"The Hole in the Rock" (Northland) EarthCache

"The Hole in the Rock" is located off the northern coast of the North Island of New Zealand. It is a popular tourist attraction in the Bay of Islands region and can be found just off the northen tip of Cape Brett.

The island was named "Piercy Island" by Captain James Cook, in honour of one of the Lords of the Admiralty, but is also known as "Motukokako" Island as it was named by early the Maori inhabitants of the area.

The island is mostly barren, more of a rock than an island, and its only inhabitants are gannets and other sea birds that use the island as a nesting site.

The famous cave was created over centuries by the actions of wave and wind against the rock causing erosion and fractures in the rock that over time formed a cave all the way through from one side of the island to the other.

The Sea Cave

More than half of the world's population live in coastal regions so it should come as no surprise that coastal features provide a source of recreation and a point of interest and inquiry as to origins and formation. The coastline is an area of constant change and development by forces much more powerful and unstoppable than our own, and considerably faster than a lot of other geological processes.

A sea cave, or littoral cave, is a type of cave primarily formed by wave action from the sea and the primary process of cave development is by erosion over time. Sea caves are found throughout the world along forming along coastlines and presented as relict caves on former coastlines.

In order to form a sea cave, the wave action of the sea typically acts upon a weakness in the rock, such as a natural fault or bedding plane between two layers of rock. The driving force behind the formation of the littoral cave is erosion, which, while acting on the whole coastline, will yield more erosion activity around the area of weakness, slowly tearing out a depression which will eventually form an enclosed cave.

As the depression forms, the wave action is increased due to the dynamics of the confined space, by both direct action and grinding from rock particles borne by the waves. Adding to the hydraulic power of the wave action is the abrasive force of the rock and sand particles.

Most sea caves exhibit rough and rocky walls, indicating a violent rock fracturing erosion process as opposed to smooth surfaces that would tend to suggest a consistent flowing motion of the water rather than that of turblent waves crashing.

The Future

History tells us that the future does not look good for "The Hole in the Rock". It will eventually erode to a point where the ceiling will collapse and the outer side of the cave will probably become a stack or a stump, before it too eventually erodes into the sea. This process will likely take many more centuries or millenia, so there should be plenty of time yet to visit "The Hole in the Rock".

More About Motukokako

The northern part of Piercy Island (Motukokako) presents a well exposed example of a skarn containing babingtonite and ilvaite. Large crystals (3-5mm in size) of babingtonite occur in a lead-zinc skarn in tertiary limestone with associated ilvaite, garnet, hedenburgite, epidote and axinite.

This represents New Zealand's best ilvaite exposure, with the best of three babingtonite occurrences. This is classified as a site of international geological importance. It is also classified as a highly vulnerable site, with the major hazard being identified as the over-collecting of minerals.

This Earthcache

In order to log a 'find' on this earthcache, you will need to visit the published coordinates, take a photo of yourself and GPS (the GPS must be clearly visible in the photo) and answer some questions as below. The terrain rating of this cache is 5 because access is by water only - either by private craft, tour or kayak if the weather is good - in any case special equipment is most definitely required!

Questions/Tasks:
1. Approximately how wide at sea level is the cave?
2. Approximately how tall is the cave from sea level?
3. Approximately how long is the cave?
4. What orientation, in relation to true north, is the cave (to the nearest 10 degrees)?
5. Take a photo with "The Hole in the Rock" in the background clearly showing yourself and your GPS - upload this with your online log.

The questions above will require you to make estimates of the cave dimensions. You are unlikely to find them on the internet and they will probably not be part of any commentary, should you be visiting by way of a boat tour. The orientation of the cave can be estimated by using your GPS to mark waypoints outside each entrance, or by using a compass.

Email the answers to questions 1 through 4 to the cache owner. Do not post these answers in your online log. Your photo should be uploaded with the online log.

You can log this cache straight away after you have emailed your answers to the cache owner, no need to wait for confirmation. Please include the name of this earthcache in the email - you'd be surprised how many people forget. Also, when contacting us with answers, if you want a reply, please include your email address. Any problems with your answers we'll be in touch.

Happy (and Safe) Earthcaching!