Practical Geology.

Horowhenua Chronicle, 29 January 1921, Page 3

Practical Geology.

THE STRUCTURE OF THE MANGAHAO GORGE.

Amongst the papers read before the Geological Section at the Science Congress this week at Palmerston this week was a treatise on the structure of Mangahao No. 1 Gorge (Mangahao hydiro-electric scheme), and its bearing on the construction of the proposed dam, by Mr G. Lesli© Adkin, of Levin. The paper was very well received by the Section, which includes among its members the leading geological authorities of the Dominion. The opinion was formed that the paper was a practical example of the application of the science of geology to the advancement of industry, and Mr Ad»kin was complimented on making a valuable addition to the records of the Congress. The following summary of the paper will be read with interest:—

The Mangahao hydro-electric scheme includes the construction of two reservoirs ani a surge-chamber—all of high and practically similar elevation and the piercing <of two lengthy tunnels, but the whole scheme hinges on the resistance of the dam to be located at the spot known as No. 1 Gorge, to the sudden and powerful floodings of the Mangahao River. The valley of the Mangahao has for the most part a shingly bottom, but at a number of places short lengths of rock-bound gorge occur, andi it is one of these that has been selected! for the site of the dam. At No. 1 Gorge the Mangahao has cut through solid rock for a distance of several chains, the vertical sides being only from 15 to 20 yards apart, and as a dam-site the spot is at first sight ideal. On its eastern side the gorge is surmounted by a terrace about 70ft above the river-level, and though faced with solid rock, this is in reality composed of shingle and boulders and marks the filled-up ancient channel of the Mangahao. The modern and ancient channels are separated only by a narrow ridge of rock.

This valley structure at No. 1 Gorge was discovered by the writer in January, 1920. but apparently did, not come under the notice of the engineers till the: following August, when pits were sunk to a depth of 52ft without reaching rook-bottom. From observations in other Tararua valleys, it is probable that the ancient filled-up valley of the Mangahao has a depth of 100 ft.

The physiographic events responsible for the valley-structure at No. 1 Gorge •re briefly as follows: The ancient (original) valley of the Mangahao was tteep and V-shaped. With the advent of the Glacier Period, produced by the great elevation of New Zealand in the Early Pleistocene, when small glaciers existed on the Tararuas and the mountain tops were bare of bush and frostriven, immense supplies of rock-debris were washed down into' this main valleys, partly filling thtm up. At r. later stage the Mangahao widened its valley by cutting sideways into projecting spurs, with the result that rock-floors were formed on the same level as the surface of the shingle: and boulders that partly filled the original deep valley. In more recent times the Mangahao again cut its channel deeper, and at such spots as No. 1 Gorge, the present trench was cut into the rockfloor of the lateral corrasiou instead of into the shingle; and boulder beds of thei primary valley. The double «haracter of the, Mangahao Valley at 'No. 1 Gorge is unfortunate from an engineering point of view. Because of the enormous pressure of the waters of the future reservoir, the foundations of the projosad dam mußt be built on solid rock. Much additional expenditure of capital and labour will be entailed by the excavation, and refillios with co»«ret« of * trench across the old shingle-filled valley of the Mangahao, which at this place is about four chains in width and perhaps 100 ft in depth. It should be borne in mind, however, that it is impossible to avoid the ancient channel, so that though it is rather less favourable for the purpose than was at first anticipated, No. 1 Gorge is'perhaps the best spot that could) have been found for the damming of the Mangahao Eiver in connection with the proposed hydro-elec-trio floikeme.