Britannica
Sources used in this answer
How do rocks carried by glaciers scrape and polish valley floors?
Created from Britannica articles
using AI.
AI can make mistakes, so verify using Britannica articles.
valley
The rugged Atlas Mountains surround a valley in Morocco.
Glaciers erode valley floors through a process of abrasion, acting much like a sheet of sandpaper. While glacier ice itself is too soft to significantly erode solid rock, it carries a basal debris zoneâa layer of rock fragments ranging from fine particles to large bouldersâthat becomes embedded in the ice at the base of the glacier.12
As the glacier moves, it drags this debris across the bedrock. This action grinds, scratches, and polishes the valley floor, creating various features such as:
- Striations: Scratches on the rock surface that indicate the direction of ice movement.3
- Chatter marks: Sickle-shaped gouges or lunate fractures formed when the glacier drags a boulder over brittle rock.3
- Knob-and-tail: Protrusions formed when harder parts of the rock resist erosion while softer surrounding material is worn away.3
Over thousands of years, this abrasive mechanism can erode the substrate to a depth of several tens of metres, contributing to the formation of characteristic U-shaped valleys.14