Weathering

Rain, wind and freezing are the causes of the erosion of a landscape.

Wind erodes high places via a mechanical effect. Particles are torn away as it passes. The polishing or  abrasive power of the wind is reinforced by the presence of sand and dust.

Water has a mechanical effect (from the impact of water drops, known as the "splash effect"),  and a chemical one (dissolution of calcareous substances, like limestone).

The durations of the effects of these phenomena are quite variable -- from a few hours (avalanches, hurricanes) to several millions of years.

Note finally that gravity, which pulls all matter downward, is an omnipresent phenomenon in the levelling of terrain.