Minnesota

• Glaciation


  75000 years ago, a series of lobes of ice extended from the main Laurentide ice sheet and spread across Minnesota;

  Lobes advanced and retreated many times, transporting and depositing till;

  different origins of the lobes resulted in tills with different characteristics and materials.
• The Laurentide ice sheet in the Quaternary period (2 million years ago to the present) formed the present landscape of Minnesota.
• the Laurentide ice sheet was centred in what is now the Hudson Bay.
• The Laurentide ice sheet was 5 million square miles wide, and 2.5 miles thick.
• Metamorphic gneiss

  Crop out along the Minnesota River Valley dating back 3600 million years;

  Volcanic and sedimentary rocks began forming 2700 million years ago;

  Volcanic rock formed massive layers of sedimentary rock;

  tectonic compression formed a mountain range in Northern Minnesota;

  many volcanic rocks have metamorphosed into greenstone.
• Minnesota‘s oldest rocks lie in alternating belts in the northern half of the state and much of the Minnesota River Valley.
• High mountains were eroded over time by the ice sheet, with the highest mountains current being 500 to 700 metres.
• Ellipsoidal basin

  created by erosion;

  now studded with thousands of lakes, such as Upper and Lower Red Lakes in northern Minnesota;

  Arrowhead region (NE) is the deepest point of the basin, as earlier tectonic tilting of the landscape exposed weak shale rocks which eroded rapidly.
• the far SE of Minnesota was not as extensively covered by the ice sheet, and so has steeper hills and deeper valleys.
• Lake Agassiz


  WAS a proglacial lake partly in PRESENT Red River valley (Minnesota and North Dakota);

  Formed by glaciers in the north blocking the natural northward drainage of the area;

  at maximum it covered 440000 km squared (equivalent to present day Black Sea);

  resulting discharge helped the adjacent Mississippi River to form a very large valley in southeastern Minnesota;

  River that drained from the lake was Glacial River Warren which flowed over the top of a recessional moraine at Brown’s Valley;

  When the lake drained, it left behind fertile silt for farmland.
• Wadena Lobe


  Advance from NE Canada and reached just south of Minneapolis;

  Till is red and sandy, being derived from sandstone and shales;

  first deposited the Alexandria moraine;

  formed the drumlin fields spanning Otter Tail, Wadena and Todd countries;

  formed the Itasca moraine.
• Des Moines Lobes

  tan coloured till that is clay-rich and calcareous because of the shale and limestone rocks at its source to the NW;

  in the SW, Prairie Coteau has an end moraine.
• Many till deposits in W Minnesota have been found by borehole drilling to be more than 100 m thick; In the SW, boreholes 160 m depth deep still had not reached bedrock.
• ground moraine with reddish iron-rich sediments extends from St Cloud to NE; glaciers produced formed a set of terminal moraines which extend into the Twin Cities (Minneapolis and St Paul).
• the last advance of the Rainy and Superior Lobes left a coarse-textured till contains abundant fragments of basalts, gabbro, granite, red sandstone, slate and greenstone strewn across the NE of Minnesota and as S as the Twin Cities.
• the lakes in the arrowhead region lie in deeply eroded shales