What term refers to a large valley filled with sand, clay, and other deposits?

The term that accurately describes a large valley filled with sand, clay, and other deposits is "sedimentary basin." A sedimentary basin is a natural depression in the Earth's crust where sediments accumulate over geological time. These basins can be formed through various geological processes, including tectonic activity and erosion.

Within these basins, sediments such as sand and clay are deposited by water, wind, or ice, creating a diverse range of geological formations. The accumulated sediments can vary in composition depending on the environment and the sources of the materials.

The other terms mentioned do not encapsulate the definition of a large valley filled with deposits accurately. For example, a floodplain refers specifically to the flat area adjacent to a river that becomes inundated during floods, while an alluvial plain is more about the area formed by the deposition of sediment from a river or stream. A geologic formation generally refers to a body of rock with a distinct and recognizable set of characteristics, which may not specifically pertain to sediment accumulation in a valley context. Thus, sedimentary basin is the most precise term for the described geological feature.