multiple till deposits and other glacial features indicate several major episodes
of glaciation during the late Cenozoic Era. During each of these episodes, large
ice sheets covered most of northern Europe and northern North America , and
glaciation in mountain areas of the world was much more extensive than at
present. At the peak of glaciation about a third of the Earth's land surface was
glaciated (in contrast to the 10 percent of the land surface currently under
glaciers). Warmer climates prevailed during interglacial episodes.
The glacial ages also affected regions never covered by ice. Because of wetter
climate in the past, large lakes formed in now-arid regions of the United States.
Sea level was considerably lower.
Glacial ages also occurred in the more distant geologic past, as indicated by
late Paleozoic and Precambrian tillites.
Permafrost is ground that has remained frozen for two or more years and can
reach thicknesses of up to several hundred metres in high-latitude regions.
Ground ice, ice wedges, pingos, and patterned ground are found in areas
underlain by permafrost. Thermokarst is the subsidence and collapse of the
ground surface caused by thawing of permafrost. Dealing with permafrost is a
major issue for urban development in Canada's north. Chapter 13-Waste
Mass wasting is the movement of a mass of debris
(soil and loose rock fragments) or bedrock toward the
base of a slope. Movement can take place as a flow,
slide, or fall. Gravity is the driving force. The
component of gravitational force that propels mass
wasting is the shear force, which occurs parallel to
the slopes. The resistance to that force is the shear
strength of rock or debris. If shear force exceeds
shear strength, mass wasting takes place. Water is an
important factor in mass wasting.
A number of other factors determine whether
movement will occur and, if it does, the rate of
The slowest type of movement, creep, occurs mostly
on relatively gentle slopes, usually aided by water in
the soil. In colder climates, repeated freezing and
thawing of water within the soil contributes to creep.
Landsliding is a general term for more rapid mass
wasting of rock, debris, or both. Flows include creep,
earthflows, mudflows, and debris avalanches.
Earthflows vary greatly in velocity although they are
not as rapid as debris avalanches, which are turbulent
masses of debris, water, and air. Gelifluction, a
special variety of earthflow, usually takes place in
arctic or subarctic climates where ground is
permanently frozen (permafrost). A mudflow is a
slurry of debris and water. Most mudflows flow in
channels much as streams do.
Rockfall is the fall of broken rock down a vertical or
near-vertical slope. A rockslide is a slab of rock sliding
down a less-than-vertical surface. Debris falls and
debris slides involve unconsolidated material rather
than bedrock. Landslides also take place underwater.
The larger ones of these are vastly bigger than any
that have occurred on land. Chapter 18-Beaches
Wind blowing over the sea surface forms waves, which transfer some of the
wind's energy to shorelines. Orbital water motion extends to a depth equal to
half the wavelength.
As a wave moves into shallow water, the ocean bottom flattens the orbital
motion and causes the wave to slow and peak up, eventually forming a
breaker whose crest topples forward. The turbulence of surf is an important
agent of sediment erosion and transportation.
Wave refraction bends wave crests and makes them more parallel to shore.
Few waves actually become parallel to the shore, and so longshore currents
develop in the surf zone. Rip currents carry water seaward from the surf zone.
A beach consists of a berm, beach face, and marine terrace. Summer beaches
have a wide berm and a smooth offshore profile. Winter beaches are narrow,
with offshore bars.
Longshore drift of sand is caused by the waves hitting the beach face at an
angle and also by longshore currents.
Deposition of sand that is drifting along the shore can form spits and
baymouth bars. Drifting sand may also be deposited against jetties or groins
or inside breakwaters.
Rivers supply most sand to beaches, although local erosion may also
contribute sediment. If the river supply of sand is cut off by dams, the
beaches gradually disappear.
Coasts may be erosional or depositional, drowned or uplifted, or shaped by
organisms such as corals and mangroves.