What defines a canyon?

A canyon is characterized by its deep, narrow valleys with steep cliffs, which is why the correct choice is the one that describes these distinctive features. Canyons are typically formed through processes such as erosion over long periods, often by rivers that cut through rock and soil, resulting in the steep, rugged walls usually associated with canyon landscapes.

The other options do not convey the same geological characteristics. Flat valleys with wide openings suggest a more gentle topography rather than the dramatic elevation changes of a canyon. High mountain terrains, while perhaps related to mountainous regions where canyons might be found, do not specifically describe the defining steep-sided and narrow nature of a canyon. Lastly, river valleys with gradual slopes imply a more gently sloping landscape that lacks the steep cliffs synonymous with canyon formations. Thus, the defining feature of a canyon rests in its deep, narrow profile with steep cliffs, distinctly portraying the dramatic contrasts in elevation that make them unique geological formations.