Topic: Pilot Point - overview

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SECTION SEVEN

The Tongaporutu River and the Family of Rocks.

OVERVIEW

THE TONGAPORUTU RIVER

The Tongaporutu River is bounded by Pilot Point on the northern side and the Three Sisters Beach on the southern side.  The Tongaporutu baches are to landward (east).  The Tongaporutu River is the most substantial river on the Tongaporutu coastline.  Travelling inland, the main body of the river eventually reaches its source at Mt Damper.

The Tonga River, as I call it, migrates back and forth across the estuary between Mammoth Rock (Pa Tangata) and Pilot Point.  However, the river doesn’t follow a straight line before it vroom vrooms into the sea.

The river tends to follow a winding, snake-like route to the sea.  This usually starts on the seaward side of the Tonga Reserve, (where the toilet is).  Thus, part of the river can be right over on the southern side of the estuary, between the Tonga Reserve and Mammoth Rock, but actually exits into the sea over on the Pilot Point side of the estuary.

For recording purposes however, I will only record which side of the estuary, either Pilot Point or Mammoth Rock, that the river actually exits into the sea from.  For example, as shown in PHO2008-1534 taken from the Pilot Point overlook on 11.12.2005, part of the river runs close to the southern side of the estuary on the Mammoth Rock side, but it actually exits into the sea on the northern, Pilot Point side of the estuary.  Further, the river may be between the middle of the estuary and Mammoth Rock, or between the middle of the estuary and Pilot Point.  Whenever I state “Mammoth Rock” or “Pilot Point”, the river is on that side of the middle, to whatever degree.

This fluid ‘handedness’ determines which side of the estuary the sand banks or bars build up on.

At Pilot Point there is a substantial sand dune area to landward.  To the rear of Mammoth Rock and along the first third of the Three Sisters beach, there is a low lying dune/coastal forest area with higher land to the rear.  Up until June 2008, this dune was connected to Mammoth Rock by a sand/land bridge.  According to the Gibbs family, this land bridge had been in place for at least 50 years.

Both of these dunes differ from each other and are detailed in Section 9 on Beaches, and Section 10 on Flora and Fauna.

 

THE FAMILY OF ROCKS

The family of rocks are located close to the Pilot Point arch.  They are situated mostly on the Tonga River side of the arch, with the outer members of the family being more to seaward.

I initially decided to document them from time to time to see if they moved around at all.  Most cliff collapse material eventually breaks up and is destroyed by the sea.  Large boulders closer in though often live longer.  How long is dictated by the specific location’s unique topography.

The family of rocks, though fairly close in, do not fall into this category.  When the Tonga River is over on the Pilot Point side, it envelops the ones furtherest out.  Despite the pulling power of the river, these rocks appear to be anchored here.  I believe they are survivors of a previous arch collapse.

At the Point, arches form, in time are destroyed, then the surviving debris forms a ‘family of rocks’.  At this location in the Tonga River, (obviously when it is over on the Pilot Point side), I believe an eddy forms.  That is, the tides and currents swirl around a moveable ‘dead spot’, much like the eye of a hurricane.  The rocks, though sitting on bedrock, may slightly slide around, the propellant being liquefied sand.  I have actually stood on one of these boulders and felt it move.  The higher the sand covering (of the rock), the more anchored in place it is.  The lower the sand covering, the more likelihood of sliding.  From what I can tell, the rocks do not appear to travel far or overturn; they just swivel around on their bases.  Perhaps their size prevents either occurring.

Eventually, as Pilot Point retreats to landward, then the oldest rock family members get buried and all evidence of past collapses is then lost.  If one follows a line roughly south-west and used sonar (or whatever), then if I am right, some of these older family members could be found.  Just as some rock stacks like the Three Sisters form a succession line, so too the family of rocks.

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