2 
The N.Z. Journal of Science and Technology. 
[Mar. 
the top of which is 60-80 ft. above the normal level of the river in the gorge, 
and as the crest of the proposed dam is to go still higher its eastern wing 
will extend across the top of the terrace to the steep valley-side. The high- 
level terrace presents a face of solid rock to the gorge, but notwithstanding 
this fact the terrace is a built feature, and marks the site at this point of 
the ancient valley of the Mangahao—an old gorge which was subsequently 
filled up with shingle, boulders, and other river-borne detritus (fig. 2). 
This unexpected structure of the Mangahao Valiev at No. 1 Gorge was 
determined by the writer during a visit to the spot on the 11th January, 
1920, when the accompanying sections (figs. 3, a , b, c, d ) were worked out. 
At that time the dense virgin bush, which came down to the margin of the 
narrow river-bed and overhung the rocky gorge, was untouched by the 
bushmams axe, so that the extent of the high-level terrace could be only 
Fig. 1. —The Mangahao No. I Gorge, with bush cleared and other preliminary works 
preparatory to the construction of the dam in progress. View looking down¬ 
stream. High-level terrace on right. 
inferred. On the evidence, however, of an exposure of high-level shingle 
and boulders on the right bank of the Mangahao immediately above No. 1 
Gorge, and a knowledge of certain valley-structures in other Tararua river- 
valleys, where considerable recent changes in the river-courses have taken 
place, revealing detritus-filled ancient valleys, it seemed certain that a 
similar change had taken place at the present Mangahao No. 1 Gorge, and 
that it diverged from and converged with the ancient valley of the river 
at its upper and lower ends respectively. 
The validity of the above conclusions as to the structure of the 
Mangahao Valiev at No 1 Gorge was substantiated on a subsequent visit 
to the locality on the 12th December, 1920. The clearing of the bush 
preparatory to the construction of the dam had revealed the extent of the 
high-level terrace, and a series of test shafts along the line of the proposed 
dam its composition of detritus, not solid rock. One of the test pits, 
located on the terrace-top near the middle of the ancient valley, had been 
