This cache was part of a Geotour: as the geotour has now finished, the codeword will no longer be sent as part of the confirmation that your Earthcache answers are correct. Please do not ask.
At the posted coordinates itself, you are at a belt of mostly sand dunes that have been added on the seaward side of an older barrier in the last few hundred thousand years. Kaipara North and South Heads are sand barriers that have built across the front of Kaipara Bay in the last 2 million years, creating New Zealand's and the Southern Hemisphere's largest harbour - the Kaipara.
This belt of dunes is currently eroding, creating sea cliffs of fairly soft sand. In the cliffs you can see evidence of how this belt of sand has been added. At various places there are lenses of black lignite. These lignites are dead plant material that accumulated in swamps within the sand dunes at times when the dunes were stabilised and had become covered in trees. The swamps were located in flat sections of small stream valleys or in depressions between sand dunes.
Lignite lenses can be seen near the base of the cliffs just to the north and south of the access to the beach. It is called a lens as it is thicker in the middle and thinner at the edges, much like a lens in a magnifying glass.
The thin lignite lens to the north contains a 5 cm thick white layer of fine glass which is volcanic ash that was erupted from the centre of the North Island in a huge eruption cloud and blown north to here, where it fell out of the sky and was preserved in the swamps.
In the sand seen in the cliffs north of the beach access there is evidence of at least five periods when the dunes were stabilised and became partly or fully covered in vegetation.
In some places the sand has layering within it at an angle of 20-30 degrees to the horizontal (called cross-bedding). These layers record the moving steep front of a sand dune being blown along by the winds.
What is lignite?
Lignite is formed from peat at shallow depths and temperatures lower than 100°C. It is the first product of the formation of coal and is intermediate between peat and coal. Lignite contains about 60 to 70 percent carbon.
Most lignites are geologically young compared to coal, generally having formed during the Mesozoic and Cenozoic Eras. The Mesozoic Era began 252.2 million years ago, following the conclusion of the Paleozoic Era, and ended 66 million years ago, at the dawn of the Cenozoic Era, which is our current era.
Types of Rock
There are three kinds of rock. Igneous rocks form when molten rock (magma or lava) cools and solidifies. Sedimentary rocks originate when particles settle out of water or air, or by precipitation of minerals from water. They accumulate in layers. Metamorphic rocks result when existing rocks are changed by heat, pressure, or reactive fluids, such as hot, mineral-laden water. Most rocks are made of minerals containing silicon and oxygen, the most abundant elements in the Earth’s crust.
How to solve this Earthcache:
- Based on the information above and your view of the lignite lenses, do you think the lignite is an igneous, sedimentary or metamorphic rock? Why do you think so?
- Look for sand with layers at a high angle - can you decide which way the sand dune was being blown?
- Based on the information above and your view of the lignite, do you think the lignite was formed in the Mesozoic or Cenozoic Era?
- At GZ you will be able to find pieces of lignite that have eroded from the cliffs. Describe in your own words the texture of the lignite. How does it compare to black coal? Do not pick any lignite from the cliff under any circumstance please. If no lignite is crumbled from the rock, simply describe what you see.
- Please take a photo of yourself, your GPS, a personal item or a paper with your caching name with a lignite lens clearly showing in the background. (You must provide a photo for logging, so I know you saw it yourself).
Confused?
Don’t be! The important thing is to visit the site and find the lignite deposits. Send your answers and I will contact you if any of your answers aren’t correct. Most importantly, enjoy this beautiful place.