THE BURIED FOREST AT ARAPUNI

Otago Witness, Issue 3870, 15 May 1928, Page 76

THE BURIED FOREST AT ARAPUNI

By

J. E. A.

(Special for the Otago Witness.) The hydro-electric power works at Arapuni have been responsible for the creation of several scenic attractions. The Waikato River, held up by the huge dam, makes a peaceful detour by way of its ancient bed, instead of thundering through the Arapuni Gorge, as was its age-long habit. But, like a captive creature of the wilds held in leash against its will, its pent-up energy bursts forth with redoubled violence. Coming to the Waititi Valley, within sight of its old channel, it makes a break for freedom, leaping a cliff and forming a magnificent waterfall 70ft high. The whole volume of the river, on its first liberation, commenced a work of tremendous erosion, scouring from the peaceful little valley such quantities of pumice sand, stones, and other detritus, that for a while the Hora Hora works, some six miles further down tire river, seemed in danger of being choked with the debris. Fortunately this menace has been overcome, but not without the expenditure of much labour and several interruptions to the supply of power. There were several who foresaw the results of the liberation of the river below the spillway, and the opinion was expressed in some quarteis that the impetuous torrent would rapidly cut its way back along its old channel, and so render the dam abortive. It is reassuring to be told by those most competent to judge that there is no danger of such a development, the rocky ledge of the present waterfall forming a sufficient barrier to further erosion at this point. Between the fall and the reunion with the regular riverbed the scene is a weird one. Where was once a scrub-covered flat the soil and gravel have been removed to a great depth, i evealing a forest of dead trees, standing as they grew in a former geological age, their roots still embedded in the old land surface. Many of them will doubtless fall over a s the river undermines them. Their tops are shorn off at an almost uniform level, but as the roots sprang from the uneven surface of the old land, the actual measurement of the tree trunks ranges from 10ft to 40ft in height. The timber consists of the usual present-day forest admixture, including rimu, kahikatea, and rewarewa. The last-mentioned, it may be noted, is net a swamp timber. Some of the rimu trunks measure Oft in diameter, and most of them still retain their bark. The rewarewa is reduced to a fibrous, cellular state, and all the timber shows slight signs of carbonising thiough its long burial. A good many of the stumps still hold on their truncated tops cushions of the peaty composition with which they were covered. The whole area of the forest had been buried in pure sand and pumice, amongst which were occasional pieces of charcoal, which, like a good deal of the pumice, may have floated from a distant source.

A person passing over the arid, flatbottomed Waititi Valley a few months ago would have found no’ indications that a forest lay below, unless by carefully examining the bed of the creek which flowed swiftly along its northern side. Here a few old tree stumps could be seen, but the stream had worn into the older rocks, ryalitic bieccia and tuffs, which had checked further erosion.

Prior to the deposition of the gravel this was a wide valley of considerable depth, whose bottom and sides were forest-clad, and along whose bottom flowed a stream, about the size of the present one. To trace the next stage is doubly interesting, because it serves as an index to the more recent deposits in the whole Waikato Valley. Vast accumulations of pumice and sand overspread this part of the basin, smoothing over spurs and valleys and creating a wide river flat, through which the river meandered, the old bed thus formed being the present headrace to the penstock tunnels. The uniformity and wide extent of- these deposits suggest tiiat a volcanic eruption in the upper reaches of the Waikato covered a wide area with easily eroded material such as. may be found covering the region around Mount Tarawera..-

The Waikato River has been the subject of much-speculation in regard to its original course and outlet. A widelyheld theory is that it formerly discharged into the Firth of Thames, but that by a process of “ beheading ” it broke through the Maungatautari Gorge and found it's way to the west coast. It must be borne in- mind, however, that a gorge is necessarily older than the mountains through which it now seems to cut. If the Waikato even had another course it was at a very remote geological period.

There is evidence that at the time of the immense deposits of pumice and sand above-mentioned the present Waikato Plain was a deeply eroded valley, partly bush covered, and bordered by steep slopes which here and there presented cliffs and escarpments. The burying of the forest at Arapuni was probably contemporaneous with the discharge into this valley of vast, deposits from the volcanic belt. The light alluvium wa§ the foundation of much of the dairying, land in the Cambridge district, the subsequent elevation of the region causing the river

gradually to recede into its present channel, leaving the veil-known terrace formation as seen at the present day.