The Mystery at Rustlers' Fort (The Wilderness Mystery Series)

Phil and Buzz are teenaged cousins who join Buzz’s naturalist parents and little sister, Kitsy, on a summer-long research trip to the North Rim of the Grand Canyon. It is the height of the 1950s uranium boom in the Southwest, and the boys sneakily get hold of a Geiger counter to hunt for the element, not realizing that mining is illegal in the national park. Still, the Geiger counter helps to solve a mystery that has plagued the campers all summer long: the constant presence throughout their adventures of a strange character they call "The Dinosaur." Along the way, the boys are introduced to the canyon’s natural history, from Kaibab squirrels to mountain lions, bats, and more.

The recurrent themes of the books in the Wilderness Mystery Series are natural phenomena—caves, canyons, mountains, sand dunes, and forests—and a sense of the past as seen through archaeology. In many of the narratives, events of long ago are seen to have left traces of their passing. Notwithstanding the fact that the books were written in the 1950s, the progressive Franklin Folsom (alias Troy Nesbit) had refreshing views of women, Native Americans, and the environment, and he was prescient in having his characters often oppose corporate and government efforts to develop wilderness areas.