Topic: Strata - Three Sisters Beach
The Three Sisters beach consists of a low-lying dune coastal forest area at its northern end, or first third if you like. The remaining southern two thirds consist of the more typical interbedded mudstone and sandstone cliffs. The beach here has no inter-tidal upper zone.
To the rear and just north of the Three Sisters, there is a common garden variety through cave that houses some Maori petroglyphs, more commonly referred to as the Maori cave carvings. They are located at the southern end of the cave and are high up on the cave wall, starting at around 12 feet above ground level.
30 July 2003
This is the Little Sister, part of the iconic Three Sisters rock stacks. Sadly, the Little Sister was destroyed in the ferocious alpha storm I recorded on the 29th September 2003.
13 August 2003
PHO2008-131 (Example). Also, PHO2008-120
CLIFF SEQUENCING. Immediately south of the Three Sisters’ blind corridor cave entrance, there was a small ‘ROOM’ in the cliff wall. PHO2008-131 is more a location shot, taken as part of the cliff sequencing.
To the rear of the Inner Sister, the cave housing the Maori carvings is visible in the lower right of photo PHO2008-120.
28 August 2003
This is the cliff section that houses the Maori cave carvings.
23 January 2004
PHO2008-526, PHO2008-527, PHO2008-1204
Photos PHO2008-526 and 1204 show how particularly soft rocks can dissolve during heavy rain. (Also detailed in Section Two on Weather). Photo PHO2008-527 shows a ‘flowing rock’ formation at the base of Mammoth Rock that I particularly liked.
5 April 2004
I was quite taken with this unusual small Room in the cliff. (First shown on 13.8.2003). If you sat inside it, it would give the impression of being in a room with a view. To the rear, the room was decorated with green algae ‘wallpaper’.
24 July 2005
The ‘room’ in the cliff wall first observed on 13.8.03, has now been destroyed.
21 August 2005
I photographed the MAORI CARVINGS with my 135 mm macro telephoto lens. This is the first time I have photographed them. This is because I had to wait for the right light conditions. Being high up in the cave, I needed for the sun to be at a low angle in the sky. Also, I needed for the light to be diffused so as to eliminate harsh shadows.
9 September 2006
A very low 0.1m tide was due at 4.51 pm. There was a slight north-easterly breeze and it was raining steadily. After I had finished on the Four Brothers Beach, I returned to the Three Sisters Beach at the end of the day. Just past the Sisters (north), while trying to set up a shot which ultimately didn’t work, I was struck by three dark, shark-like dorsal fins protruding from the water just beyond the wave-line. I knew they weren’t sharks as they didn’t move, but wondered what they were.
After I had photographed the Sisters I want down to the shore-line to have a closer look at the ‘sharks’ fins’. I wondered if they might be part of an ancient shipwreck due to their positioning. From what I could see, they appeared to be the remains of ancient fossilized trees, all virtually identically pointed like black shark dorsal fins. It was too dark by now to take a good image. Also, I was too stuffed to change the roll of film. I filed it away in my mind to take a photo on another occasion. As they only seem to be visible during extremely low tides, low beach states and calm weather, I may have a long wait. (See also Section Eight on Beaches).
As of 5 September 2010, I still haven’t taken a photo of them.
15 July 2007
According to a small article in The Taranaki Daily News on 3 July 2007, photographer Harley Betts noticed that the MAORI CARVINGS HAD BEEN DESTROYED by erosion. I presumed that there had been a cliff collapse. However, upon finally seeing for myself, I discovered that the cliff that housed the Maori carvings cave hadn’t collapsed; rather, the particular section of cave wall that contained the Maori petroglyphs had been preferentially carved out by a single rogue wave. The rest of the cave appeared undamaged. I photographed the area where the Maori carvings had been. (This loss has also been noted in Section Four on Cliffs).
I also photographed the remains of the room in the cliff wall located immediately south of the Three Sisters Cave. This was observed to have been destroyed on 21.8.05.
24 July 2008
SUPER-STORM EVENT. Super-Storm Two. This storm took out the sand/land bridge that had connected the Three Sisters dune to Mammoth Rock. This had been in place for at least 50 years.
18 August 2008
PHO2011-1293 shows part of the dune in the process of being cut back, plus rocks on the heavily scoured out beach in Zone One. PHO2011-1289 shows the beach scoured right down to bedrock in Zone Two.
12 November 2008
PHO2011-1366, PHO2011-1367, PHO2011-1400
CLIFF SECTION COLLAPSE. Although this is detailed in Section Four on Cliffs, I have included it here because of the sheer scale of the collapse. Due to its size, I had to think of a new description for this type of collapse. (This cliff section collapse also gave birth to a New Sister rock stack. See Section Six for more information).
The larger the size of the collapse, the more likely it will trigger a Ripple Effect. These Ripple Effects can cause ‘daughter’ cliff collapses in the near vicinity. This is because the outward travelling shock-waves are powerful enough to breach the gaps that separate neighbouring fracture lines that under normal circumstances, proof their host cliff from ‘next door’ collapses. However, if some of the neighbouring fracture-lines are inter-connected, then the end result can be compounded.
