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Our places: Banks Peninsula Horomaka

Image: Google Earth

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Natural History context

Some 15 million years ago the collision of two tectonic plates forced up the mountains that today form the spine of Te Waipounamu South Island. Over time, glaciers bulldozed paths through the mountains and wide rivers carried the eroded material to the eastern coast, building land out over the shallow waters and creating the Canterbury Plains. Then 12 million years ago, Lyttelton volcano emerged from the seafloor. Many more eruptions would follow, new lava flows burying old, then a few million years later a second major volcano, Akaroa erupted beside it. (Fig. 1).

Time passed. The silt and the gravels carried to the coast by the huge braided rivers eventually reached the volcanic islands, joining them to land that would later be called the Canterbury Plains. The volcanoes whose hearts had opened to the sea to form two harbours, became known as Horomaka or Te Pataka o Rakaihautū, the Banks Peninsula.

Before humans settled the area around 900 years ago, the peninsula was almost entirely forested. During centuries of Maori occupation about a third of the forest was removed, mostly by fire.

Europeans arrived, settled on the peninsula and founded the city of Christchurch just to the north. Within decades less than 1% of old-growth native forests remained. The great trees had either been cut down for wood or burned to make way for pastures to graze sheep and cattle. Hares and rabbits devastated what little remained of the native plants that once helped provide life-supporting ecosystem services, including their all-important ability to regulate the climate and store massive quantities of carbon.
Birds, reptiles, and invertebrates, each playing a role in the once thriving ecosystems, were easy pickings for the predators that had been introduced: rats, stoats, possums, hedgehogs, and the cats and goats and pigs that went feral.

Yet in spite of this systematic devastation:

“As I was doing a detailed botanical look at the peninsula, I thought it would be completely trashed, but there’s so much still here.” – Hugh Wilson Fools & Dreamers: Regenerating a Native Forest

Exposed to snowy blasts from Antarctica and dessicating nor-west winds, the species that evolved to live in this diverse landscape are keen to return and help provide the ecosystem services we all need to thrive in Canterbury’s changing climate. The geography of the peninsula makes it an ideal candidate to become a pest-free mainland island. All they need is a fighting chance.

The geographical  people of the Banks Peninsula are now giving it to them through the Banks Peninsula Conservation Trust, Rod Donald Trust, and projects such as Hinewai Reserve and the Akaroa Area School restoration of Te Umu Te Rehua Reserve. Marine reserves, marine protected areas, and taiapure—estuarine and coastal areas that are significant for food, spiritual, or cultural reasons for Maori—have also been established (Fig. 2).

Fig. 1 Banks Peninsular volcanic sequence (Image: Hartung)
Fig. 2.= Marine protected areas and taipaure (Image: Rod Donald Trust)

Kaitorete Spit

The sand spit is a geological late-comer, composed of the silt and the gravels carried to the coast by the huge braided rivers after the glaciers melted at the end of the Last Glacial Maximum around 20,000 years ago. Back then, sea levels were around 120-130 metres lower than today, and Banks Peninsula wasn’t a peninsula. The twin Akaroa and Lyttleton volcanoes were in the middle and the Canterbury Plains. The coast as much as 50km east of where it is today.

As the climate warmed, melting ice caps and glaciers drained into the ocean. The Rakaia and Waimakariri River (whose mouth shifted many times between Te Waihora Lake Ellesemere and where it is today) delivered sand and gravels to the coast, but not enough to keep pace with rising sea levels. By 4,500 years ago, the climate, and with it global sea levels had stabilised close to where they are today. Braided rivers continued to carry sand and gravels to South Canterbury’s beaches, but here, powerful southerly waves and currents eroded the sand and pushed it north. When it reached the Banks Peninsula, some of it built up, helping to formed the Kaitorete Spit, closing off what we now call Te Waihora Lake Ellesemere (Fig. 3).

Today, the once-wide braidplain of the Rakaia River has been forcibly narrowed. Confined by stopbanks and drained of water for agriculture, it’s no longer able to replenish the coasts with enough sand and gravel to keep pace with hungry waves and rising sea levels. The existing coastline is being eaten away. Currents push some of this eroded material along the coast, visible in the light-coloured sediment-filled swirls just offshore (Fig. 3). Some material continues to be deposited on the 5,500 ha Kaitorete Spit. How long this hell the spit will stay abreast of accelerating sea level rise is unclear, but protecting the native vegetation that binds the sediment in place will play an important role and continue to provides homes to the unique endemic invertebrates and reptiles that live there.

Fig. 3 Kaitorete Spit (Image: Google Earth)

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