The Turtle Mountains are geologically diverse. The southern half of the Turtle Mountains consists of an uplifted core of older Precambrian igneous and metamorphic rocks. The northern portion is overlain by Tertiary volcanic rocks (mostly basalt) intruded by small plugs of hypabyssal Tertiary rhyolite. Some small exposures of Precambrian granite occur in the northern half of the Turtle Mountains. These granites are found in the rugged, broken country just north and east of Mohawk Spring and are host to several mines and prospect pits. Although the southern half of the Turtle Mountains is comprised mostly of Precambrian igneous and metamorphic basement rock, a small exposure of Mesozoic granite occurs just to the north of Horn Peak.
The range is cut by several fault zones that trend northwest-southeast. The great majority of mines in the Turtle Mountains are hosted in Precambrian igneous and metamorphic rock.
The southern half of the Turtle Mountains is underlain by the same Precambrian basement rock as that of the Whipple Mountains, which lie only 15 miles to the east. The southern edge of the Turtle Mountains does show mineralization but not to the extent of the Whipple Mountains. Nevertheless, the area below Horn Spring probably deserves a detailed search with a metal-detector. Prospectors