Put simply, the larger the collapse, the further the shock-waves can travel, the greater the area that can be affected. This can happen over a period of time, ranging from a few hours after the initial event to more than a year as daughter collapses can trigger grand-daughter collapses and so on in a spill-over Ripple Effect. The separation distance in terms of both length and time between the original cliff collapse event and the final grand-daughter collapse, can be so great that at first glance they can appear to be disparate, separate events.
Added to this is that a cliff collapse usually ‘favours’ one direction over another. That is, although superficially the collapse may appear to have occurred ‘front on’, it is more likely that the initial cliff failure occurred off to one side, then radiated outwards and downwards along the path of highest susceptibility or least resistance. This can explain why subsequent daughter collapses tend to occur on one side and not on both sides as one might assume. If the cliff collapse was triggered by, say, a massive storm event, the storm’s angle of hit would in a big way account for the handedness of subsequent shock-wave travel.
For example, though the prevailing weather conditions may come from the south-west, the most potent rain-bearing storms hit from the north-west. Furthermore, they hit the Tongaporutu coastline at a more direct angle. If we follow this through to its logical conclusion, then using the cliff section collapse as an example, any daughter collapses should occur on the opposite side – to the right (south) of the original impact zone.
In this instance, I believe this has subsequently proved to be the case. Though the massive cliff section collapse occurred in November 2008, I believe it was caused by the Super-Storm Event that occurred in July 2008. These storms triggering a fatal weakening of already weak key fracture lines, thus leading a few months later to the catastrophic cliff section collapse.
Following on from this, there was a full cliff face collapse just south of Elephant Rock on 11.1.09. Then, on 25.5.09, there was a full cliff face collapse immediately to the rear of Elephant Rock. Specifically, it was immediately north of the 11.1.09 collapse.
As of October 2009, there have been no further cliff collapses, but the original cliff section collapse site is still unstable with ongoing loss of material, some of which is quite substantial.
At cliff tops, the sand and volcanic ash soil mix is quite loose. This is due in part to the wobble factor. The wobble factor is caused by shock-waves radiating upwards and outwards from waves smashing into the base of the cliffs. The cliff bases present as solid, unyielding structures that are subject to cracking and fracturing. These fracture lines radiate upwards. The porosity of the top soil strata makes it highly vulnerable to top down collapse because it is affected by both bottom up and top down erosion causal agents.
14 December 2008
PHO2011-1436 (Example only)
8 February 2009
Of the cliff section collapse that I documented on 12.11.08, the only part of the original cliff to have survived is part of the outer wall that housed the defunct Maori Cave Carvings. The gap between it and the new cliff wall boundary is around 25 feet.
19 September 2009
PHO2011-1678, PHO2011-1679, PHO2011-1680
At the northern end of the cliff section collapse site, there were two interesting rock features. One was a low rock bridge. This was in the actual bedrock. Due to the bedrock being exposed for extensive periods of time, the wave action has hollowed out the interior of one of the bedrock bases and sculpted a low rock bridge several feet in length. The other interesting feature close by were two roughly conical rocks close together, that when lined up with the Middle and Inner Sisters, resembled two smaller versions of the Sisters rock stacks.
I also photographed some interesting caterpiller-like rocks on the beach at Zone One.
18 October 2009
Close to the first arch, there is a low, narrow, east to west facing a broken caterpillar-like rock formation that houses a petrified log or parts of a tree trunk.
Other sites that contain fossilized trees are: Te Kawau Pa, Gibbs’ Fishing Point, specifically to the rear of the fishing ledge, and Twin Creeks. They are detailed in Section 10 on Flora and Fauna, flotsam etc.
15 November 2009
It was raining with a north-westerly wind. I was up at Tongaporutu with Pam Murdoch, Dawn Bowen and Mark Robbins, members of the Taranaki Geological Society. It was a planned outing. The weather was wet and miserable.
At the end of the high sand cliffs, (part of the dune area of the Three Sisters beach), and just before the arch, I noticed a rock stratum just above the beach that was a lovely powder blue colour. Pam thought it was the rain that brought the colour out. I agreed. I haven’t noticed it before. It really glowed in the wet light. Pam went to investigate and was surprised to find that it was composed of a very soft clay. A bit like a softer version of Playdough. I took a photo of the blue clay seam on its own, then another photo with Pam in it to give an indication of size. I believe that this is the same type of clay as the nodules of brilliant blue clay early European travellers found in the papa. These nodules were called ‘pukepoto’ and were prized by Maori as a pigment. (As notated from the Coasters book, page 254).
2 December 2009
The lovely soft powder blue rock seam that I first noticed on 15.11.09 is still there, but it is partially buried beneath a higher sand covering. The light is still ‘wet’ with fog and low cloud. It will be interesting to see how it changes in bright sunlight conditions – if at all.
16 January 2010
The powder blue rock seam retains its lovely colour in fine conditions. Currently it is only partly visible due to high sand cover.
18 September 2010
MEGA-STORM. Though this section is primarily concerned with the more tangible features of the coast, this storm was of such a magnitude that I just had to include a couple of images here. PHO2011-1863 shows part of the dune, while PHO2011-1866, though also photographed from the gap at the dune, looks across towards Pilot Point. The more powerful the storm, the more sea foam it generates. This was the biggest individual storm I have ever documented to date.