1. Missing commas: There should be commas after "feet" and "miles" to separate the descriptive phrases from the main noun phrase.

2. Missing capital letters: "Stone Mountain" and "granite" should be capitalized because they are proper nouns.

3. Missing period: The sentence needs a period at the end.

Here's the corrected sentence:

Standing 850 feet above plains with a circumference of five miles, Stone Mountain is the largest exposed mound of granite in the world.