(a) The Brocken Range and Environs.
The Brocken Range is an easterly offshoot of the Maungaraki Range, which runs south-westward, parallel with the east coast and ten to twelve miles distant from it. The Brocken Range splits off near Rewa Hill and lies about halfway between the coast and the Maungaraki Range. The portion investigated lies to the south of the Fernyhurst Road, and includes the prominent peaks Te Maipi, Pukekowai, and Puketeitei rising to elevations of 1500–1600 ft. above sea-level. These peaks are jagged and precipitous “taipos”; and Te Maipi is almost inaccessible except from the south side. They form open grass country with small patches of native bush near the tops. The range is composed of hard, moderately coarse, indurated and shattered greywacke (P.7599, 7604*) which Ongley (1935) has correlated on lithology with the Taitai formation of the Waiapu Subdivision. The line of peaks seems to follow strike ridges of harder components of the formation.
This greywacke also forms the lower country on the east side of the range between it and the Kaiwhata and Te Maire Streams. Here again are the peculiar ridges and “castles” resembling basalt plugs or necks (see Fig. 3). The reason for their resistance to erosion is not known, as on examination their component material appears to be identical with that of the surrounding rocks. Such, for example, is the “Sugarloaf,” 780 ft. high on the north side of Totara Stream (P.7598).
On the north bank, near the junction of Totara and Te Maire Streams, a few small, well-rounded pebbles of porphyritic igneous rock (P.7617) were discovered in the weathered sandstone, thus supporting, but not proving, the correlation with the Taitai formation, which farther to the north contains numerous bands of igneous conglomerate.
[Footnote] * The numbers prefixed by “P” refer to specimens in the rock and mineral collections of the N.Z. Geological Survey.
Fig. 1. East side, Brocken Range from Sugarloaf Te Maipi in centre, Pukekowai to left with Baneepeth between Te
Stream in totoground
Fig. 2. West side, Brocken Range, from Te Maipi. Pukekowai to left, Tertiary limestone scap running from middle distance to extreme right and Red Hill teschenite in low rounded hill near right-hand border. Maungaraki Range in the background.
Fig. 3. Sugarloaf Hill, a prominent greywacke residual near Ngahape. Brocken Range in the background.
Text Fig. 1. Looking east up Kaiwhata Valley, The sill forms the prominent outcrop intersecting the opposite slope and forming a V pointing down-valley.
The most interesting feature and the main purpose of the trip was to discover the source of the teschenite pebbles and boulders found by McKay (1884) in the Upokongaruru Stream near Red Hill, on the Brancepeth run, and briefly described by Sollas (1906).
Between Pukekowai and Te Maipi is a low pass about 1200 ft., high over which runs a well-formed track. Passing from east to west, one descends into a hollow formed by the headwaters of the Upokongaruru Stream, evidently part of McKay's “crater” (1884), but in fact, merely the breached hollow between two greywacke strike ridges.
Abutting sharply against the greywacke about half a mile northwest of Pukekowai and on the south side of the Upokongaruru is a ridge of dark-brown, moderately soft quartz sandstone (P.7611), capped by 4–5 ft. of extremely hard coquina (P.7600) (a detrital limestone composed chiefly of molluscan shells) dipping at 25° to the west. This latter rock closely resembles the Ugly Hill limestone (Ihungia) of the Dannevirke Subdivision.
On the north side of the stream the igneous intrusion was discovered. This is evidently McKay's Red Hill; and it is composed entirely of coarsely crystalline basic igneous rock, a melanocratic teschenite (P.7616). From the scattered boulders of igneous rock aligned in a ridge on the south side of the Upokongaruru, it appears that here is a small dyke of teschenite which has altered sediments of the Taitai formation (P. 7606, 7613).
The main intrusion appears to be roughly circular. The rock on decomposing gives rise to a green, sandy soil (P.7610), and the
mass extends for about 20 chains north of the Upokongaruru Stream to a point almost due west of Te Maipi, where it is seen in contact with the greywacke.
The coarse texture of the igneous rock indicates a hypabyssal intrusion, the age of which is doubtful. The intrusion has certainly altered the older greywacke and argillite, but does not appear to have affected the younger (? Ihungia) sandstone and limestone, which seem to have attained their present position by fracturing along a line west of the Red Hill igneous mass. Hence the teschenite is post-Taitai (Upper Mesozoic) and probably pre-Ihungia (Lower Miocene) in age.