Here's why:

* Glaciers are powerful agents of erosion: As glaciers move, they carry rocks and sediment embedded in their ice. These rocks act like sandpaper, grinding against the bedrock beneath the glacier.

* Striations are scratches: The scratches left behind by these rocks are called glacial striations. They indicate the direction of glacial movement.

* Deposition is a separate process: While glaciers can deposit sediment (forming moraines, outwash plains, etc.), the formation of striations is a direct result of the glacier's erosive force.

Therefore, glacial striations are a clear sign of erosion caused by the movement of a glacier.