What is a cliff?

A cliff is best defined as a vertical expanse of rock, which typically forms along coastlines where erosion has removed the softer material, leaving behind a steep, sometimes towering face of rock. Cliffs can result from various geological processes, including weathering, erosion, and tectonic activity. This striking feature is characterized by its height and slope, often leading directly to water bodies like oceans or rivers.

In contrast, a steep hill with vegetation may resemble some cliff aspects but does not specifically identify the vertical rock face inherent to cliffs. Coastal plains and flat areas of shore refer to low-lying, flatland regions adjacent to the coast that do not possess the steep characteristics of a cliff. Thus, the answer correctly identifies the defining feature of a cliff as a vertical rock face, highlighting its unique geological nature.