Geology.

The Tararua Mountains represent that position of the main axis of New Zealand mountains running in N.N.E.–S.S.W. direction, which extends from the Manawatu Gorge to Cape Turakirae, being some 150 km. long and 25 km. wide. On the east side they are bounded by the Wairarapa Plains, which rise to about 300 m. towards the middle of the mountains and about the low hills of the main divide, descending plains continuing northward of this. On the west side they are bounded by the Tasman Sea and the Manawatu Plains, which for the most part lie well below 100 m. at the foothills. The hills above the Manawatu Gorge are only about 450 m. high. From here they rise gradually towards the middle until they are about 1500 m., with Mount Arěte, 1504 m., in the centre of the watershed, and Mitre, 1570 m., which is off the main range, the highest peak. Continuing southward, the main range often drops well below 1200 m. until Mount Hector, 1529 m., a secondary centre of the system, is reached. From here the range rapidly drops to below 500 m., at Rimutaka Saddle, and then continues under the name of Rimutaka Range, gradually rising towards Cook Strait with Mount Matthews, 939 m., the highest peak overlooking Palliser Bay.

The mountains are built almost entirely of non-fossiliferous Trias-Jura sandstone (mostly submetamorphic greywacke) with some shales. There are small patches of younger limestone at the foot of the hills at Manawatu Gorge and along the plains on the east side together with some calcareous mudstone. The extent of these is, however, almost insignificant. The apparently recently elevated range in the vicinity of the Cook Strait is composed largely of black, slaty shales which crumble readily and run into extensive screes. The mountains, it would appear, were generally elevated in recent geological times some 600 m. with further upward warping in their middle part. Throughout there are still numerous places of uneroded tableland from which streams often plunge hundreds of metres into deep gorges. Valleys with more or less extensive river flats are not uncommon. Narrow rugged ridges as well as more gentle ones with numerous tarns are frequent, while many flat-topped summits are boggy.

Climate.

Tararua Mountains are subject to intense moisture-saturated N.W. winds. On the average, for about five days the wind blows from N.W. or N., on three days from S.E. or S. The winds are more evenly divided in July, while in January N.W. wind is more than twice as frequent as S.E. The average velocity of S.E. winds is much lower than that of N.W. The lowest mean monthly velocity at Wellington occurs in July, about 4.3 m.p.s., and the highest in October, about 6.2 m.p.s. The velocities must, of course, be much higher in the mountains. Kidson (1930) indicates on his maps that at about 1200 m. the average number of rainy days is about 200. The rainfall at this elevation exceeds 250 cm., while in the neighbourhood of 800 m. it is only 100 cm., the rainfall becoming more frequent and heavier with increasing altitude. It is fairly evenly

distributed throughout the year. Occasionally snow falls as low down as sea-level. Above 1200 m. it persists almost continuously for about five months, from June till November. At these heights frosts occur even in the warmest parts of summer. The coldest month is July with mean sea-level temperature about 8.5° C., and the warmest, January with mean sea-level temperature about 16.7° C. The crests of the ranges are fog-bound for some 200 days a year. For two years careful records were kept, with the assistance of Mr. A. J. Hilkie, of Wellington, on the fog conditions as they could be observed from Wellington and Palmerston North. The following table shows the weather prevailing on the high crests of the central portion of the ranges during the 1932–33 season. Any day with more than 4 hours of clear weather is shown as a fine day (C), days with continuous heavy fog are marked (H), while days with intermediate conditions are marked (L). That direction of wind which prevailed during the greater part of a day is recorded as if it actually prevailed during the whole day. Calm days are credited with the wind that prevailed on days preceding them. Calm days, or days with changeable winds, however, are of too rare occurrence to affect the general accuracy of the table.

The seasons preceding and following 1932–33 indicate that the weather during the period was exceptionally fine. Thus, in 1931, there were only forty-six days with fine weather on the crests and about fifty-five in 1935. No accurate record was kept for lower ranges at either end of the mountains, but continuous observations indicate that the foggy conditions prevail there as frequently as in the higher central ranges. The southern end tends to be covered with fog more frequently during S. or S.E. winds. The same is the case with the hills north of Mount Bruce.

Man's Influence.

Extensive areas on the Tararua Mountains below 600 m. in many places have been converted into pastures or, as is the case in many places where man was not sufficiently judicious, it is laid waste, whole hillsides running into slips. Timber milling is in progress in a number of places. In others stock has full access to the otherwise untouched forest. The vegetation of these places was largely reconstructed from scattered, more or less well-preserved, remnants or else left as gaps, probably never to be filled. This particularly applies to the low range branching towards Wellington.

The vegetation, where it has not been touched by man, is remarkably free from introduced species. Almost the only places subject to invasion are river-beds where Trifolium repens, Holcus lanatus, Dactylis glomerata, Nasturtium officinale, Agrostis spp. are of common occurrence. Most of the other species of nearby cultivated areas also occur here. None are found in the forest except perhaps occasionally along regularly used tracks and about mountain huts. There is only one species, Hypochaeris radicata, found above the forest line, but even this seems to be able to compete against native plants only with the help of deer.

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