Limestone chips are used to surface many of the paths on Manawatu walkways, including the newer parts of the Gorge walk such as the Tawa Loop at the western end of the Gorge.
Limestone in this area: If you stand at the given co-ordinates with your back to the barrier fence and look across the road, especially to a bluff to your right, you are looking essentially at limestone. Up and in behind the area of the bluff is an old limestone quarry that served the area for much of the 20th century.
Below the bluff, the railway used to go through three short tunnels, two of them close together ( the bend is still called TwinTunnels). As reported in the Wairarapa Daily Times, June 29, 1909, a passenger train ran into a limestone rock slide between tunnels the night before. Fortunately no one was injured, but it was speculated that work such as blasting in the quarry just opposite may have helped destabilise the rock. In 2008 the tunnels were removed to facilitate larger trains (milk trains in particular). To do this the whole bluff had to be cut back and stabilised. 240,000 cubic metres of rock (limestone) and spoil were removed. Some was dumped in the old quarry. In front of you, across the road, are some mounds of limestone removed during this tunnel 'daylighting' and awaiting use.
NB: By December, 2014, the mounds have reduced a little. As long as you are at the given co-ords, you are at the right place, and you can certainly still see the bluff to your right, as in the photo.
By October, 2015, vegetation is continuing to grow over parts of the mounds and some of the bluff, although you can still easily tell they are there. For a better overall view of the bluff itself you can cross the nearby single lane Ballance Bridge and look back from co-ordinates S 40 20.366 E 175 49,106. There is plenty of room to park/turn around. The best view is on the bridge itself, but ,of course, there is no parking there.
What is limestone? It is a sedimentary rock consisting of the shells and bones of tiny marine fossils made of lime or calcium carbonate (CaCO3). Limestone contains at least 50% calcium carbonate. All limestones contain some mud or sand.
Limestone is abundant in NZ, most formed in the Oligocene and Miocene periods. This formation occured 20-30 million years ago when much of NZ was submerged under shallow seas and the climate was warmer. Layers of shells and bones accumulated on the sea floor. They were compressed and cemented together as sediments (eg: sandstones, siltstones) were deposited on top. With the folding of tectonic forces, the limestone was fractured and uplifted to form various limestone landscapes such as the dramatic karst landscapes like Castle Hill, and the Waitomo Caves.
Limestone is particularly subject to erosion by water. Rainwater is slightly acidic (containing dissolved carbon dioxide). Running through soil increases its acidity. Carbonate rocks like limestone are slowly dissolved by this acidic water. In time typical features of limestone country develop, eg: underground caves and rivers and streams, cliffs, fluted rock outcrops, springs, sink holes.
Uses: A major use of limestone in NZ is as an argricultural fertiliser, when finely crushed to a powder. It is also used as a roading aggregate, and as one of the two main materials used to make cement (the other being marl - a calcareous clay). Other uses include in the production of pulp and paper, steel, glass, gold-ore, petrochemicals and pesticde, and in sewage and wastewater treatment.
Manawatu Gorge limestone: The old quarry contained broken shell limestone. This is "an impure, sandy, muddy or pebbly limestone consisting of broken shell material" (Mineral Commodity Report 21, pg 3). It was mainly used for agriculture and roading.
References: Geology rocks & minerals - Auckland University. flexiblelearning.auckland.ac.nz
Mineral Commodity Report 21 - Limestone... www.nzpam.govt.nz
NZ limestone origins www.sciencelearn.org.nz
Te Ara Encyclopedia of NZ www.teara.govt.nz
Wairarapa Daily Times, June 29, 1909 paperspast.natlib.govt.nz
To log this cache: You must visit the published co-ordinates and answer the following questions. Send your answers to the cache owner and WAIT for approval to log the cache.
1. To be designated limestone, what must at least 50% of the rock be made of, and what does this component originate from?
2. How long ago were NZ's limestones formed?
3. Briefly describe one dramatic NZ karst landscape NOT mentioned in the cache's given information.
4. Describe how limestone is processed for, and used in, agriculture in NZ.
5. Take a photo of your GPS ( include yourself if you're happy to do so) with the bluff in the background. Do not send this in with your answers. Include with your log when you have the go-ahead.
NB. While it is optional, we would appreciate it if you would include a photo showing the site has been visited. Thank you.