CHAPTER 4 GLACIERS AND ARID LANDSCAPES
A. Types of glaciers
• definition: “thick ice mass that originates on land from accumulation,
compaction, recrystallization of snow”
• “dynamic erosional agent”, but moves slowly (few cm/day)
• most glaciers @ high latitudes or in mountains
• mountain glaciers are “alpine”
- stream of ice flows downvalley
- narrow and long (up to 60 miles long)
• continental ice sheets much larger than mountain glaciers
-Greenland ice sheet in N. Hemisphere
° 1 mile thick
° 17 million km2
-Antarctic ice sheet in S. Hemisphere
° 3 miles thick
° 14 million km2
- combined area = how much % of Earth land area?
B. How glaciers move
Fig. 4.2
• mechanism is flow by
1. Plastic movement within ice below 165 ft (50 m)
2. Slippage at base of ice, along the ground
• above 50 m depth, ice is brittle, called “Zone of fracture”
this ice carried by ice below
this is zone where crevasses can develop due to tension

flow fastest in glacier where? Fig 4.3

Zones of Accumulation and Ablation (wasteage) Fig 4.2
-accumulation: (net gain) snow thickens, drives flow
- wasteage (net loss): glacier melts here, icebergs created
through “calving”
- icebergs by the thousands calve off Greenland &
Antarctic every year
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• ice budget dictates advance, retreat, or stationary position
-budget is balance between accumulation and wastage
• glacial ice always moves forward, carrying sediment as if it’s on a
conveyor belt.
C. Glacial Erosion
• huge erosional agent, especially in past Ice Ages
• as Sediment transporter, “has no equal”
-once ice picks something up, it stays in transport until it
reaches terminus
• How glaciers erode
-plucking; freeze-thaw in cracks loosens blocks of rock
-abrasion; bedload of rock fragments in base of ice scours
ground
° produces glacial “rock flour”, v fine grained
° produces striations on bed rock; allows for mapping
of flow patterns
• Mt. Glaciers produce variety of features Fig. 4.6
-glacial troughs (U shaped valley created from V shaped) Fig 4.7
° main glaciers deep, tributaries produce hanging valleys
Fig 4.8
- cirques -- three sided bowl at head of valley - site of
accumulation
- aretes are bedrock edges of cirques
- horn is a peak surrounded by cirques
most famous “horn” is
??
-fjords - u shaped valleys that become sea inlets (Norway, Alaska,)
Fig. 4.11
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D. Glacial Deposits
• types: many and varied, “drift” encompasses all
origin of term from belief these deposits were “Drifted in” by
floating ice during an ancient flood
- 2 main types of glacial deposits
1. laid down by glacier itself “till”
2. laid down by meltwater from glacier “stratified drift”
“till”is unsorted; different grain sizes dumped together
“stratified drift” is sorted like any deposit in running water;
usually stratified drift deposits are sand and gravel, because fine silt
& clay is carried away, and boulders stay near terminus
“glacial erratics” - boulders that come from somewhere else. They
help geologists trace the path of the glacier.
• depositional features of glaciers
Fig. 4.15
-”moraines” ridges or layers of till
“lateral” - along side of a valley wall (parallel to
movement)
“medial” - joining of 2 lateral moraines (parallel to
movement)
“end” - at terminus (perpendicular to movement)- 2
types
1. “terminal moraine” @ farthest advance - 10s to
100’s
miles high
2. “recessional moraine” - periodic stationary points
during retreat
“ground” - horizonal layer laid down during retreat levels the ground by filling in low spots
- outwash plains
Fig. 4.15
° sand and gravel deposits adjacent to end moraines
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features of outwash plains include:
° “kettles” - depressions once filled with blocks of ice while
sediments deposited around them; often the site of lake or pond
° “drumlins, drumlin fields” - assymetric hills, steep side facing
direction from which ice advanced,
point like arrows in flow direction
molded in the zone of flow in active glacier,
advancing over previously deposited drift
° “eskers” - ridges of sand and gravel from streams flowing
in tunnels beneath the ice, near terminus
° “kames” - compositionally like eskers, but sedimentsare in
piles, rather than long sinuous deposits
E. Glaciers of the Past
Fig. 4.18
• “Ice Age” a complex sequence of glacial and inter-glacial stages
• Almost 30% of Earth land area covered by ice at maximum
• Latest began 2-3 million years ago (“Pleistocene epoch”)
• Older glaciations at 2 billion (Precambrian),600 million
(Cambrian), and 250 million (Permian) years ago
F. Indirect Effects of Glaciers
• Ice advance/retreat forced animals, plants to migrate
• Rivers changed course
• Crustal rebound associated with removal of weight of ice
• worldwide sea level changes
-Sea level falls when glaciers large
in the recent past,
-East Coast 60 miles to east of NYC
-France, England connected (English Channel dry)
-Land bridge across Bering Strait
-Sea level rises when glaciers retreat…… Global Warming might do
WHAT??
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G. Deserts
Fig. 4.21
• term literally from “deserted” or “unoccupied”
• Distribution and causes of deserts
- largest % of Earth land surface (30%)
-2 climate types
1. desert (arid)
2. Steppe (semi-arid ; transition from desert to more
humid
• primarily low and mid-latitude
• air currents don’t produce much precipation
• far removed from oceans (Asia, Africa, Australia or often in rainshadow)
Fig. 4.22
H. Role of Water in deserts
• ephemeral (intermittent) vs perennial streams
arid climate
humid climate
• erosional impactof short-lived flooding can be big, though infrequent
• streams in deserts not fed by groundwater, usually small and don’t
make it to sea
• exceptions such as Nile and Colorado originate outside the deserts
they cross
I. Great Basin - Evolution of a Landscape
• Look at Basin and Range province
-interior drainage never reaches sea
- erosion lowers mountains and fills basins
- sediment apron forms from coalescing fans
J. Wind Erosion
• negligible in humid regions - moisture & vegetation holdthings
• extensive in arid areas –
little moisture or vegetation in 1930’s Dust Bowl of Great Plains;
a trigger to erosion was plowing under of upper vegetative cover.
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• wind has similarities to water, but differences too…
-wind can’t tranport coarse material
- wind not confined to channels
• “deflation” is transport of loose material that ultimately lowers or
deflates ground level
• “saltation” describes movement of wind’s bed load by skipping or rolling
• “blowouts” – depressions caused by deflation
• “desert pavement” - coarse grained pebbles and cobbles left after
fines
removed after 100’s of years – can easily be
disrupted by 4x4s, for example
K. Wind Deposits
• sometimes significant
• 2 types:
1. extensive windblown silt (loess)
2. mounds and ridges of sand (dunes)
-loess - sources are deserts and glacial strat. drift
in US, much loess picked up from glacial drift deposits.
-sand dunes
° as sand accumulates in dunes, it traps even more
sediment
° assymetric profile - leeward slope is 34°, angle of repose
of sand grains on slip face
° dune migrates as sand climbs upwind ward side, falls
down lee side
° preserved slip faces create cross beds Fig 4.29