No one knows exactly when humans first paired nuts with dried fruit and thought, "Gorp! I must have this as I journey forth afoot." We do know, however, that the salty-sweet concoction now called trail mix became commercially available in the 1960s and that makers have been refining recipes ever since.
http://sierraclub.typepad.com/greenlife/2010/03/mixed-reviews-our-trailmix-taste-test.html