What geological condition is known to promote leaching?

Leaching refers to the process by which soluble materials are washed out from the soil or other material, often as a result of water movement through it. The correct choice emphasizes the significance of gravel deposits in this context. Gravel deposits typically have large, interconnected pore spaces that allow water to flow freely. This water can carry away soluble nutrients and other materials from the gravel, promoting leaching.

In comparison, clay soils have small particles that pack tightly together, reducing porosity and water movement, which limits leaching. Fertile plains, while potentially conducive to good agricultural practices, do not inherently promote leaching unless the soil is specifically designed to facilitate water movement. Rocky outcrops, similar to clay soils, can contain areas where water accumulates but are not conducive to leaching due to their solid structure and limited pore space. Therefore, gravel deposits are the geological condition specifically known to enhance the leaching process.