Primary agent is gravity, but gravity acts in concert with running water.
Types of Weathering Mechanical or Physical - the breakdown of rock material into smaller and smaller pieces with no change in the chemical composition of the weathered material. Chemical - the breakdown of rocks by chemical agents. Obviously the chief chemical agent is water which carries dissociated carbonic acid. Mechanical Weathering Expansion and Contraction - the thermal heating and cooling of rocks causing expansion and contraction. Frost Action - Water freezes at night and expands because the solid occupies greater volume.