Cajon Canyon is a long valley in the northeastern San Gabriel Mountains of California that ends in a box canyon whose steep surrounding slopes make access difficult in its upper reaches. Originally called El Cajon De San Gabriel De Amuscopiabit, it took the shortened Spanish name El Cajon, meaning “the box,” from this enclosed landscape. The canyon extends from its mouth in Cajon Pass near Cajon Junction at about 1,991 feet to its head near Mountain Top Junction at roughly 4,910 feet. With State Route 138 running its length between Interstate 15 and the Angeles Crest Highway, it is both a distinct mountain landform and an important traveled corridor.