You are searching for an ammo box. It's right at the edge of the tree line.
The Civilian Conservation Corps was a program during the Great Depression to put people back to work. Young men worked on civic projects across the country in order to be able to send money home to their families. The nearby picnic shelter and other structures in the park are evidence of their hard work.
A park pass is required for entry to the park. This cache was placed with the permission of the park manager.