2. Minnesota

Question 1

Describe past glaciations in Minnesota:

Answer 1

• Present landscape formed from glacial activity in quaternary period (2.5 million years)
• Several advances and retreats of ice sheets
• Laurentide ice sheet centres Hudson Bay and extended/retreat many times during Quaternary period

Question 2

Describe Laurentide ice sheet:

Answer 2

• 75,000 years ago a series of lobes extended from the Laurentide ice sheet spreading across Minnesota
• Resulted in deposition of tile with different characteristics

Question 3

Describe the geology of Minnesota:

Answer 3

• Part of the Laurentian Shield
• Oldest rocks lie in alternating belts in northern half of the state + Minnesota River Valley - belts are volcanic and sedimentary rocks w granitic material between belts
• Metamorphic gneiss crops out along the Minnesota River Valley dating back 3,600 million years.
• Volcanic and sedimentary rocks began their formation 2,700 million years ago as a result of lava escaping through rifts in the sea floor - many volcanic rocks have metamorphosed into greenstone
• Tectonic activity folded rock formations and caused faults as well as mountain ranges in the north

Question 4

Evidence of mountain erosion:

Answer 4

High mountains were worn down -> highest peaks are now only 500-700m

Question 5

Evidence of ellipsoidal basins:

Answer 5

• A large ellipsoidal basin was created + is now studded with thousands of lakes eg Upper and Lower Red Lakes in northern Minnesota
• Arrowhead region of the northeast, the erosional basin was deep as the earlier tectonic tilting of the landscape exposed weal shale rocks which were eroded much more rapidly than resistant volcanic rocks around them. -> lakes of the region lie in the deeply eroded shales

Question 6

Evidence of striations:

Answer 6

Ice sheet advancement abraded striations into bare rock outcrops of gneiss and greenstone -> their alignment indicating direction of ice advance

Question 7

Evidence of drumlins:

Answer 7

Wadena Lobe formed drumlin field that spanned Otter Tail, Wadena and Todd counties

Question 8

What is the characteristic of the till deposited by the Wadena Lobe?

Answer 8

Advanced from northeast Canada and reached just south of Minneapolis - till deposited by this lobe is characteristically red and sandy, having been derived from the red sandstone and shales to the north and northeast

Question 9

Evidence of terminal moraine:

Answer 9

Terminal moraine extended from northwest St Cloud into Minneapolis and St Paul

Question 10

What type of till did the last advance of the Rainy and Superior lobes leave?

Answer 10

Left a coarse-textured till containing fragments of a salts, gabbro, granite, red sandstone, slate and greenstone strewn across the northeastern half of Minnesota and as far south as the Twin Cities

Question 11

How thick are till deposits?

Answer 11

• In the west of Minnesota have been found to be more than 100m thick as a result of borehole drilling
• In the southwest, boreholes 160m deep had still not reached bedrock.

Question 12

How were proglacial lakes formed?

Answer 12

The edge of ice sheet + associated lobes dammed the natural drainage of area creating a number of proglacial lakes

Question 13

Describe the largest proglacial lake:

Answer 13

Lake Agassiz:

• Glaciers to the north blocked the natural drainage of the area and as ice melted this formed Lake Agassiz
• Covered 440,000 km2
• Overflowed the watershed at Brown’s valley and cut through Minnesota river valley
• River form Lake Agassiz = Glacial River Warren
• When lake drained it deposited fertile silt deposits producing Red River Valley = rich farmland