Laurentide Ice Sheet Flashcards
1. Where are Minnesota’s oldest rocks located?
Minnesota’s oldest rocks lie in alternating belts in the northern half of the state and much of the Minnesota River Valley.
- What types of rocks are found in the belts of Minnesota?
The belts consist of volcanic and sedimentary rocks, while granitic rock materials lie in the areas between the belts.
- Along which geological feature can metamorphic gneiss be found in Minnesota?
Metamorphic gneiss crops out along the Minnesota River Valley.
- How old are the volcanic and sedimentary rocks in Minnesota?
The formation of volcanic and sedimentary rocks began 2700 million years ago when lava escaped through rifts in what was then the sea floor.
- What were the main characteristics of the glaciation in Minnesota?
Around 75,000 years ago, lobes or tongues of ice extended from the main Laurentide ice sheet and spread across Minnesota. These lobes advanced and retreated multiple times, transporting and depositing till across a wide area.
- How did the glaciation impact the landscape of Minnesota?
PT1
The Laurentide ice sheet, over 1km 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 500-700m.
- What were the erosional impacts of the Laurentide ice sheet in Minnesota?
- The erosional impact of the Laurentide ice sheet created a large ellipsoidal basin, studded with thousands of lakes, such as Upper and Lower Red Lakes. In the Arrowhead region of the northeast of 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 rocks around them. Thus the lakes of this region lie in 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 the ice advance.
- The far southeast of the state was not extensively covered by the ice sheet, retaining a more varied landscape of steep hills and deep valleys.
- How did the Laurentide ice sheet affect the rivers in Minnesota?
Most of the rivers draining the southeastern area of Minnesota are tributaries of the Minnesota and Mississippi Rivers.
What is the Wadena Lobe?
The Wadena Lobe is a glacial lobe that advanced from northeast Canada and reached just south of Minneapolis.
What is the characteristic composition of the till deposited by the Wadena Lobe?
The till deposited by the Wadena Lobe is characteristically red and sandy, being derived from the red sandstone and shales to the north and northeast.
What are the major landforms formed by the Wadena Lobe?
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.
What is the composition of the ground moraine extending from St Cloud northeastward?
The ground moraine extending from St Cloud northeastward consists of reddish iron-rich sediments.
What are terminal moraines, and where are they located in Minnesota?
Terminal moraines are ridges of debris deposited at the furthest extent of a glacier’s advance. In Minnesota, a set of terminal moraines extends from northwest of St Cloud into the Twin Cities (Minneapolis and St Paul).
What is the composition of the till left by the Rainy and Superior Lobes in Minnesota?
The till left by the Rainy and Superior Lobes is coarse-textured and contains abundant fragments of basalts, gabbro, granite, red sandstone, slate, and greenstone.
What is the composition of the till deposited by the Des Moines Lobe?
The till deposited by the Des Moines Lobe is colored tan to buff and is clay-rich and calcareous because of shale and limestone rocks at its source to the northwest.
