What is the term for a mass of small loose stones that form or cover a slope on a mountain?

The term that accurately describes a mass of small loose stones that form or cover a slope on a mountain is "talus." Talus refers specifically to the accumulation of rock debris at the base of a slope that has been weathered and fragmented from the surrounding rock. This typically occurs due to processes like freeze-thaw cycles and gravitational movement, which break apart larger rocks into smaller fragments.

In contrast, "scree" commonly refers to a similar accumulation of small, loose stones but is often associated more broadly with debris that collects on a slope, regardless of its origin. While "scree" and "talus" are sometimes used interchangeably, "talus" is a more precise term for the situation described in the question.

The terms "cobblestone" and "gravel" identify specific types of rock materials. Cobblestones are larger, rounded stones often used in construction and road surfacing, while gravel typically refers to unbound, loose rock particles of a range of sizes, including those that can be much larger than those found in talus or scree formations. Therefore, they do not adequately describe the specific geological phenomenon occurring on mountain slopes that the question refers to.