How Glaciers Sculpted North America

Ice sheets up to 3 km thick covered much of Canada and northern US during the Last Glacial Maximum (~26,000–19,000 years ago). Their movement rewrote the continent's topography through six mechanisms:

1. Erosion

Ice + embedded rock = giant rasp. Carved U-shaped valleys (Yosemite, Glacier NP), fjords (Puget Sound, Maine coast), cirques, arêtes, horns (Matterhorn-type peaks).

2. Deposition

Melting ice dumps till (unsorted sediment) as moraines (terminal, lateral, medial, ground). Long Island, Cape Cod, Oak Ridges Moraine (Toronto) are glacial debris.

3. Striation & Grooving

Bedrock scratched by debris—parallel grooves reveal ice flow direction. Visible in Central Park, Kellys Knob (Australia), Canadian Shield.

4. Plucking & Quarrying

Ice freezes into bedrock fractures, rips blocks away. Creates cirques (amphitheater heads of valleys) and roches moutonnées (asymmetric hills).

5. Polishing

Fine sediment under pressure buffs bedrock to glassy smoothness—diagnostic of glacial passage.

6. Deglaciation Landforms

• Eskers: Subglacial stream deposits (sinuous ridges)
• Kettle lakes: Buried ice blocks melt → depressions fill (Minnesota's 10,000 lakes)
• Outwash plains: Meltwater streams spread sand/gravel (Cape Cod, Long Island south shore)
• Drumlins: Streamlined till hills (Rochester, NY; Clew Bay, Ireland)

North America's lakes, soils, coastlines, and fertility maps largely reflect this icy architecture.