Previous Work in the Area.
The district has been little studied in the past chiefly on account of the difficulty of access. The area here described forms part of four survey districts, Otahoua, Rewa, Wainuioru, and Kaiwhata in the Wellington Land District.
The first record of geological investigation is that given in 1861 by James C. Crawford, the Wellington Provincial Government geologist. He described the precipitous and jagged hills “on a line parallel to the East Coast, and perhaps at a distance of ten miles from it” as composed of stratified and tilted sandstones. The Brocken Range is composed of a group of these “taipos,” so-called by the Maoris, who held them in superstitious dread as the dwellings of evil spirits. Crawford noted that they were intruded by igneous rocks, which he called “trachytes,” and also described pebbles of the same rock from the bed of the Upokongaruru Stream, which flows from the western slopes of the Brocken Range. He noted that this “trachytic” rock decomposed into an iron sand, and “in it Mr. Haast has discovered a speck of gold with the microscope.”
In 1883, McKay visited the East Coast District of Wellington Province, and a year later published a short paper on the igneous rocks. In it he mentioned a later, fuller report; but this does not seem to have been published. He noted the frequent occurrence of igneous rocks on the coast from Castle Point southwards, and drew attention to the “syenitic and porphyritic” rocks brought down by the Kaiwhata River. He searched in vain for the source of these igneous boulders, but on information received from Mr. Beetham. M.H.R., he found a large outcrop of the rocks in the headwaters of the Upokongaruru Stream on the eastern part of the Brancepeth run. This, however, did not explain the presence of coarse, crystalline igneous rocks in the Kaiwhata River, for the Upokongaruru is a tributary of the Pahaoa River, a stream farther to the south. McKay recognized this fact, and added a note that Mr. Beetham had informed him that such igneous rocks traversed the old sandstones on the north-east. An examination of the Te Maire Stream or the Totara
Stream would have shown him that the igneous boulders do not come down either of these valleys, the only channels along which they could reach the Kaiwhata River from the Brocken Range.
The explanation was partly given by McKay's son, W. A. McKay, in 1899, when he recorded the presence of igneous intrusions and tuffs first, in the Kaiwhata river-bed about a mile below its junction with Bismarck Creek and second, near the confluence of the Te Maire and Kaiwhata. W. A. McKay spent two or three days in the Kaiwhata Valley in very bad weather during the early part of 1899, and he has given a good account of his work there.
The present writer found another completely independent intrusion of coarsely crystalline igneous rocks, which crosses the Kaiwhata Stream a quarter of a mile east of Ngahape.
The regional examination by the officers of the N.Z. Geological Survey in Eketahuna Subdivision to the north has not yet been extended into the Brocken District; but much of the geology of the latter can be correlated with that of the former area.