The biggest concentration is on the southern shore between Koutu and Kauwhare Points. The Koutu boulders, like the better known Moeraki Boulders in Otago, are nearly perfectly spherical. One third of the Moeraki Boulders are less than 1.0 metre in diameter, and the other two-thirds from 1.5 to 2.2 metres. The Koutu Boulders are many times that size. Many are around 3 metres in diameter, and they include some giant specimens as large as 5 metres in diameter.
The Koutu boulders are excellent examples of concretions. The word concretion comes from Latin and means “grown together”. Concretions form within layers of sedimentary strata that have already been deposited on the sea floor. They usually form early in the burial history of the sediment, before the rest of the sediment is hardened into rock. They quite often form by the precipitation of a considerable amount of cementing material around a nucleus, often organic, such as a leaf, tooth, a piece of shell or a fossil. The boulders are composed of the same material as the surrounding rock and they form when the cementing mineral binds grains of the sediment into a cohesive mass.
Depending on the environmental conditions present at the time of their formation, concretions can be created by either pervasive or concentric growth. In the case of pervasive growth, precipitated minerals infill the pore spaces of the host sediments simultaneously throughout a volume, cementing it into a mass which eventually becomes a concretion. In concentric growth, the concretion grows as successive layers of mineral accrete to its surface, similar to the formation of oyster pearls. This process results in the radius of the concretion growing with time. They may continue to grow over millions of years. It has been estimated that the largest of the Koutu boulders may have taken 5 million years to grow.
The concretionary cement often makes the concretion harder and more resistant to weathering than the host stratum. Eventually the seabed has been uplifted to form coastal cliffs that have eroded over time, exposing the boulders which have tumbled down onto the beach. This process continues, and semi exposed boulders can be seen protruding from the cliffs along the beach at Koutu.
Concretions are found in a variety of rocks, but are particularly common in shales, siltstones, and sandstones. They often look as if they do not belong to the stratum in which they were found. They vary greatly in size and appearance, and can be classified according to various characteristics. The Moeraki and Koutu boulders are examples of septarian concretions, which are also cannonball concretions.
Cannonball concretions are large spherical concretions, which resemble ... wait for it ... cannonballs.
Septarian concretions are concretions containing angular cavities or cracks, which are called "septaria". The word comes from the Latin word septum; "partition", and refers to the cracks/separations in this kind of rock. The cracks are highly variable in shape and volume, as well as the degree of shrinkage they indicate. Although it has commonly been assumed that concretions grew incrementally from the inside outwards, the fact that cracks that radiate out from the centre are narrower towards the edges is taken as evidence that in septarian concretions the periphery was stiffer while the inside was softer, presumably due to a gradient in the amount of cement precipitated.
Septaria usually contain crystals precipitated from circulating solutions. While many of the cracks in the Moeraki Boulders have been completely filled with brown calcite, yellow calcite, and, in rare cases, dolomite and quartz, the Koutu Boulder cracks appear to have little of this precipitation.
Koutu Boulders Earthcache
For this Earthcache you will visit some of the Koutu boulders and take some measurements. You will require a measuring tool. Parking is available at the posted co-ordinates. Follow the Koutu Loop Road to the next road on the left, Waione Road. Turn into Waione Road and after about 100m you will arrive at the parking area. Walk to the beach and turn to the right.
You will be required to perform tasks at various points along beach. At first you see mainly broken boulders, which have washed furthest down the harbour from the source. Then some small intact boulders are scattered on the beach, and the further along the beach you go, the bigger the boulders get.
The Koutu boulders are best viewed 2 hours either side of low tide and access along the beach is easiest during these times. However the boulders for the earthcache tasks are near the high tide mark and should be accessible up to 2 hours either side of high tide, though some rock scrambling may be required.
To claim this Earthcache, please complete the following tasks:
Wp1 (S 35 28.007 E 173 25.021) Cross section of boulder
Here you will see half a boulder embedded in the sand with the cut face exposed. This cross section shows several distinct layers.
Task 1. How many layers can you see?
Wp2 (S 35 27.896 E 173 25.009) Small boulder
Here you will see a spherical boulder, one of the few intact boulders at this end of the beach.
Task 2. Measure the height and circumference of this boulder (To the nearest 10 cm).
Wp3 (S 35 27.469 E 173 24.882) Split boulder showing the nucleus
Here you will see a boulder that has split cleanly in two. It clearly shows the ‘pearl seed’ around which the boulder has grown.
Task 3. Measure the diameter of the upright face of this boulder horizontally across the flat surface.
Wp4 (S 35 27.478 E 173 24.879) Two big boulders
Here you will see two large boulders.
Task 4. Measure the circumference of the one nearer to the sea (On the right in the photo).
Task 5. Estimate the height of the other boulder (On the left in the photo).
Task 6. Take a photo of yourself with your GPS, in front of any of the Koutu boulders not included in the measurement tasks. Post this photo with your log.
Email your answers to the cache owners and go ahead and log your visit. However logs that are not accompanied by emailed answers or do not have the required photo uploaded may be deleted. Please DO NOT include answers in your online log. Online logs with answers will be deleted.
Interested in seeing some of the real giants of the Koutu boulders?
A second access to the harbour and the really big boulders has recently been provided by Council. Drive to the end of Waione Road,and where the road forks take the right hand fork, Cabbage Tree Bay Road. Drive about 750 metres, almost to the end of the road. Just past the entrance to number 76 you will see where a small parking area has been created on the right hand side of the road. Park your vehicle here. On the left of the parking area (looking towards the Harbour) there is a path which leads down to the beach. It is a bit steep but walkable depending when the grass was last cut. Once on the beach, walk to the right for about 30 to 40 minutes and you will see some of the real giants among the Koutu Boulders. It is best to do the walk within two hours either side of low tide.