Minnesota

Question 1

what is the main ice sheet in Minnesota?

Answer 1

Laurentide, Hudson Bay
grew + retreated many times with climatic changes throughout the Quaternary, during colder periods it extended southward across the upper Midwest

Question 2

what is the geology like?

Answer 2

• belts of volcanic + sedimentary rocks -> granite rocks lie in the areas between the belts
• metamorphic gneiss crops out along the Minnesota river Valley
• volcanic + sedimentary rocks began their formation 2700 mill years ago, when lava escaped through rifts in what was the sea floor
• volcanic debris released into nearby seas later forming massive layers of sedimentary rock on the sea floor
• tectonic activity folded many of these rock formations + formed faults
• tectonic compression created a range of mountains several km high in N Minnesota

Question 3

what impact did the lobes extending from the main Laurentide ice sheet have?

Answer 3

they advanced + retreated a no. of times, transporting + depositing till across a wide area. the different origins of these lobes resulted in tills with different characteristics + materials

Question 4

how thick is Laurentide ice sheet?

Answer 4

1km

Question 5

what erosional impact did the lobes have?

Answer 5

high mountains worn down, so that today highest peaks = 500-700m
a large ellipsoidal basin was created + is now studded with thousands of lakes eg Upper + Lower Red Lakes in N Minnesota
as the lobes advanced they abraded striations in bare rock outcrops of gneiss + greenstone, their alignment indicated the direction of ice advance

Question 6

where was the erosional basin particularly deep?

Answer 6

in the arrowhead region as the earlier tectonic tilting of landscape exposed weak shale rocks which were eroded much more rapidly than the resistant volcanic rocks around them -> lakes of this region lie in deeply eroded shales

Question 7

what was the erosional impact of the Laurentide ice sheet?

Answer 7

considerable, shaped the overall landscape
however, the continental ice sheet erosion does not produce the specular landforms associated with valley glaciers + alpine glaciation

Question 8

what type of till did the Wadena lobe deposit?

Answer 8

red and sandy from the red sandstone + shales to the N + NE

Question 9

what depositional landform did the wadena lobe create?

Answer 9

drumlin fields

Question 10

where are the set of terminal moraines?

Answer 10

extend form NW of St cloud into the twin cities

Question 11

what did the rainy + superior lobes leave?

Answer 11

a coarse textured till containing fragments of basalts, gabbros, granite from across NE half of Minnesota as far south as twin cities

Question 12

what type of till did Des Moines lobe deposit?

Answer 12

coloured tan to buff + is clay rich + calcareous bc of shale + limestone rocks as its source in NW

Question 13

in the west how thick are the till deposits?

Answer 13

100m

Question 14

how did the ice sheet + its lobes affect the drainage of the area?

Answer 14

dammed the natural drainage which created a number of proglacial lakes

Question 15

what was the largest proglacial lake?

Answer 15

Lake Agassiz

Question 16

how was the proglacial lake formed?

Answer 16

• glaciers in the N blocked natural drainage northward
• as ice melted the lake developed south of the ice, at its max. it covered 440,000km2
• the water overflowed the watershed at Brown’s Valle, drained through the traverse gap + cut the present Minnesota river valley
• > staggering amount of discharge -> helped form a very large valley in SE Minnesota

Question 17

what river drained from Lake Agassiz?

Answer 17

glacial river warren -> flowed over the top of a recessional moraine at Brown’s valley, when the lake drained
-> left fertile silt deposits which produce the rich farmland of the Red river valley