1. Erosional forces like gravity, wind, water, and glaciers cause erosion by removing and transporting surface materials from one location to another through processes like mass movement. 2. Mass movement occurs as gravity pulls materials like rock and sediment down slopes, sometimes causing rapid events like landslides but sometimes occurring so slowly they are barely visible. 3. Glaciers are large masses of snow and ice that erode land as they move and later deposit sediments elsewhere, leaving behind features like moraines, eskers, and glacial till.