1921.] 
Adkin.—Mangahao Hydro-electric Scheme. 
3 
sunk, up to the above date, to a depth of 52 ft. through shingle and 
boulders with no appearance of an approach to rock bottom. 
It may be of interest to give a resume of the physiographic events 
responsible for the occurrence of such valley forms and structure as are to 
be found at Mangahao No. 1 Gorge (fig. 1) and at numerous other places 
in the Tararua river-valley. The primary valley of the Mangahao, in 
common with the other principal river-valleys of the Tararua Range, 
dates from the breaking-up of the pre-Miocene peneplain* by the 
block - faulting and, in the Tararua area, the accompanying complex 
warping of the Kaikoura deformation, and from the resultant inaugu¬ 
ration of the river-svstems of the penultimate and present erosion cycles. 
[V 
-Primary valley of 7 Vfcj.vT.ga-K ao R\v*e v* 
120 ISO 
i so 2-iQ 2fo 2^o j>o o Teel 
Go jo so so *oo Ya»“d5 
Fig. 2. —Section of Mangahao Valley at No. 1 Gorge, showing relationship of the present 
channel to the primary valley of the Mangahao River. F.L. = Flood-level 
of 8th December, 1920, 18^ ft. above normal. 
Fig. 3. —The evolution of No. 1 Gorge, Mangahao Valley, a. The Mangahao Valley 
in the late Tertiary period. b. The valley partly filled with a deposit of 
shingle, &c., in the early Pleistocene, c. The valley-plain widened by lateral 
corrasion in the early Pleistocene at a later stage. d. By rejuvenation in 
more recent times, the river cut its present channel into the rock floor 
instead of into the valley-fill of the primary valley. 
* J. M. Bell, 1910, Trans. N.Z. Inst, vol. 42, p. 538; and J. Hendeeson, 1911, 
Trans. N.Z. Inst., vol. 43, pp. 312-13 
