Which type of soil is least likely to contain rock fragments?

Loess is primarily composed of silt-sized particles that are deposited by wind. This type of soil is typically characterized by its fine texture and lacks significant quantities of rock fragments. The formation of loess occurs in environments where strong winds can carry and deposit these fine particles, often resulting from the weathering of rock and transportation over long distances. The absence of larger rock fragments is a defining characteristic of loess deposits, making it distinct from other soil types that have varying amounts of gravel or coarse materials.

Colluvium, on the other hand, is often found at the base of slopes and tends to accumulate rock fragments due to gravitational movement and erosion of surrounding geological features. Outwash is formed by the melting of glaciers and typically contains a mix of sand, gravel, and larger rock fragments as water carries these materials away from the glacier. Till is a type of unsorted glacial deposit that comprises a mix of clay, silt, sand, and a significant amount of rock fragments, deposited directly by glacial ice.

Therefore, the distinction comes from the process of formation and the predominant particle size, with loess being the least likely to contain rock fragments compared to the other soil types listed.