Geology
Whangarei Limestone is a glauconitic sandy limestone fromed during the Oligocene age (between 24 and 36 million years old). Outcropping in a number of places around the Whangarei area, a significant and notable outcropping of this limestone can be found along the coastline between Langs Beach and Waipu Cove.
In its unweathered form, the rock is greyish white in colour and quite strong with thin (around 1 mm thick) extremely closely bedded lenses of calcareous glauconitic sandstone. Exposed to the elements, the sandstone lenses weather out giving the characteristic sharp layered pancake appearance. Elsewhere in New Zealand this phenomenon can be seen famously at Punakaiki in the South Island.
In addition to the horizontal pancake appearance of the rock, this limestone also exhibits widely spaced (typically between 0.5 and 2 metres) subvertical joints. These joints generally weather out, due to percolation of acid rich groundwater through the calcite rich rock, to cavernous crevasses.
The horizontal layers of resistant bands of limestone are separated by softer, thin, mud-rich layers. This type of layering, found in limestones worldwide, is called stylobedding. Although the origin of stylobedding has been debated, it is mostly agreed that this layering is not original bedding, but a secondary feature caused by compaction and may relate to the water temperature at the time the sediment was laid down and compacted to form the rock.
This Earthcache
To claim a find on this earthcache you will need to visit the published coordinates. You will need to take some measurements and make observations and then draw some conclusions about the limestone rock formations. Your answers must be sent to the cache owner before you log your find online (by clicking on our profile at the top of the page). Don't visit at night as you won't be able to see anything and therefore you won't learn anything!
1. What is the typical thickness of the "pancake" layers in the limestone rock?
2. Why is the "pancake" layering more prominent closer to the water?
3. Describe the difference in appearance between the horizontal layering/jointing and the vertical/subvertical jointing in the rock?
4. Take a photo of the limestone formations you observed during your visit and upload this with your online log (don't send to the cache owner).
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, please tick the box to include your email address - this is simply a courtesy to make it easy for us to reply. Any problems with your answers we'll be in touch. Note, the photo requirement is mandatory.
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First to sample the lovely layercake:
@McArgister
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