Topic: Caves - Follow-ups
PLEASE NOTE: Dates recorded AFTER 18.9.2010 will be FOLLOW UPS ONLY as the Tongaporutu Project is now completed.
The follow ups will mostly be limited to the Four Brothers Beach. However, should anything major occur at any of the other locations, then they will also be recorded.
FOLLOW UPS TO THE MAIN TONGAPORUTU PROJECT
The Four Brothers Beach
At the Twin Arches Cave, at the northern entrance, there had been a full cliff face collapse. This had completely blocked off the northern entrance. This site has experienced collapses of varying sizes in the recent past. The largest, though not actually observed until 28.9.2008 due to the beach being inaccessible, most probably occured on 20.7.2008 during Super-Storm One.
Over time, that massive roof collapse material washed away to once again reveal the northern entrance. This latest massive collapse, which had a live flax plant atop it, was mostly likely instigated by the Mega-Storm I observed on 18th September.
Down at Cathedral Cave there had been a massive new collapse. Specifically, part of the cliff wall collapsed immediately above the cave’s roof and on the landward side of the cave’s entrance. As with the Twin Arches Cave collapse, I believe that this was also instigated by the Mega-Storm on the 18th September.
5.12.2010 PHO2012-0591-0593, 0595
At the Twin Arches Cave, it appeared that another substantial cliff collapse had occurred. This has extended the debris field almost out onto the open beach. The source of the collapse appeared to be the part of the cliff that extends out from the cave’s northern entrance on the seaward side. (PHO2012-0591 and 0595)
Shortly afterwards I went around to the smaller, main southern entrance. This consists of two entrances. The one on the seaward side is a much smaller, narrower corridor cave. This remained intact. The landward entrance leads to the cave proper. A gap open to the sky revealed that the cave had lost approximately two thirds of its ‘mass’ or roof. From what I could see, the blind corridor that leads landward from the main cave remains intact, although there was cliff collapse material on the floor.
The Twin Arches Cave, or what remains of it, could calve off to form a fairly substantial new rock stack in the near future.
Down at Cathedral Cave, I photographed a young couple whom I met up with on the beach. I asked them to pose near the cave’s entrance and cliff collapse material for scale. They were from the United Kingdom. He was Ricky Gellatly, an Oceanographer, and she was Ava Zecevic. Both had seen me in the short film running down at Te Papa in Wellington.
I had come up to Tonga especially to see Carol MacKenzie. She does not have much longer to live. Though I did do some limited photography, it was not the primary reason for my trip.
The weather was stormy with big seas and big winds. Fortunately, most of the active rain band had passed. On the MacKenzies farm, I battled my way against the wind to the cliff-top directly above the north-western entrance to the Twin Arches cave system. Due to the pounding seas, a lot of the substantial cliff collapse debris had been washed away. I could see the sea rushing into the cave from the south-western entrance.
I took a vertical photo looking down on the cave, but the resultant image may not be sharp due to the ferocious wind. (It did end up being sharp – PHO2012-0601).
At the Picnic Table Overlook I obtained a good, if wild view of the Cathedral Cave and the rapidly disintegrating cliff fall debris pile. From what I could see, another crack has appeared in the landward adjoining cliff face. Also, the cave entrance appears to be in the process of being ‘eaten back’. This seems to be following the ‘eating back’ of the Wall, specifically the northern end that culminates with the prominent ship’s bulb feature at its base. (The Wall being on the Gibbs’ Fishing Point side of the cave, or to seaward).
Again, I had come up to Tonga to see Carol MacKenzie. Later on in the afternoon, I walked across to the cliff top that overlooked the Twin Arches cave system. I hadn’t brought my camera up with me so couldn’t photograph what I saw.
A lot more material had come away, most likely triggered by the weather bomb of tropical origin that hit last Sunday, the 23rd. On the seaward part of the cliff, opposite the mainland cliff, a large chunk had come away. It was literally cut in half along a fracture line that I had observed earlier. Specifically, this outer cliff ‘finger’ is being preferentially eroded in a north to south direction towards the Twin Arches. On top of the ‘bridge that links the mainland cliff, passes over the cave and across to the seaward cliff finger, at the bridge’s narrowest point, it was only about five feet across.
More soil and cliff remnants continue to collapse on both the mainland cliff face, the bridge cliff face and the cliff finger cliff face. A flax plant near the narrow part of the bridge is next in line to erode out.
Though I had come up to see Carol, I also wanted to document any changes to the dune on the Three Sisters Beach, check on the New Sister and up here on the MacKenzies’ farm, check on the Twin Arches cave system. To access this overlook I had to climb through a fence. I noted that part of the fence is only about 18 inches away from a sheer drop to the bottom of the cliff. Anyone not knowing what they were doing could easily fall over the cliff. I am grateful to the MacKenzies for access to this site.
