Hole in the Rock (Wellington) EarthCache

The northern, western and southern coasts of Kapiti are exposed to strong marine erosion and are steeply cliffed. Examples of this can be found along the route to this Earthcache

The divide separating drainage to the north-west from that to the south-east lies close to the western cliffs, and from its crest, regularly spaced ridges slope to the south-east, separated by deeply cut stream valleys.

Due north from Pukerua Bay, Wellington, shows the regular east-sloping ridges of the southern portion of Kapiti Island. The form of Kapiti is attributed to dissection of a tilted peneplain, and the strait between Kapiti and Paraparaumu to synclinal structure, Waikanae delta and Tararua foothills.

Kapiti Island from Paraparaumu Beach shows the regular pattern of eastward-flowing consequent streams dissecting the warped (monoclinal) surface represented by regularly sloping ridges and interfluve remnants. Rangatira Point is on the coast beneath the summit.

The lowest raised beach has been ascribed to the activity of the 1855 earthquake, but it is much older, for its deposits are consolidated and weathered.

The Wellington coast, both from the south and from the north, the regular eastward-sloping profiles of the Kapiti ridges suggest derivation from a warped or tilted peneplain, by the activity of consequent streams. The view of Kapiti from the mainland at Paraparaumu supports this hypothesis, for there seem to be residual, slightly dissected interfluve surfaces on some of the ridges. In the north, streams and ridges have an easterly direction, in the centre of the island the ridges slope south-eastward, and in the south they slope to the south-south-east, an approach to a radial pattern. Kapiti may thus be part of an elongated anticlinal dome developed by warping of a peneplain.

The two Fishermens Islets, off the south-east coast of Kapiti, are stacks some 50–75 ft. in height, and Tokomapuna, abut a mile off-shore, is a low islet surrounded by an extensive shallow reef.

There is strong evidence, from the nature of the schistose rocks, that a long-established fault zone coincides with the eastern coastline of Kapiti, but no certain evidence that late faulting had influenced the island's physiography. On the contrary, a hypothesis of minor sea cliffing, producing a nick in an east-sloping monocline, dissected by consequent streams, seems adequate to explain the observed features of the coast. Although the mainland coast opposite is in places strongly cliffed, and has been considered a fault coast, certain views of it hint at the possibility that it, too, may be a much modified monocline. Whether or not this is so, it would seem reasonable to consider the sea-filled depression between Kapiti and the North Island a synclinal structure separating the great Tararua anticline, on the east, from a minor fold, on the west, of which Kapiti Island is the only emergent.

Since undeformed Pleistocene sediments (Otaki formation) occupy parts of the syncline a little north of Kapiti. the folding may be attributed to the late-Tertiary, Kaikoura movements, without implying thereby any great precision of geologic dating. Occurrence and relations of the schistose rocks: Metamorphosed rock, superficially resembling the schists of Otago and Marlborough, crops out at Rangatira,

Other exposures were found protruding through the boulder-strewn surface of the raised beach, between the strand line and the higher land to the west.

On the coast south of the Rangatira foreland, greywackes and argillites are exposed, locally massive and irregularly jointed, elsewhere showing a regular and consistent interbedding of alternating dark and light, or fine and coarse, argillites and sandstones. Large beach boulders that cannot have been transported far consist of conglomerates and finely banded argillites.

Pug zones, in which the rocks are greatly sheared and brecciated, occur at several points, but these nowhere resemble the phyllonites. Most bedding planes and shear planes within pug zones strike parallel to the bedding of the phyllonites, but the dip is steeper (70° or more) to the west.

In all the Kapiti phyllonites the s-planes are clearly parallel to the original bedding.

The zone of phyllonite may perhaps best be considered a “horse” of altered rock formed in an early phase of disastrophic movement, brought to its present position in a later phase of faulting. The sediments of Kapiti are certainly of pre-Cretaceous, possibly of pre-Triassic age, and the formation of the phyllonites may date from the post-Hokonui (Lower Cretaceous) orogeny. The movements which faulted the phyllonites against unaltered sediments cannot be closely dated, but preceded the formation of the peneplain which truncates the structures in the old rocks at Kapiti. This peneplain was deformed before the deposition of the Pleistocene Otaki formation, and, in common with some other peneplains recognised in New Zealand, is believed to be later Tertiary in age. Structural relations: In the past, few have speculated on the relationship between Kapiti Island and the structures of adjacent parts of Cook Strait.

Probably the structure of which Kapiti is a part had a similar intermittent growth, but there is no evidence whether the warped and dissected peneplain of Kapiti is a pre-Pliocene surface, or a surface developed during stillstand at a Pliocene or post-Pliocene date.

If Kapiti is part of a crestal swell on an anticlinal fold, continuous for 150 miles from the Kaimanawa Range, an extension of the structure farther south-west may be sought. Submarine conditions in Cook Strait are not favourable for the preservation of structural features of the sea bottom. Tidal currents passing through the strait at speeds of up to 3½ knots are periodically strengthened by southerly and northerly gales. On the sea bottom within the strait, foul bottom, “ledge rock,” coarse detritus, and even derived Pliocene mollusca (locally), attest the strength of the bottom currents. Under such conditions, destruction of structural irregularities, and their replacement by a topography related to bottom scour, are to be expected, and these processes must have been accentuated if, as is generally believed, sea level retreated about 300 ft. during the last glaciation.

TO LOG THIS EARTHCACHE answer the following questions and take a photograph:

At the GZ you will find the Hole in the Rock which has being created from years of erosion, you will need to bring along with you a tape measure and a camera and answer the following to claim your Earthcache. Please take note of the weather conditions when doing this cache

Please do not post your answers in your log entry. Email your answers to us to receive permission to log, then upload your photo with your online log entry. No Night Visits, you won't see anything in the dark and your photograph is very important, It will be interesting to see the different pictures as the weather conditions and seasonal changes.

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