Here, wave erosion has formed a tunnel through a weak point in the rock. The waves also compress the air at the end of the tunnel. Eventually these processes have extended the tunnel until it formed a hole at the far end.
While photographing the blowhole I also noticed a couple of Superb fairywrens flitting about some washed up kelp. I assumed they were after flies and other insects which had been attracted to this rotting seaweed. The Fairywrens were also, now and then flying into the entrance of the blowhole tunnel right in amongst the surf spray. If you look at the photo, they were flying to a point at the upper left corner of the tunnel entrance and maybe several feet in. I have no idea what they were doing in there.
This has been formed in much the same way as the blowhole only it has been a large sea cave as opposed to a smaller tunnel. Eventually the roof of the cave at the landward end has collapsed leaving a natural and rather spectacular arch.
In the photo below, the safety fence on the far side gives a bit of scale. A short walk from here there is a lookout where photo #1 was taken.
The Devil's Kitchen is very close the the Tasman Arch. This also would have been a cave but the entire roof has been lost. Cracks in the rock have have caused the weak point for this erosional process and this is also the reason for the vertical sides to this formation.
Here I shall quote verbatim from a National Parks pamphlet which says:
Remarkable Cave is a striking example of dolerite contact with sediments folded in the cave roof. It has formed as the result of the intersection of two large sea caves, eroded along two zones of weakness over many thousands of years. It is called ‘remarkable’ because some say the opening resembles the map of