TTWWD – what happened here?

We went to the East Coast and had a look at the Pacific Ocean. It was living up to its name that day, calmly rolling in and rolling out. Dangerously deceptive though, the beach shelves deeply at the water edge and there are dangerous rips, or undercurrents; definitely no swimming: you might end up in North Island or worse.

We started at Wakanui beach, embraced by two headlands, the beach spreads in either direction for several miles, in one direction towards the Ashburton River at Haketere and in the other towards the mighty Rakaia.

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Between the headlands the ground is flat, this is the Canterbury Plain. The pebbles on the beach, near the sea are larger, mostly, than the ones nearer the land? This is the reverse of the shingle beaches in England, isn’t it? Aren’t the larger stones further back?

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So then I took a closer look at the beach shelf itself. Where I was standing there was a two foot shelf at the back of the long beach I have just described. Made up, to all intents and purposes, of sand and stone. Fragility hardly describes it, this is not stone with pebbles, I could break it up with one hand and manipulate it so that the stones fell out and I had a handful of dark grey sand.

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So what happened? Was this a glacier? A riverbed? What took off the top part of this bit of land? Either side there are landslips visible in the higher stretches, presumably made of a similar sandy construction.

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We went from there to Haketere. This is the mouth of the Ashburton River, another braid river running down from the foothills of the Southern Alps. Here, there is an example of a Hapua (or lagoon)

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You can see the sand bar (actually it is many pebbles) stopping the river at its lowest levels from reaching the sea, so it gathers in pools behind the bar until it slops over a lower point, or breaches a weak point, then gradually wearing it down to a wider and wider gap, until in full spate it breaks up the bar, washes away the accumulated matter and flows evenly out to sea, the pebble bar then builds up slowly again and the whole process is repeated.
Bearing in mind that in full flood these rivers are carrying between 3000-5000 cubic metres of water a minute, for only one or two days a year, that must be something to see.

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