Final answer:
Glacial striations are fine, parallel scratches on rocks that were once beneath a glacier. They are formed when rocks and boulders carried by the glacier scrape and scratch the underlying bedrock.
Step-by-step explanation:
Fine, parallel scratches on rocks that were once beneath a glacier are called glacial striations. Glacial striations are formed when rocks and boulders are dragged along the bedrock by the movement of a glacier. The immense weight and pressure of the ice cause the rocks to scratch and scrape the underlying rocks, leaving behind parallel grooves.