Explanation:

- The passage describes a valley largely shaped by natural forces such as river waters, rainfall, and winds.

- Blank (3) requires a word that describes the size or nature of the mountain mentioned, which is notably distinct from its surroundings.

- Option 1: "humongous" conveys a sense of being very large or immense, fitting the description of a "giant monolith."

- Option 2: "small" contradicts the description of a "giant monolith."

- Option 3: "neat" doesn't fit as it describes tidiness rather than size.

- Option 4: "tiny" also contradicts the idea of a "giant monolith."