A glacier slowly moves across the land, picking up and carrying bits of rocks down the side of a hill. weathering or erosion?

Erosion.

Explanation:

Erosion is the movement of material on the earth's crust from one location to another. Weathering is the breaking down of rocks and such due to external or internal factors, but does not necessarily move the material to another location.

With this in mind, the glacier carries rocks with it as it travels and is therefore erosion.