Till (Eratics)
Eractics are formed by glacial ice erosion resulting form the movement of ice. Glaciers erode by multiple processes: abrasion/scouring, plucking, ice thrusting and glacially-induced spalling. Glaciers crack pieces of bedrock off in the process of plucking, producing the larger eractics. In an abrasion process, debris in the basal ice scrapes along the bed, polishing and gouging the underlying rocks, similar to sandpaper on wood, producing smaller glacial till. In ice thrusting, the glacier freezes to its bed, then as it surges forward, it moves large sheets of frozen sediment at the base along with the glacier. Galcially-induced spalling occurs when ice lens formation with the rocks below the glacier spall off layers of rock, providing smaller debris which is ground into the glacial basal material to become till.
Yeager Rock is a 400-metric-ton eratic on the Waterville Plateau, Washington. Underneath the rock there is a glacial till.