Overview
Sun. May. 9.
Lovely day, calm + [and] fine. Breakfasted at 8.30 and at 9. set out on another visit to the Oruawharo Ridge. Takapau is situated on a river-built plain of fine yellow water-worn gravels – these are exposed in the high terrace of the Porangahau River due east of rly. [railway] stn. [station] - crossed P. [orangahau?] at junction of the Te Pine Pine Stm. [stream] + ascended backslope of Rangitoto Limestone Ridge. At the foot of the backslope the gravel beds wedge out + are seen to rest on bluish papa (clay marls). 1/4pl. [ate] photo (1) of limestone scarp at one of the airgaps with slope covered with huge blocks derived from its wastage. On gaining the crest of the ridge I could clearly see that the ridge is the western limb of a plunging anticline – in the backslope I ascended the limestone strata dip west, in the end of the ridge near Takapau–Hatuma Road they dip north + in a small hill behind Oruawharo Homestead they dip NE [Northeast] or E [East]. From the R.L. [Rangitoto Limestone] Ridge I took a photo 1/2 pl. (2) of the Ruataniwha Plain with Takapau in centre, terraced trenches of Porangahau + Te Pine Pine Stms in foreground + Ruahine in background. On reaching the small watergap just south of the 3 airgaps I noted what appears to be an unconformity between the main limestone strata + an overlying shelly limestone, the two sets of strata being less widely separated at the crest of the ridge than at the foot of the backslope. Passed
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the next watergap + proceeded along the Maharakeke–Porangahau divide. The Continuous Escarpment dies out on the south side of this divide, but there is another (discontinuous) escarpment situated between its linear continuation + the R.L. Ridge. Reached southern end of ridge T of the Oruawharo Ridge – from this point two spurs or divides diverge, one being that along which I had come + the other composed of "rubbly" greywacke, leading to O. peak L. Screes occur on the outer slopes of O. ridges T + SN, but none on the slope leading down to the saddle between them. The lower rounded ridge with the hard greywacke "boulders" appears to merge into the lower western slopes of the ridge connecting T + L. The weathered forms of the two types of greywacke are totally different; one being closely-jointed, rubbly + fairly-hard; the other is a soft yellow sand rock with wide spaced regular joints. Reached the top of gorge north of O. peak A at 11.25 In one place just here the "rubbly" greywacke appears to dip. E.80°, but as there was no distinctive bed to go by the line measured may have only been a joint plain. Down the gorge I noted a probable fault, two sets of thin-bedded rock coming together at a sharp angle. Further down but before it passes beyond the greywacke area the gorge of the stream becomes a mere crack fairly deep + there is a very steep drop. Left stm. [stream]
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at this point and turning SW, [Southwest] ascended a narrow crumbling spur to summit of peak A. 1/4 pl. photo (3) looking NE [Northeast] from crest of A showing rejuvenated tributaries of Maharakeke. Reached summit of A at 12.15 + lunched. On the east side of the Oruawharo Ridge + much closer to it than the RL Ridge on the west, there is a limestone scarp facing west + cut into sections by the streams that similarly cut the Oruawharo Ridge. Besides the westward-facing limestone scarp just east of peak A + noted on a previous visit to this locality, there is another just west of peak B + another west of W, this last being surmounted by trig stn. [station] 1283ft. This discontinuous limestone scarp extends along the one line of "strike" at different altitudes + evidently undulates in a north-south direction, or, there are two bands of limestone – the former idea seems the more probable. In the gorge of the stream on the south side of peak B, this limestone seems to lie unconformably on the "rubbly" greywacke. Ascended this gorge which is deep + narrow + cut through the greywacke but although there are extensive exposures of bare rock, there are no distinctive beds to indicate dip + the whole is much weathered + minutely jointed. Further up I found a "slip" showing many thin alternations of greywacke + slate, all much weathered,
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and the beds undulating + irregular but apparently vertical in attitude. Finally I came to my original observation of the dip – a wide band of slatey rock enclosed between greywacke, its dip seemed to be 58° W. Followed the left-hand branch of the stream upwards + crossed its headwaters to the crest of the Continuous Escarpment. While following the crest of this ridge northwards I had an inspiration as to the solution of the several topographical problems hereabouts – an inspiration based on to-days observations:- The post-Hokonui deformation crumpled + uplifted the the Jura–Triassic strata to form the Tararua + Ruahine Ranges in their primary form . In addition ridges of Jura–Trias [sic] rocks were buckled up in the region to the east of the main axis of elevation. These eastern fold ridges were most numerous in the south Wairarapa district + the most northerly of them was the original Oruawharo Ridge. Subsequently, denudation reduced this ridge to a serrated form so that it formed a linear series of peaks with cols between. During the Tertiary submergence these higher portions of the ridge of the ridge existed as a linear series of islands. The Tertiary sea-bottom with its load of limestones, clay marls, sands +c [etc]was then uplifted by the first movements of the Kaikoura deformation + buckled into folds by the
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later movements, the pressure being from the NW [Northwest] to the SE.[Southeast] The Tertiary beds were thrown into folds of a more noticeable form than the ancient compressed Jura–Triassic rocks. The axis of one fold, an anticline plunging northward, lay on the western side of + parallel to the Oruawharo Ridge. The consequent streams on the eastern flank of this fold passed through the cols in the Oruawharo Ridge + later cut the present gorges. Subsequent streams opened out the crest of the fold, + the harder limestone strata formed the ridges. The consequent stms [streams] on the western flank of the fold cut the gaps as erosion progressed, through the Rangitoto Limestone Ridge. The above hypothesis, or theory since there is good evidence, solves the problems (1) of the plunging anticlinal fold of which the R.L. Ridge + adjacent escarpments form the western limb, not coinciding with the axis of, + having as its core, the Oruawharo Ridge; (2) the gorges cutting across the high Oruawharo Ridge; (3) the gaps through the R.L. Ridge; (4) the lack of gaps in the Continuous Escarpment – the inferior position in the fold of Tertiary strata of the hard band forming the crest of this ridge was the factor which caused it to become the waterparting throughout the later phases of the denudation of the fold.
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This theory of the ^'origin of the' topo.[graphical] + phys.[ical] features of this locality appears to fit all the known facts + thus seems to be the correct explanation. It does not appear to be possible to solve the ^'detailed' structure of the Oruawharo Ridge itself but this does not affect the application of the above theory. The old rocks of this ridge seem to have acted to some extent as a buffer against which the Tertiary Strata were compressed to form a somewhat sharper fold than usual hereabouts. Proceeding I followed the Porangahau–Maharakeke divide + continued through the larger watergap + on down to the road leading to Takapau – reached hotel at 3.5pm. After tea Moody + I went to evening service at Presbyterian Church - small building + audience Rev.[erend] Hodson preached + gave it to the congregation "hot + strong", condemning them for their half-hearted Christianity + their worldly desires – great stuff for an outsider. On return to hotel, found a company of gentlemen in Commercial Room playing poker with matches worth 3.d each.