These are cancellation stamps that can be collected from visitor centers while visiting National Parks in the US. The ones below are the National Parks I've been to where I've had them stamped, in alphabetical order.

Arches National Park cancellation Black Canyon of the Gunnison cancellation Bryce Canyon cancellation Canyonlands cancellations
Capitol Reef cancellation Death Valley cancellation Grand Canyon cancellation Great Basin cancellation
Great Sand Dunes cancellation Joshua Tree cancellation Kings Canyon cancellation Mesa Verde cancellation
Rocky Mountain cancellation Yosemite cancellation Zion cancellation