The Ohau Moraines are some of the most sensitive recorders of ice-age climate change in the Southern Hemisphere. 

Lake Ohau sits in a ‘depression’ with mountains on either side. These mountains bare scars cut into their rock face. These scares were formed by glacier movement – a river of ice – scraping along the valley sides as it went. These scars record the varying height of the glacier and the extent of its length down valley. At the end of Lake Ohau we find hummocky landforms. These are the terminal moraines left behind as the glacier retreated.

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Glaciers are sensitive to climate change. By studying glaciers and their associated landforms, scientists can have a better understanding of past climate change.

Here are some basic facts:

How does a glacier move?
A glacier can be thought of as a river of ice – all be it a slow one (but some also have surges – just because nature is like that – it doesn’t always do what we expect!). As more snow falls in the head of the glacial valley (cirque), it compacts into ice. Overtime the glacier moves down gradient (under gravity and its own weight) – similar to a river! As the glacier moves the dense ice cuts and scrapes along anything it comes into contact with:

• The sides of the valley (that’s why glaciers produce ‘u’ shaped valleys as opposed to ‘v’ shaped valleys associated with rivers). You can often see scrapings along the valley walls as is visible at Lake Ohau

• The bottom of the valley (at the base of the glacier)

• The end of the glacier

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Glaciers carry the materials they have eroded - either on, in or under the ice. These sediments are eventually deposited as moraines when the glacier retreats.

Specifics of Ohau Moraines

Ohau Moraines are terminal moraines. Terminal moraines are a mix of a wide range of sediments and can include boulders, rocks, pebbles, sands and silts. The Ohau terminal moraine location records the extent of the last glaciation – approximately 18000 years ago. From this point the glacier at Ohau retreated and has left behind a large depression that is now occupied by Lake Ohau. The Alps to Ocean cycle trail traversers these moraines. By travelling over the
moraine you get a sense of being on and surrounded by a geologically young landform. This landform has its origins high up in the valley from much older (basement) rock – rock that was formed before the break up of Gondwana that has been on its own journey. The basement rock that forms the moraines has experienced Gondwana breakup, metamorphism, tectonic uplift, glacial erosion and deposition, and its story has not finished.

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To learn more you can listen to the Sasha Say’s podcasts on Ohau Moraines here.

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Recent research
Krause, M. A. (2017). Characterising change in post-glacial climate using seismic imaging of
Lake Ohau sediments (Thesis, Master of Science). University of Otago.

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Please be aware the following hazards following: water, cycling traffic, and uneven surface. Tsunami risk - If an earthquake is strong or long, get gone.