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Glacial Systems
• An open system of flowing ice
– Water input as snow
– Transformed into ice
– Ice flows under pressure (starts 60 m thk) – Water leaves by evaporation and melting
Types of Glaciers
• Glaciers are divided into 2 subsystems
I) Valley glaciers
• Ice streams that flow from mountain areas
down valley
II) Continental Glaciers
• Huge ice sheets (commonly 3000 m thick)
• Cover most of underlying terrain
Sediment Input: Erosion
• Powerful agents of erosion
– Up to 0.35 mm/yr; during Pleistocene approx. 20 m bedrock eroded in Canada
– Ice wedging (glacial plucking)-freeze/thaw
– Abrasion
• Glacial striations form parallel to the direction
of ice flow; glacial grooves; polish
Ice Flow
• Ice is brittle
– Differential flow velocity creates tensional stress
– Crevasses form as a result of differential flow
• Generally forms at a right angle to flow
Sediment Output – Evaporation, Melting
• Removal of ice from the system – Melting is the major process
• Influenced by temp, climate factors, surface debris on ice
• Streams may exist on the ice surface or subglacial
– Calving occurs where glaciers enter the sea
Valley glacier geomorphic features:
– Cirque- bowl shaped depression; head of
glacier
– Arete- A sharp crested ridge bounded by 2 cirques
– Horn - A sharp peak bounded by 3 or more cirques
– Hanging valleys - characteristic U shape valley of a smaller glacier
– Terminal moraines – sediment deposited at glacier terminus
– Outwash plains- moraines reworked- braided streams
Continental Glacier Systems
• Most important glacial system – Strongly modify entire landscape
– Cause crustal subsidence – Modify/destroy previous drainage systems – Continentsized ice sheets
– Antarctica (4500 m thick!) and Greenland
Continental Glacier Systems
• Geomorphic features
– Till – def.: unsorted, unstratified gravels, sands, silts, clays - deposited beneath ice
- Drumlins – streamlined, tearshaped hills
- Eskers - melt water stream depositssinuous in shape
- Kettles –potholes from ice blocks
– Proglacial lakes
– Terminal/recessional moraines; outwash
Pleistocene Glaciation
• Global event that disrupted hydrologic cycle
– Began 2-3 mya; retreated 15-20,000yrs ago – 4 major periods of glaciation
– >100 m drop in sea level
• Modified drainage systems
– Pre-glacial drainage: NE through Canada
– Glaciation produced the Mississippi - Missouri - Ohio river system
• Created numerous lakes
– Ice scours bedrock
– Large lakes created along margins
– Moraines create closed basins, plug valleys
– Repeated rise and fall of sea level during glacial and interglacial periods
• Topography of continental shelf provides evidence; sea level dropped more than 100 m
– Rain fed lakes developed in arid & semi- arid regions – playa lakes
– Lake Bonneville -> Great Salt Lake
– Continents continue to rebound post glaciation
• Severe climate change shifted ecology
– Plants died and animals migrated
• Species displaced ~ 3200 km
• New migration routes created by lower sea level – Great Britain connected to Asia;
Asia & N. Am. Connected by land bridge across Bering Strait
– Areas covered by ice depopulated
– Numerous extinctions especially in large mammals