This dimension names what actually drives a given physical process, water, wind, ice or gravity. Wikipedia's own list of erosion's agents, "rainfall; bedrock wear in rivers; coastal erosion by the sea and waves; glacial plucking, abrasion, and scour; areal flooding; wind abrasion; groundwater processes; and mass movement processes," shows how many different forces can cause the same broad kind of change. Browsing by caused by groups processes that share a common underlying trigger.