Roadside Geology, Organ Pipe Cactus National Monument
Hunt CB. 1972. Roadside Geology, Organ Pipe Cactus National Monument
Highway 85 crosses Organ Pipe Cactus National Monument from north to south, a paved road about 22 miles long and passing Park Headquarters. The hills just east of the North Entrance (alt. 1,780 feet) are capped by late Tertiary or Quaternary basaltic lavas. On some maps these hills are included with the Gunsight Hills although other maps restrict that name to lower hills of granite east of the lava capped ones. About southeast from the entrance can be seen the north end of the Ajo Range composed almost wholly of middle Tertiary volcanic rocks. The range is deeply dissected and the skyline is jagged, because these volcanic rocks do not have a protective cap of the resistant basaltic lavas.