Nice sentence! It's almost right, but we can make it sound a little better for readers.
- Why change it? When two adjectives describe the same noun (like big and stony), we usually put a comma between them: "big, stony bumps."
- Better wording: The word "stony" is okay, but "rocky" is a more common word when we talk about mountains.
- Earth's surface: You can say "Earth's surface" or "the Earth's surface." Both are correct.
Corrected sentence (simple):
Mountains are big, rocky bumps on Earth's surface.
Another clear way to say it:
Mountains are large, rocky rises on Earth's surface.
Great job writing about mountains! These small changes make the sentence flow more smoothly.