4 
The N.Z. Journal of Science and Technology. 
[Mar. 
The primary valley of the Mangahao was acutely Y-shaped in its trans¬ 
verse portions, while its longitudinal portions, though also V-shaped, 
were more widely opened. In the earlier stages of the epeirogenic uplift 
of the country in the early Pleistocene the Mangahao Valley was pro¬ 
bably still further deepened (fig. 2, a), but later, during the climax of 
elevation, in the period generally termed the Glacier Period—a period in 
which the North Island is now known to have participated*—extensive 
alluviationf of the Tararua river-valleys took place (fig. 2, b). A super¬ 
abundance of coarse debris derived from the very extensive areas lying 
between the timber-line and the mountain-summits during the Glacier 
Period was delivered to the main valleys by their headwaters and laterals, 
filling them to a considerable depth. The alluviation ceased when an 
adjustment of load to volume and gradient had been effected. Then 
lateral corrasion became predominant, and parts of the valley-slopes were 
shorn away, especially where a shoulder of the valley-side projected into 
the convex side of a bend in the valley. This process produced rock 
floorst of equal grade and elevation to the surface of the valley-fill of the 
primary valley (fig. 2, c). In the final rejuvenation of, and downcutting 
by, the larger rivers— e.g., the Mangahao—the present trench, at such 
spots as No. 1 Gorge, was cut in the rock floor formed by the lateral corrasion 
instead of into the detritus of the partially-filled primay valley (fig. 2, d). 
Above No. 1 Gorge the ancient and modern channels of the Mangahao 
coincide ; at the upper and lower ends of the gorge they diverge and con¬ 
verge respectively ; at the gorge itself they are separated by a narrow wall 
of rock, and the line of the ancient channel is marked by the high-level 
terrace. 
The interesting but-—from an engineering point of view—unfortunate 
physiographic structure of Mangahao No. 1 Gorge will entail the expendi¬ 
ture of much additional labour and capital in connection with the con¬ 
struction of the dam. In view of the enormous pressure of the impounded 
waters and of the periodic devastating floods, a solid-rock bottom for the 
foundations of the dam will probably be deemed a necessity. If the high- 
level terrace had been cut in solid rock, instead of being merely faced with 
that material, the foundations of the eastern wing of the dam could have 
rested on its upper surface. The presence of the old filled channel will 
necessitate, if a solid-rock foundation is considered indispensable, the exca¬ 
vation of a trench across the high-level terrace—a trench some 3 chains or 
4 chains in length, about 25 ft. in width, and of perhaps 100 ft. in depth ; 
and when this has been done the trench wall require to be filled with concrete 
before the eastern wing of the dam appears above the present ground- 
surface. The high-level terrace stands at a height of 60-80 ft. above the 
present river-level, and, as the ancient channel was probably excavated 
to a rather greater depth than the present one, the estimate of the depth 
of rock-bottom in the middle of the ancient channel at 100 ft. below the 
terrace-top is probably not excessive. 
Although the structure of the Mangahao Valley at No. 1 Gorge is less 
favourable for the construction of the future dam than was perhaps at 
first anticipated, the impossibility of avoiding the old detritus-filled valley 
should be borne in mind, and thus it is doubtful if a better spot than No. 1 
Gorge could have been found for the damming of the Mangahao River in 
connection with the proposed hydro-electric scheme. 
* G. L. Adkix, 1912, The Discovery and Extent of Former Glaciation in the 
Tararua Range, Tracis. N.Z. Inst., vol. 44, pp. 308-16. 
f See also A max, 1920, Trans. N.Z. Inst., vol. 52, p. 188. 
t See also Adktn, 1911, Trans. N.Z. Inst., vol. 43, p. 516, and plate xix, 
section C-D ; and 1919, Trans. N.Z. Inst., vol. 51, pp. 112-13. 
