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Glaciers, The Ice Age & Paleovegetation pkg. Lectures from March 17th and 22nd, Including the three broad topics in the title and all their subdivisions.

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University of Toronto St. George
Sarah Finkelstein

Glacial Processes and the Quaternary Ice Age//Chp. 17 th Glacial Processes Agents of Erosion: Wind/water/ice act to transport materials that’ve been loosened by physical/chemical weathering and mass wasting. Ice: PWRful AoErosion b/c can move massive amnts. of sediment -Ice axn restricted to glacial periods -Glacial vs. Interglacial Periods > Glacial: significant area of Earth covered in ice; much glacial erosion potential > Interglacial: we’re in one. Ice sheets have retreated north to Greenland/mountains/Arctic , but ice is still available in high latitudes/elevations What is a glacier? -Large mass of flowing land ice; plastic body that flows. -Ice on land under pressure begins to flow, becoming a glacier -Mass balance: glacier fed (accumulates) from precipitation in form of snow at the top, and ablates (melts) at the bottom. > Can’t form in dry, high temperature areas. >Glaciers form in conditions of low temperature AND adequate snowfall/ppt. >Firn: Beginning of a glacier. > In certain areas, snowfall from winter doesn’t melt in summer and continues to accumulate. Persisting snow is compacted over time into glacial ice (firn), due to intense pressure from the heaviness of piling precipitation. How do glaciers move? - Under pressure, ice loses its rigidity. Because of this pressure it becomes plastic, able to flow, and moves downslope like water (distinguished from normal brittle ice as we know it). Can flow downslope over landscapes without breaking. Will flow downslope direction despite accumulation or ablation. -Severe erosion: Glaciers can be 1000s of metres in height; weight transports huge amnts of entrained material as it flows, plucking material from ground downslope to the lowlands Mass Balance -determines whether a glacier is growing or shrinking -If there’s more ablation than accumulation, there is a retreat of the terminus (end/bottom of slope). Equilibrium States -Glacier is in equilibrium when ablation = accumulation (flow is continual however) >Changes in ablation/accumulation rate correspond to glacier advance/retreat and a new equilibrium x Glacial retreat, Quelcccaya Ice Cap in Peru (5400 m above sea LVL): procession of glacier up the valley in proglacial lake >Proglacial lake: contains meltwater off the glacier, gated by sedimentary border left over from glacier’s movement Glacial Processes and the Quaternary Ice Age//Chp. 17 th Alpine glaciers – rivers of ice that begin formation up in mountains then flow downslope, excavating… > …distinctive U-shaped valleys -3 stages of glacier-created landforms: 1). mass wasting: basic rivers flowing down mountainsides, v-shaped valleys formed in mountainsides. 2). glaciations: environment changes and glaciers begin forming during glacials. >Erosional landforms of Glacials: cirques: are bowl shaped depressions of heads of valleys, representing where glaciers are formed, and where they first start excavating those mountainside valleys arêtes: when cirques form adjacent to each other in different orientations horns: when at least 3 cirques form on diff. faces of mountain >Depositional landforms of Glacials x will only form in lowlands, not in uplands b/c of steep slopes and mass-wasting moraines: endpts. of glacier movement, after it has crushed a lot of sediment on its way down a slope and deposits crushed material at its base (terminus), as ablation sets in 3). ice recession: main glacial trough is much steeper than the tributaries, creating ‘hanging valleys’ where waterfall forms b/c of steepness. >Features resulting from ice recession: tarn: mountain lake that forms during ice recession Paternoster lakes: series of cirques with tarns inside that form next to each other in the shape of rosary beads fjords: U-shaped valleys carved out by glaciers, then flooded by ocean after glaciers recede. Their formation is tied to sea LVL. Ice sheets - If temperatures stay cold enough in summer, ice accumulates when in flows into lower elevations/latitudes (lowlands), and become coalitions of ice sheets that can blanket entire landscapes. This happened during Pleistocene in Canada. -Only ice sheets in the Antarctic (~14 mill. km ) but also in Greenland (~1.7mill. 2 km ) exist today, and they cover entire continent(s) besides their coasts > Because we’re in an interglacial, no glaciers in lowlands, but yes glaciers in Antarctic/Greenland March 22 ndlecture starts at Continental glaciers Continental glaciers: The importance of depositional landforms -Entrainment (plucking, abrasion…) transport and deposition of eroded material in lowlands from uplands Glacial Processes and the Quaternary Ice Age//Chp. 17 th Ice-marginal landforms - much sediment/water from glaciers so typically braided steam situation flowing from glaciers during glacial, onto outwash plains (land that braided streams flow onto at terminus of glaciers). -meltwater from glaciers becomes impounded behind topographic features; marginal lake ( lakes formed next to margin of glaciers). Deltas: sometimes when glaciers ablate with lots of meltwater, flow creates landforms and becomes stream flowing under landforms. When stream hits lake, cross-sectional area becomes big so flow velocity decreases and heavier sediments are deposited. Unlike till plain, deltas are stratified due to certain stream competencies. e.g. Nile River >Delta kame: Eventually, delta turns into a pile of sand and becomes Delta “kame” b/c water isn’t flowing around it anymore. Till plain: sediment left behind by glacier, usually ends up having deranged drainage system. >Few important things; till deposits only found where glacier was actually sitting on the landscape. Material that was entrained & transported over time. Till is, as a result, unsorted/unstratified sediment. x Moraine: subset of till; represents the terminal position of the ice. >ridge/rim where glacier used to be/its extent; ablation starts and this ride of debris becomes the ridge, the moraine. Moraines trace out the edges of glaciers. > can be many moraines in a given area with previous glacial action e.g. many in Ontario >Many types; medial, lateral, recessional, terminal. >Glaciers scrape the landscape and keep adding till to themselves; when they melt, moraine is the ridge/rim leftover. >Most of sediment material is very coarsely-grained. Can’t grow much on top. >During Pleistocene, moraines would be deposited and picked back up numerous times, erasing previous moraines and creating new ones, creating the complex topographic outlook of Southern Ontario. Kettle: blocks of ice get stuck in till, then sediment forms around block of ice, which eventually forms a kettle lake when melted Glacio-Fluvial sediments: deposited by fluvial flow of meltwater from glacier. Fluvial = “flow/rivers” etc. Stratified/sorted sediment. Glacio-lacustrine sediments: deposited by proglacial lakes. Lacustrine = “Comes from a lake”. Stratified/sorted, esp. finely grained sediment. Glacial Drift Drift: Generally thick material that has been deposited by ice, can be thin too. Examples of glacial drift: >Till (unsorted/unstratified), Glacio-fluvial (meltwater;sorted),Glacio-lacustrine (proglacial lakes;sorted) -Loess: wind deposited sediment Glacia
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