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Volume 72, 1942-43
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– 350 –

(b) The Kaiwhata Valley.

The Kaiwhata River drains the low country situated between the Brocken Range on the west and the coastal range on the east which runs south-south-west from the Mataikona River in Eketahuna Subdivision. The Kaiwhata Stream drains the western slopes of this latter range and flows westwards to Ngahape, where it is joined by the Kopi, Te Maire, and Totara Streams. From this point for a further six miles it travels south along the general strike of the country as a stream of considerable size; again it turns at right angles, and cutting through the coast range in a deep gorge, discharges on the east coast about eight miles south of Uruti Point.

The drainage system is now deeply entrenched owing to uplift, whilst deforestation has greatly increased erosion. In the lower Kaiwhata Valley south of Ngahape there is a prominent, high-level terrace representing a former erosion level, protected now by the rock barrier of the coastal range, which is composed of white, pink, and grey fine argillites of the Whangai formation.

Time permitted of only a rapid reconnaissance of the Kaiwhata Stream east of Ngahape; but the information gathered may be of value.

At the stream junction about two miles east of Ngahape the stream cuts through well-bedded, fine, micaceous, black argillites and sandstones (P. 7612), which are twisted and shattered by faulting. The rocks strike about south-west and dip generally to the north-west but are often vertical. These beds are probably of Tapuwaeroa or even Raukumara age, and their attitude suggests a strong fault between this point and the coast range.

Downstream the dips flatten somewhat, and the strike swings to the west. Half-a-mile west of the junction, near the old woolshed on the north bank, the first of a series of conglomerate bands appears with pebbles of greywacke up to 2 in. in diameter. No igneous pebbles were seen. This is followed by a set of fine bedded sandstones, 25 ft. thick, dipping 40° to the north and overlain in turn by another coarse conglomerate. A small fault here brings fine black mudstone into juxtaposition with the conglomerates. This mudstone contains Inoceramus (Callistoceramus) bicorrugatus Marwick (P.7603), the characteristic fossil of the Mangaotane mudstone (Raukumara Series).

The succession continues downstream with alternating mudstones and sandstones and occasional thick conglomerate bands, the grade of the pebbles in the latter appearing to decrease as the beds become younger.

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The Kaiwhata Sill (See Sketch Map).

In the Kaiwhata Stream, about 20 chains upstream from the Ngahape bridge, the stream intersects a large sill of hard, crystalline, dolerites weathering in cuboidal blocks (P.7602). The sill appears here to be about 20 ft. thick, though contacts with the neighbouring rocks were not seen.

The reason for calling this igneous mass a sill is due to its attitude seen from the hills above the stream. It stands out as a hard erosion remnant amongst the softer sandstones and mudstones, and is seen to conform closely with the bedding of the latter, and follows the inclined V-shape of the valley. That it is not a flow is indicated by outliers above the next succeeding sedimentary rocks.

The writer followed the sill exposures for about a mile and a half to the north-east, but owing to the limited time did not go further, though the indications were that the sill is exposed for a long distance farther north-east. McKay (1899) noted that an intrusion crossed the Kaiwhata River a mile below the junction with Bismarck Creek; this is on a projection of the line of the Kaiwhata sill and two miles south of its intersection with Kaiwhata Stream.

The igneous rock forms prominent buttresses, and in exposures to the north of Kaiwhata Stream is extremely coarse-grained with long fibrous crystals of feldspar and clinopyroxene (P.7607, 7615).

Downstream from the sill the mudstones and sandstones continue, and at the junction of the Kaiwhata and Kopi Streams there is a thick bed of soft greensand (P.7609). Westward to Ngahape the beds are highly glauconitic.

West of the Kaiwhata and Te Maire, greywacke and indurated sandstone form prominent hills such as the Sugarloaf. This is strong evidence for a fault running approximately north and south along the line of the Kaiwhata Valley; but owing to the lack of data its position is ill-defined.

Ngahape Intrusion.

On the Kaiwhata Valley road about 200 yards north-west of Ngahape School is a large outcrop, 100 yards long and 30 ft. high, of black, zeolitic, moderately fine-grained variolite displaying pillow forms in one or two places (P.4886–4893; P.7608). It intrudes sandstone and argillite (? Tapuwaeroa), pieces of which form large horn-felsed xenoliths in the cliff exposure (P. 7614). The rock is deeply weathered and is traversed by thick veins of calcite (P.7605). This mass was first noted by W. A. McKay (1899), and was recently visited and sampled by Mr. I. J. Pohlen, of the Soil Survey Department. The upper part of Cemetery Hill, of which the igneous rock forms the lower slopes, is composed of pink and yellow fine argillite.