ice sheet - Minnesota

• what impact do ice sheets have on a landscape?

  - when the ice sheet flows (basal sliding & gravitational flow) across soft sediment it makes the landscape bumpy

  - bumps are called sub-glacial bedforms

  - drumlins are an example

  - ice flows within the ice sheet via internal deformation
• what are the details of glaciation that formed the ice sheet?

  - 75000 yrs ago a series of lobes or tongues of ice extended from the main Laurentide ice sheet and spread across Minesota

  - the lobes advanced and retreated repeatedly transporting and depositing till
• what are the depositional impacts on the landscape?

  - wadena lobe, advanced from Northeast Canada and reached just south of Minneapolis

  > till is red & sandy from the red sandstone and shales to the N and NE

  > first deposited the Alexandra moraine, the drumlin fields spanning Otter tail, wadena and Todd counties and finally the Itasca Moraine

  > ground moraine with reddish iron rich sediment extend from st cloud NE. glaciers produced formed a set of moraines

  > last advance of the rainy and superior lobes left a coarse - textured till containing abundant fragments of basalts, gabbro, granite, red sandstone, slate and greenstone

  > des moines lobe deposited till that is tan to buff and clay rich

  > till deposited more than 100m. 160m in SW
• describe the proglacial lakes formed

  - 12000 years ago the glaciers melted back to the topographic divide. The glacial meltwater was trapped and accumulated creating lake Agassiz (covered about 123,500 miles, 400 feet depth)

  - drained in a variety of different directions

  - overstepped a moraine dam and an outlet river was created, drained south and carved the Minnesota river valley
• what were the erosional impacts on the landscape?

  - Laurentide ice sheet is over 1km thick in some places

  - highest mountains were wore down, such today the highest peaks are now only 500-700m high

  - a large, elipsoidal basin was created by the erosion and is now studded with thousands of lakes

  - arrowhead region of NE the erosional basin was particularly deep as the earlier tectonic tilting of the landscape exposed weak shale rocks which were eroded more rapidly

  - as the lobes advanced they abraded striations in bare rock outcrops of gneiss and greenstone

  - the far SE was not so extensively covered by the ice sheet and so retains a more varied landscape of steep hills and deep valleys
• describe Minnesota's geology


  - oldest rocks lie in alternating belts in the Northern half of the state. The belts are of volcanic and sedimentary rock. Granite lies between

  - metamorphic gneiss crops date back 3600 mill yrs. Sedimentary 2700mill yrs. Lava escaped sea floor