The Secret of the Missing Footprint — Phyllis A. Whitney

THE RIDDLE OF THE ROCKS --

When her parents flew off to Europe, leaving Marcie behind, she was sure she would have a miserable summer. But that was before she met Timothy Rainbow. With Timothy around there was always excitement. Timothy knew special places in the woods and the secrets of the old quarry, with its strange collection of prehistoric rocks.

So when Uncle Evan's prize raindrop rock was stolen, Timothy and Marcie decided to play detective. Then some of Timothy's special stones disappeared too. Who would want to swipe a bunch of rocks, even if some of them did have dinosaur footprints? And what was the thief going to do with them? Marcie had to discover the culprit, but when she finally caught him, she found herself in big trouble…