Topic: Caves - Overview

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“CAVE”  According to the dictionary, a cave is an “Underground hollow, ...  smash in (person’s) head ...”

Sea-caves are intoxicatingly dangerous places that are subject to rapid change.  And they can smash in a person’s head if one is unlucky enough to be inside such a cave when the roof collapses!

Caves can either be ‘blind’, as in having one entrance, or ‘through’, having two or more entrances.  Basically, caves are like houses.  And like houses, they come with a roof and walls, plus one or more entrances.  Unlike most houses however, they can have a cornucopia of arches, columns/pillars and overhanging bowls/domes, but not necessarily all at the same time in their evolution.

Cliffs give birth to sea-caves.  Usually they are carved out by the sea, starting life as small cracks or fractures.  At Tongaporutu there are several different types of sea caves.  How they are created (and subseqently destroyed), is dependent upon their particular location on the Tongaporutu coastline.  Specifically, whether a cave is associated with a promontory, a cove or in the cliff proper.  All of these features are affected differently by wind/wave action, dependent upon their angle of orientation to the prevailing weather patterns.

The prevailing wind/wave direction along the Tongaporutu coastline is from the south-west.  However, their angle of orientation relative to the coastline means that they tend to ‘hit’ at roughly a 75 degree angle.

One could also assume that the heaviest rain events also originate from the south-west.  However, this is not the case.  The heaviest rain events tend to be delivered from the north-west.  The angle of impact from this direction is closer to 90 degrees.

What this means is that, localizing factors accepted, sea-caves are predominantly carved out in a south to north direction and are preferentially eroded out from the north to south direction.  Thus sea-caves have an overall ‘handedness’ in both their creation and destruction.

Obviously, destructive wave action occurs from both directions, and ocean currents also play a part.  These currents are subject to seasonal changes and El Nino or La Nina weather patterns.  All of which can have a temporary reinforcing or a reducing effect. These abberations aside, for the most part, the above trends hold. 

However, due to the length of the coastline covered, from Te Kawau Pa down to White Cliffs, and more importantly its curvature, then these angles will differ slightly.  Another factor is that although the cliffs are basically composed of the same building materials of interbedded sandstones and mudstones, they do exhibit local variations of layering, folding and density.

These local variations play out in the types of caves, or if any caves at all, are formed.  For example, at White Cliffs, (just the part of it that I have documented), there are no caves at all.  Sometimes arches, when situated close to caves, can be considered as part of a cave system.  Two such examples are the Twin Arches Cave System on the Four Brothers Beach and the Pilot Point Cave System at Pilot Point.

There are two common types of caves at Tongaporutu.  These are determined by their structure and/or material, and their shape/geometry.  Specifically, the degree of fracture and curvature (3-D), or the degree of hardness and flatness (2-D).  For the most part, both of these cave types tend to be small in size.

The first type of cliff, (3-D), of which the Three Sisters Beach and the Four Brothers Beach have good examples of, create fracture type blind caves.  These start out as small, localized hollowed-out areas at the base of the cliff that preferentially enlarge over time.

 

Sometimes, instead of just becoming an increasingly larger blind cave that tracks inwards, (mainly east), the forming blind cave can track parallel to the cliff.  Further, if the particular cliff section the cave is forming in is protruding, then the cave can become a through-going, corridor cave with two entrances.  The Maori carvings cave on the Three Sisters Beach is a good example of this.  They are housed in a small protruding part of an even larger protruding cliff section.  This is in keeping with fractal scaling, (which applies to coastlines).  Fractal scaling means that something looks the same at all scales.

 

Some cliff promontories such as the Point, (it separates the Three Sisters Beach from the Four Brothers Beach), can be quite substantial.  These can also host equally substantial through-going arches or caves.  The Passageway Cave, (located at the Point), is a good example of a through-going cave.  It also has an added feature of a blind ‘room’.  Several of the larger caves also exhibit this feature of being both through-going and blind.  The most spectacular example of this type of cave is located at Te Kawau Pa.

 

Arches form the same way as caves.  For the most part, they are fairly insubstantial.  There are some notable exceptions however, such as the Pilot Point Arch.  To further complicate things, there are hybrids of arches and through-going corridor caves that can best be termed as through-going arch/corridor caves.

 

The second type of cliff (2-D), of which the cliffs immediately north of the Middle Rock Stack at Twin Creeks are a good example, create smallish, bowl type blind caves.  Due to the relatively straightness and length of the particular cliff wall that they form in, these caves are always blind, (single entrance).  Beach Two has examples of both fracture type blind caves and bowl type blind caves.  Virtually all are insignificant features.

 

At Tongaporutu, there are three other cave types.  Though they are created in a similar way to either of the two common type of caves just detailed, they are subject to additional creative and destructive forces that make them site unique. 

 

They are:

 

1.      Twin Cave Complexes.  (The Three Sisters Cave System and the Twin Arches Cave System).

 

2.      Wave energy transference created cave.  (Cathedral Cave).

 

3.      Opposite handedness (dome entrance) caves.  (The Passageway and Twin Creeks).

 

These  particular caves, along with the others, are fully described under their relevant headings.

 

Stand inside any cave and the first thing you notice is how much material there is stacked above your head.  The second thing you notice is how loud everything is.  You know you shouldn’t be there, but there is something hypnotic about caves.  Maybe it harkens back to when we used to live in them.  Maybe it’s a mix of primeval fear and fascination.  You know you shouldn’t be there.  But you are.

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