How do glaciers create scratches and grooves in bedrock?

Glaciers create scratches and grooves in bedrock through a process known as abrasion. Because pure ice is relatively soft and lacks the shear strength to erode solid rock, it relies on a basal debris zone—a layer of rock fragments ranging from fine particles to large boulders embedded in the ice at the glacier's base.12

As the glacier moves, it acts like a sheet of sandpaper, dragging these embedded rock fragments across the underlying bedrock. The resulting marks, often called "tool-marks," vary based on the size and shape of the debris, the pressure applied, and the hardness of both the tool and the substrate. These features include:13

• Striations: Scratches visible to the naked eye, ranging from microscopic to several centimetres deep and tens of metres long.14
• Chatter marks: A series of crescent-shaped gouges or lunate fractures formed when a boulder is dragged across a hard, brittle surface.4
• Knob-and-tail: Protrusions formed when harder rock sections protect softer rock on their lee side during abrasion.4

These marks are essential for geologists, as their orientation helps determine the historical direction of ice movement.4