An eastern spur of the
Southern Alps, the Two Thumb Range runs in a predominantly north–south direction for approximately . It and the smaller, mostly parallel,
Sibbald Range branch from the Southern Alps close to Mount D'Archaic, northeast of
Aoraki / Mount Cook. The two ranges are separated by the valley of the
Macauley River and form a barrier between the valleys of the
Godley River and
Lake Tekapo to the west and the
Rangitata River to the east. Much of the eastern side of Lake Tekapo is separated from the range by the lower Richmond Range branch. As for the Richmond Range branch of the range, several smaller ranges branch off towards the east including the Black Mountain Range from near Achilles Low Peak, and the Brabazon Range from Mount Caton. The Two Thumb Range's peaks diminish in size towards its southern end, which is immediately to the east of the town of
Lake Tekapo, after which it splits into two less significant ranges, the Rollesby and Albury ranges. At the saddle between the Two Thumb Range and these ranges,
State Highway 8 crosses
Burkes Pass on its route between Lake Tekapo and
Fairlie. The
South Opuha,
Havelock, and
Coal Rivers have their sources within the Two Thumb Range, as do numerous smaller watercourses, the most notable of which is Forest Creek, a tributary of the Rangitata River.
Peaks Names The Thumbs twin peaks in the range have been described as "twin peaks like two giant thumbs … a famous mid-Canterbury landmark." for the highest peak of Achilles, where Achilles is an unofficial name, located as the saddle between two peaks, that are called High and Low peaks. Further there is a Mount Achilles in Otago. Most of the Two Thumb Range consists of unfoliated meta
greywackes, with areas of
pumpellyite-
actinolite. There is current and
Quaternary displacement east of the
Alpine Fault that has been ongoing for less than 5 million years that has resulted in uplift of the Two Thumb Range. The northern range is uplifting as part of the Southern Alps while distinct faulting structures are known in the southern portion. To the east of the ranges are two series of north-striking
Fox Peak faults and to the west the northeast-striking Forest Creek faults that parallel the Alpine Fault at about the Mount Musgrave level in the southern range. A fault is inferred to separate the
Round Hill area from the rest of the ranges. It has been postulated that the southern Two Thumb Range has uplifted about in the last 1.5 to 2 million years. ==Recreation==