Laurentide Ice Sheet Flashcards
1
Q
How thick was the ice sheet?
A
4km
2
Q
When was the most recent advance of the Laurentide ice sheet?
A
100,000 years ago
3
Q
What was the last advance called?
A
Wisconsin Glaciation
4
Q
As the ice was so thick and heavy, what did this do to the ground?
A
Caused the isotatic lowering of the surface landscape
5
Q
Current Geology
A
- Minnesota’s oldest rocks lie in alternating belts in the northern half of the state and much of the Minnesota River Valley.
- The beits are of volcanic and sedimentary rocks, granitic rock materials lie in the areas between the belts.
- Metamorphic gneiss crops out along the Minnesota River Valley dating back 3600 million years
- Volcanic and sedimentary rocks began their formation 2700 million years ago, ago, when lave escaped through rifts in what was then the sea floor.
6
Q
Describe the Glaciation
A
- Around 75,000 years ago, a series of lobes or tongues of ice extended from the main Laurentide ice sheet and spread across Minnesota (Figure 2.21).
- These lobes advanced and retreated a number of times, transporting and depositing til across a wide area.
- The different origins of these lobes resulted in tills with different characteristics and materials.
7
Q
Describe the Erosional impact
A
- The Laurentide ice sheet, over 1 km thick in places, and its lobes had a massive erosional impact on the landscape.
- The high mountains were worn down, such that today the highest peaks are now only a modest 500-700 m.
- A large. ellipsoidal basin was created by this erosion and is now studded with thousands of lakes, such as Upper and Lower Red Lakes in norther Minnesota.
- In the Arrowhead region of the northeast the erosional basin was particularly deep as the earlier tectonic tilting of the landscape exposed weak shale rocks which were eroded much more rapidly than the resistant volcanic rocks around them. Thus the lakes of this region lie in the deeply eroded shales.
- As the lobes of ice advanced they abraded striations in bare rock outcrops of gneiss and greenstone, their alignment indicating the direction of ice advance.
- The far southeast of the state was not extensively covered by the ice sheet and so retains a more varied landscape of steep hills and deep valleys.
- Most of the rivers draining this area are tributaries of the Minnesota and Mississippi Rivers.
- The erosional impact of the Laurentide ice sheet was therefore considerable and shaped the overall landscape.
8
Q
Describe the Depositional Impact
A
- The Wadena Lobe advanced from northeast Canada and reached just south of Minneapolis. The till deposited by this lobe is characteristically red and sandy, being derived from the red sandstone and shales to the north and northeast.
- The Wadena Lobe first deposited the Alexandria moraine, formed the drumlin fields spanning Otter Tail, Wadena and Todd counties, and finally formed the Itasca moraine.
- Ground moraine with reddish iron-rich sediments extends from St Cloud northeastward. The glaciers produced formed a set of terminal moraines which extend from northwest of St Cloud into the Twin Cities (Minneapolis and St Paul).
- The 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 strewn across the northeastern half of Minnesota and as far south as the Twin Cities.
- The Des Moines Lobe deposited till that is coloured tan to buff and is clay-rich and calcareous because of shale and limestone rocks at its source to the northwest.
- In the southwest, Prairie Coteau has a fine example of an end moraine. Many of the till deposits in the west of the state have been found by borehole drilling to be more than 100 m thick. In the southwest, boreholes 160 m deep still had not reached bedrock.
9
Q
Describe the Pro Glacial Lakes
A
- The edge of the giant ice sheet and its associated lobes also dammed the natural drainage of the area. This created a number of proglacial lakes.
- The largest of these was Lake Agassiz, a small part of which occupied the present Red River valley of Minnesota and North Dakota. Glaciers to the north blocked the natural northward drainage of the area.
- As the ice melted, a proglacial lake developed south of the ice. At its maximum this lake covered 440,000 km2 (a similar size to the present day Black Sea.
- The water overflowed the watershed at Brown’s Valley, Minnesota, drained through the Traverse Gap and cut the present Minnesota River valley.
- The amount of discharge was staggering. It helped the adjacent Mississippi River to form a very large valley in southeastern Minnesota.
- The river that drained from Lake Agassiz is called the Glacial River Warren. It flowed over the top of a recessional moraine at Brown’s Valley. When the lake finally drained it left behind fertile silt deposits producing the rich farmland of the Red River valley.
10
Q
Describe the Pro Glacial Lakes
A
- The edge of the giant ice sheet and its associated lobes also dammed the natural drainage of the area. This created a number of proglacial lakes.
- The largest of these was Lake Agassiz, a small part of which occupied the present Red River valley of Minnesota and North Dakota. Glaciers to the north blocked the natural northward drainage of the area.
- As the ice melted, a proglacial lake developed south of the ice. At its maximum this lake covered 440,000 km2 (a similar size to the present day Black Sea.
- The water overflowed the watershed at Brown’s Valley, Minnesota, drained through the Traverse Gap and cut the present Minnesota River valley.
- The amount of discharge was staggering. It helped the adjacent Mississippi River to form a very large valley in southeastern Minnesota.
- The river that drained from Lake Agassiz is called the Glacial River Warren. It flowed over the top of a recessional moraine at Brown’s Valley. When the lake finally drained it left behind fertile silt deposits producing the rich farmland of the Red River valley.