After sitting a while with Carol, at around 2 pm, I went to the cliff top that overlooked the cave system. The timing was important as I needed the sun to be almost directly overhead to minimize any shadows. I took eight images from various sites atop the cliff. From what I could see, there appeared to be little change from when I was last here on the 30th January.
On the northern side of the narrowest point, (less than five feet across), some slump holes were present. Though partially covered with grass, they were visible, but only if you knew to look out for them. Also, in places there were severe soil overhangs. Standing on any of them would be to invite death. Heading back from the narrow point towards the mainland, some dead and dying pohutukawas and pepper trees were present. Though they had a magnificent view down on the cave debris field and north along the Four Brothers Beach, they were doomed to fall down the cliff and join the dead flax bush.
So far the sea has not been vigorous enough to break through the substantial debris field at the northern entrance to the cave. Because of this, the cave is not currently through-going.
As an aside, I photographed Pinocchio from the narrow point. This showed it from a never before seen different angle.
I believe that when the Twin Arches cave system is through going (to waves) again, then the erosion process will accelerate.
Though the narrow point atop the cave will most likely crumble away first, I believe the erosion process will slow when the downward collapse reaches harder rock. This also occurred at the New Sister. All the loose, less dense material has gone and only the harder cliff base remains. This in turn will fracture along specific fault lines and carve off in large chunks.
Lastly, I went over to Cathedral Cave to check on that. The large debris field was still there. It appeared as if more material had come down, but nothing major. There also appeared to be a partial cliff fall on the seaward entrance side. This is the landward side of the Wall. (PHO2012-0616).
20.3.2011 PHO2012-0516-0517, 0530
I particularly wanted to photograph the dead and dying trees above the Twin Arches Cave with the panoramic camera. The weather was mostly fine with some high cloud. I photographed at about 2 o’clock in the afternoon to limit shading on the eastern side of the cliff that leads into the cave.
Before photographing however, I checked the lie of the land to ensure that I wouldn’t be standing on an unsupported overhang. From what I could see the trees closest to tumbling down the cliff were overhanging by several feet. Little had changed since my last visit, although the debris field at the base had shrunk.
After this, I went over to the Picnic Table Overlook that looked across to Cathedral Cave. Once again I wanted to photograph this with the panoramic camera. Some dozing sheep down where I wanted to photograph were slow to leave. This was because they were relishing the shade, it being hot out of the southerly breeze.
More of the cliff face on the landward side of Cathedral Cave was looking increasingly unstable and prone to collapse. The cave’s entrance on this side, plus its attendant cliff appears to be retreating south. This may in part be due to the highly energized action of back-washing and cross-washing waves which the local topography creates here.
With it being a 0.0m low tide, despite there being a 2 metre swell running, I knew I would be able to access the beach. I particularly wanted to photograph the Twin Arches cave’s debris field and the Twin Arches with the panoramic camera. This also included more of the cliff that led up to the cave.
I didn’t photograph Cathedral Cave from the beach as I already had some good ground shots from the 13th February.
Lastly, before I reached the Oldest Brother, and before I started ‘workies’, I noticed that there had been a fairly recent cliff collapse. At the base of this there was a cave. Immediately to the right of this cave was the northern entrance to a through-going arch that led to where a permanent stream flowed down from the top of the cliff opposite the Oldest Brother. Immediately south of this overflowing waterfall there was another cave. Upon examining this more closely I discovered that it actually connected to the cave I observed beneath the cliff collapse on the northern side.
Thus there is a through-going fairly large cave and a through-going arch close by. I also noticed a substantial top to bottom fracture line on both the northern and southern sides of this mini-Twin arches cave system. The only thing it currently lacks is a large, overhanging bowl such as the one that has been destroyed at the Twin Arches Cave.
Although I didn’t photograph it on this occasion, I will see how it evolves. Whether it does evolve into a mini-Twin Arches cave system, or whether due to the substantial fault-lines, end up in a catastrophic collapse like the Pilot Point arch that I observed in July 2008.
After this past week’s mega-storm, I suspected that the Twin Arches cave system would mostly likely have been affected. As I didn’t think I’d be able to access the Four Brothers Beach, I went over the MacKenzies’ farm to the cliff-top that looks down on the cave.
A lot of material had come down from both sides of the cave, north-western side. Material, including some of the trees where I had stood and taken some panoramic photos on the 20th March, had also come down. The fence on the landward side is only a few feet away from toppling down the cliff. This was where I took the single photo from! And on the seaward side of it.
Looking down, the cave is through-going, but it appeared to have shortened considerably in length. This is due to the cave wall/roof being preferentially carved back in a north/south direction. The first casualty of course was the magnificent open-bowled dome. In short, both the landward and seaward facing cliff walls, along with the roof end wall are all in a state of on-going collapse.
8.4.2012 PHO2012-0646-0647, 0650
The Twin Arches cave has continued to lose material from its northern entrance, (PHO2012-0646) and track southwards. From its southern entrance, (PHO2012-0650), light emanating from the northern entrance is now clearly visible.
Cathedral Cave also continues to lose more material at its entrance.