What natural formation serves as evidence of glacial erosion?

A drumlin is a hill or a long, teardrop-shaped ridge that is formed by the movement of glacial ice. This natural formation serves as clear evidence of glacial erosion and deposition processes. During the advance of a glacier, it sculpts the underlying landscape, shaping the land into streamlined hills like drumlins. These formations indicate the direction of glacier movement, as they are formed parallel to the flow of the ice.

In contrast, valleys, plains, and plateaus represent different geological formations influenced by various processes, including erosion and deposition, but they do not specifically indicate glacial activity in the same way that drumlins do. Valleys can be formed by many different mechanisms, including river erosion and tectonic activity, while plains result from sediment deposition, and plateaus are typically formed through volcanic or tectonic uplift and erosion. Therefore, the drumlin uniquely illustrates the impact of glacial processes, making it the correct answer in this context.