Final answer:
Parallel scratches on bedrock in a U-shaped valley are typically caused by a glacier dragging rocks over the bedrock, indicating glacial erosion.
Step-by-step explanation:
The occurrence of parallel scratches on bedrock in a U-shaped valley most likely indicates that the area has been eroded by a glacier. These scratches or grooves, also known as glacial striations, are caused by the movement of a glacier as it drags rocks and debris over the underlying bedrock. Evidence of glacial erosion is distinct from erosional features created by streams, waves, or wind, as a U-shaped valley and striated rocks are characteristic of glaciation. Streams create V-shaped valleys, wave action results in different coastal formations, and wind can cause various desert features, none of which are typically associated with parallel scratches on bedrock.
When comparing surface features on Earth to those observed on other planets, such as Mars, distinctions can be made based on the erosional agents present. For example, gullies on Mars may indicate liquid water flows or sediment transport, but in the case of Earth's glacially carved U-shaped valleys, the presence of a glacier as the erosional force is clear.