Welcome to Christmas Beach, Herald Island a quiet area and part of the Waitemata Harbour. Access to the Waypoints is along the beach edge up to 3 hours either side of low tide is best to try and avoid wet feet (the feeling, not the TV Programme!) This will bring you to the sites of interest and observation.
We spotted some images on social media of the rocks here and thought we recognized the formations from a visit a few years ago to Akaroa Volcano, which has it's own Earthcache GC2N0HC, and the patterns were very similar.
Christmas Beach, Herald Island walkway
However, closer inspection revealed a different process had formed the rocks here in such a manner and we researched what it could be. A geologist friend of a friend in the UK revealed that it was almost certainly 'Liesegang rings', or fish-eye formations we were observing. The variety of coloured layers, and whorls of textured sandstone here is fascinating and highlighted to share with others.
So what are Liesegang rings, and how do they form?
'Liesegang rings', also occasionally referred to as "fish eyes", are features caused during depositing of sediment and the formation of sedimentary rocks (diagenesis) and later by a rhythmic precipitation (passage of fluids and chemical compounds), often controlled by gaps, fissures and cracks which act as conduits and passages for natural chemical solutions. The rock is over-saturated with regard to chemical compounds, which get between layers and succeed in cementing them together in regular concentric circle shapes. The rings may be different in the chemical composition by quality and/or show a gradual change (by quantity)or colour in special compounds, e.g., goethite (soil mineral containing iron oxide). Sometimes they form around a nucleus, or centre-point from where the bands continue to form outwards in regular patterns.
In sandstone beds and formations, interbedding occurs when beds (layers of rock) of a particular lithology lie between or alternate with beds of a different lithology. For example, sedimentary rocks may be interbedded if there were sea level variations in their sedimentary depositional environment.
Why does sandstone sometimes form with different coloured layers? Because it is composed of light coloured minerals, sandstone is typically light tan in colour. Other elements, however, create colours in sandstone. The most common sandstones have various shades of red, caused by iron oxide (rust). In some instances, there is a purple hue caused by manganese.
So what are the Earthcache tasks?
Task One. At WP1 (S 36° 46.469 E 174° 39.591) there is the best example of a Liesegang ring and is visible nice and close. In the centre you can see the nucleus that likely started the formation off. What colour is this centre-piece and how does if differ from the outer rings that surround it?
Task Two. At WP2 (S 36° 46.463 E 174° 39.601 ) there are some good examples of the different coloured layers described above caused by other natural elements. What colours do you observe here? From the above text, is there likely to have been Manganese present during the formation of the sedimentary rock?
Task Three. Take a picture at any other part of the exposed sandstone bank which doesn't show any of the answers to above tasks. This picture can be of yourself, your GPS, your caching name on a piece of paper, or even your shoe with the rockface in view.
(Photo required for logging, so I know you visited the feature).
So what if you need help?
Are you confused on a question? Don't worry! I'm looking for answers that showed me you went to the site and tried.
If there are any problems I will contact you and I am happy to help further or add some information.
So what about logging?:
Feel free to log this Earthcache immediately.
However, after 10 days, any log that is not accompanied by a message with the answers and a photo may be unceremoniously deleted.
References:
https://en.wikipedia.org/wiki/Liesegang_rings_(geology)