Formation of Lake Brunner / Kōtukuwhakaoka

At the viewpoint you are standing on is the glacial moraine. You can find around you in the bush glacial erratic’s, large blocks of rock that were suspended and then dropped by the ice.

From the lookout point you get views across Lake Brunner / Kōtukuwhakaoka to the Orangipuku valley, Mt Te Kinga and the Hohonu Range.

Mt Te Kinga and the Hohonu Range are so prominent as they are made of granite and granodiorite, rocks that formed deep within the earth from a cooling magma system. Their slow cooling formed large interlocking crystals making this rock resistant to erosion. These plutonic igneous rocks are part of the Western Province (Tuhua Intrusives) Rocks and are 100.5 – 145 million years old.

Hidden from view to the east is the Alpine Fault, the boundary between the tectonic plates of the Australian and Pacific Plates. The western side of the Alpine Fault is on the Australian Plate and is slowly moving northwards, while the eastern side is moving southwards and pushing up the Southern Alps.

Lake Brunner / Kōtukuwhakaoka formed as the ice retreated and filled. The Arnold River drains the lake, and is cut through the moraine. Lake Brunner / Kōtukuwhakaoka covers 41 km2 and has a maximum depth of 109 m.

This area is also a piece of remnant native bush. Along the track you will encounter ancient specimens of rimu, miro and kahikatea, along with pokaka, broadleaf, kamahi, quintinia, toatoa and wineberry. There are DOC information panels along the trail to aid with your plant identification.

As you walk along the path can you find the large glacial erratic?
If you look carefully in the drains cut along the road you can see the sediments deposited by the glacier and also the underlying blue grey muddy sandstone, rocks of the Blue Bottom Group aged between 11.2 6.4 million years old.
You can see a cut through the moraine in the road bridge over the train lines on the way to the marina.
What types of materials make up a glacial moraine? Are they all of the same size? Are the large rocks all of the same type? What do these tell you about the glacier? Hint if there is a rounded boulder of granite where would this rock have been sourced from?