How is an outwash plain primarily formed?

An outwash plain is primarily formed through the action of glacial meltwater. When a glacier melts, large volumes of water flow away from the glacier, transporting sediments that were previously embedded in the ice. This meltwater carries a mix of materials, including sands, silts, and gravels, which are deposited in layers as the water loses energy and spreads out over the landscape.

The characteristic stratified deposits of an outwash plain arise due to the sorting of sediments by the varying velocities of water. Coarser materials tend to settle out closer to the glacier, while finer materials can be transported further away before being deposited. This leads to a distinct layering of sediments that reflects the dynamics of glacial melt conditions.

Other processes listed in the choices do not accurately describe how outwash plains are formed. River erosion and sediment deposition pertain more to river valleys rather than glacial environments. Wind erosion and deposition are relevant to features like sand dunes but do not contribute to the formation of outwash plains. Similarly, sedimentation from lake waters refers to different aquatic environments and does not involve the glacial processes that create outwash plains. Thus, the correct answer emphasizes the significance of glacial meltwater in shaping these unique landforms.