LAURENTIDE case study - Glaciated Landscapes

Ice sheet

What is an ice sheet?

A large expanse of ice (minimum 50,000 km squared) that sits on top of a huge area.

How big was Laurentide estimated to be at its peak?

13 million square kilometres

How deep was Laurentide on average?

3 km

When did Laurentide exist?

75,000 years ago - during the Pleistocene

What are the 4 lobes of Laurentide?

• Wadena

• Des Moines

• Rainy

• Superior

How has climate influenced Laurentide?

Temperatures must be below 0 → allowing precipitation to fall as snow → allowing diagenesis to occur → allowing there to be a vast amount of glacial ice that is 3km deep → highly significant due to the scale of the ice sheet.

How has latitude influenced Laurentide?

The ice sheet covered from around 65 degrees north down to 45 → this means that some of it had polar latitude while other parts didn’t (southern sections e.g. Minnesota) → southern regions = warmer external temp, movement by basal sliding and lobes behaved like a temperate glacier.

How has geology influenced Laurentide?

There were alternating bands of volcanic and sedimentary rocks → also Gneisses → metamorphic granite and sedimentary rocks → these are folded/faulted, meaning they have weaker joints and are more vulnerable to erosion via plucking.

What was the erosional impact of the ice sheet on pre-existing landforms?

It wore down many of the mountains e.g. Eagle mountain is now only 701m tall.

What was the erosional impact on making new landforms?

Formation of ellipsoidal basins → these contain the red lakes in Minnesota and Mille Lacs lake. This erosional action hit the sedimentary rocks.

What are ellipsoidal basins?

Deep, elongated lakes formed by subglacial activity beneath ice sheets.

Smaller scale example of erosional action

Striations are visible on bare rock underneath where the ice sheet was and especially on the gneiss.

Depositional action from the Wadena lobe

Till deposited from the Wadena lobe left ground moraines with reddish iron-rich sediment, characteristic of sandstone from N to NE.

Example of a depositional landform from the Wadena lobe

Alexandra moraine and drumlin fields in Otter tail and Todd counties.

Ground and terminal moraines from St Cloud to twin cities.

Depositional action from the Des Moines lobe

Deposited till which is a tan colour and clay-rich, characteristic of shale and limestone rocks.

How deep do some of the till deposits go?

Over 160 m

Example of change over time in depositional landform

Most likely tons of drumlins present under the ice sheet. However, as the glacier continually retreated and advanced, they’d have been eroded and the only ones left are from the final movement.

Why were drumlins in particular present?

They are the only depositional landform formed as the ice advances.

How was the location of the edge of the ice sheet important for landforms + drainage?

Drainage was dammed by the lobes → naturally, should have gone into Hudson Bay, but instead water poured southwards feeding into Lake Agassiz.

How big was Agassiz at its peak?

440,000 km squared and 400 m deep

What evidence is there of Agassiz today?

Fluvial deposits → different to regular glacial till as they are stratified, sorted, and rounded.

Current changes to the landform occurring in seconds

Rock fall/weathering from Eagle mountain → doesn’t impact the system → no huge landforms to change.

Changes over millennia

Erosion of ellipsoidal basins.

Seasonal changes

Deposition rates → though less variation.