How do rock fragments carried by glaciers create striations on bedrock?

Glacial striations are created through the process of glacial abrasion, where a glacier acts similarly to a sheet of sandpaper. While pure ice is too soft to significantly erode solid rock, temperate glaciers typically contain a basal debris zone—ranging from several centimetres to a few metres thick—that holds rock fragments of various sizes, from fine particles to large boulders.1

As the glacier moves via basal sliding, these embedded rock fragments are dragged across the underlying bedrock. The hard rock debris acts as a "tool," scratching, grinding, and gouging the surface of the bedrock as it is forced over it by the immense weight and movement of the ice. This action produces striations, which are scratches ranging from microscopic to several centimetres deep. Because the tool is dragged across the surface, the orientation of these striations provides a record of the direction of ice movement in that immediate area.21