Minnesota Ice Sheets

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  • Minnesota Ice Sheets
    • Info
      • Canada-USA border, west of Lake Superior
      • Cold base glacier, but with warm characteristics because increasing temp
      • landscape is result of quaternary glaciation
      • gigantic Laurentide ice sheet advanced and retreated with climate changes
    • Laurentide Ice Sheet
      • Covered millions of km. Up to 2 miles thick in Canada + USA
      • ice sheet had signif impact on geology
      • gouged out the great lakes
    • Geology
      • oldest rocks lie beneath alternating volcanic/sedimentary belts in North
        • between these belts lie granitic rock materials
      • volcanic and sedimentary rocks formed 2.5bil years ago when lava escaped sea floor
      • Gneisses (metamorphic granite+sedimentary rocks) were established (failed to form mountain ranges)
    • Glaciation
      • 75k years ago, 4 main lobes extended from Laurentide ice sheet and spread across Minnesota, depositing widespread till
      • Land sculpted by ice over 2 mil years
      • Different origins of lobes resulted in tills with diff. characteristics + materials
    • Erosional Impact
      • massive, wore down many mountains, highest peaks are 500-700m, highest Eagle mountain (701m)
      • created ellipsoidal basin containing many lakes
        • particularly deep in arrowhead region due to weak sedimentary rock
      • early tectonic tilting exposed less resistant shales, forming lakes
      • glacial striations due to abrasion beneath ice
      • most steep hills + valleys in Minnesota are result of fluvial processes
    • Depositional Impact
      • Wadena ice lobe deposited drumlins + glacial drift in Wadena county + central Minnesota
        • drumlin field has fanlike form with long axis tending southwest, suggests ice movement from NE
          • ground moraines with reddish iron
      • rich sediments characteristic of red sandstone and shales from N Minnesota
      • Rainy and Superior lobes left coarse texture till with fragments of basalt, gabbro, red sandstone, slate and greenstone
        • reflects shale and limestone rocks lobe passed over, till deposits up to 160m depth
      • Des Moines lobe deposited tan/buff till -> clay rich
    • Pro-glacial Lakes
      • meltwater is released during interglacial periods
        • results in formation of proglacial lakes
          • edge of Laurentide ice sheet dammed natural drainage of area blocking meltwater from natural northward passage to sea
            • thereby creating pro-glacial lakes (ie Lake Agassiz, covered 440k km and 400m deep)


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