Is this a sentence fragment:

A small herd of buffalo grazed in the valley below us

If it doesn't have a period, than it is a sentence fragment.

If not, then I think it is a complete sentence.

Make sure it has a subject, verb, dependent or independent clause, and a period. (especially in the set up of your sentence)

If you have anymore questions, ask me.

Brainliest answer is always appreciated.