Koutu Boulders

Accessibility: MODERATE
Two larger boulders. Photo: Bruce Hayward
Easily accessible place to see New Zealand's largest spherical concretions.
Also see some of the sedimentary rocks that they are eroding out of.
Photo: Bruce Hayward
The largest spherical boulders (4-6 m-diameter) in New Zealand sit with many others on the shores of Hokianga Harbour at Koutu. Geologists call them concretions. They have grown within thick beds of sandstone and subsequently have been eroded out by weathering and removal of the surrounding softer enclosing host sandstone. Many of the boulders along the foreshore may have eroded out on the hillsides above over many thousands of years and have slowly slid or rolled down to the coast where we see them. The concretions form only in favourable conditions: dissolved minerals in percolating ground water start to crystallise, often around a small pebble, plant fragment or lump of mudstone, progressively growing outwards over thousands or even millions of years to form the hardened spherical concretion. The crystals grow in the small voids between the grains, cementing them together.
In some places along the 1 km or so of shoreline where the concretions can be seen there are also exposures of the sandstone host rocks that they formed in. These were deposited in deep water off the coast of ancient Northland about 70 million years ago, as Zealandia was moving away from Gondwana. There are also exposures of soft red and grey mudstone that were also deposited in the deep sea off Northland about 60 million years ago. These rocks and the concretions that formed in them are part of the Northland Allochthon - rocks that were pushed up out of the sea and slid into Northland 25-20 million years ago.
Some of the larger Koutu Boulders. Photograph Bruce Hayward.
Can you decide which boulder is the largest? What is its diameter?
Can you see any boulders still partly enclosed in the host rock in which they were formed?
Why do you think the boulders are spherical?
Many of the boulders have split apart - what can you see in the middle of many? Does this help tell you what the concretions initially began growing around and why they are spherical?
See if you can find some 'cone-in-cone' crystals (concentric cones nested inside each other) from broken concretions. If the rock is heavy it is probably the mineral siderite or it could be gypsum or calcite.
Where else have you seen large spherical concretions around Auckland and Northland? - they are likely to have eroded out of the same kind of sandstone rocks.

Turn off from Hwy 12 along Koutu Loop Rd and then on to Waione Rd. The south end of Koutu Loop Rd is sealed and the better road to use than coming in from the north. The boulders occur in the northern half of a 2 km stretch of foreshore that can be accessed from either end. The easiest access is from the carpark at the southern end on Waione Rd, then walk about 1 km north up the beach to the section of coast with the boulders. From the north end, access is down a steep driveway track from the end of Cabbage Tree Bay Rd and you arrive in the middle of some of the boulders.

Beware of the tide. The full stretch of boulders is only accessible when tide is within two hours either side of low tide. Beware of slippery rocks and the razor sharp oysters growing on some of the rocks you have to clamber over - take extreme care here.

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Accessibility: MODERATE

Only attempt at mid tide or lower, preferably when the tide is going out. From the Waione Rd carpark there is relatively easy access to the start of the boulders along a sandy beach.
From the Cabbage Tree Bay end access is down a long steep track than can be a challenge for some to come back up. There is less parking at the end of Cabbage Tree Bay Rd, but if you are fit you get access to the boulders faster. The 1 km stretch of coast with boulders varies between easy and tricky depending on the tide.

Geological Age
Zealandia Evolution Sequence
Haerenga Supergroup (Submergence): 85-35 million years ago