What is the primary composition of an outwash plain?

The primary composition of an outwash plain is derived from debris associated with glacial activity. During periods of glaciation, glaciers move and erode the landscape, picking up various types of material. When the glaciers melt, they release this debris, which includes gravel, sand, and silt. This material is then transported by meltwater streams that flow away from the glacier. Over time, these streams deposit sediments, creating an outwash plain characterized by stratified sediments typically sorted by size.

This composition is distinct from the other options. Fluvial deposits from rivers generally refer to sediments formed and transported by river activities, which are separate from direct glacial processes. Silt from wind deposition is typical of aeolian environments and does not relate to the glacial processes that form an outwash plain. Likewise, lacustrine sediments are associated with lake environments and are formed by the deposition of materials within lake basins, contrasting the settings where outwash plains develop. Therefore, the unique glacial origin of the materials in an outwash plain distinctly supports the answer regarding its primary composition.