Your answer is the wind. Let me explain what I mean:
1. So, the sediments are on the hilltop, just sitting there. Does the ground do anything? The ground is just being the ground. We can cross off the ground.
2. The hilltop isn't moving or knocking them off, something else moved them. The hilltop can also be crossed off.
3. The sediments are BEING moved but they aren't the ones moving. The wind is the one carrying them down to the bottom of the hill.
