Rivers – moving mountain debris
During glacial periods over the last two million years, the rivers of the South Island have carried very large loads of debris dumped into them by glaciers. The river valleys have filled with thick layers of gravels. During periods when the rivers carry less sediment, or where the land has been raised by tectonic activity, the rivers have cut down and removed much of the gravels. The remains of the gravels form flat-topped terraces flanking the river valleys well above the level of the present riverbeds.
Rivers – building plains
New Zealand has a remarkably rugged landscape – the rapid uplift of the land to form mountains has left it with few flat areas. The most extensive lowlands, such as Hawke’s Bay and the Canterbury Plains, have been created by rivers depositing vast quantities of sediments eroded from upland areas. As rivers have emerged from the confines of valleys, they have dumped their sediment load, forming great spreading fans of sand and gravel. For example, through many glacial and interglacial periods, the Canterbury Plains have built outward from the front of the Southern Alps, eventually reaching Banks Peninsula, which was once a volcanic offshore island.
Wind-blown dust is a common sight along New Zealand’s huge gravel-bed rivers, and during glacial advances the rivers were carrying even more fine material ground up by the glaciers. Thick layers of this dust, known as loess, have accumulated in many areas of the New Zealand landscape.
Sea-level ups and downs
During the Quaternary period, as huge ice caps built up in the northern hemisphere, water became locked up in the ice and the sea level dropped by more than 100 metres. When the ice caps later melted, water was returned to the oceans and the sea level rose.
A single island
At the height of the last glaciation, about 20,000 years ago, areas of sea floor at the shallow northern end of Cook Strait were above sea level. It would have been possible to walk between the North and South islands.
When the sea was lower during glacial periods, more of the continental shelf was exposed and the coastline was seaward of its present position. When the sea rose, it flooded back in over the coastal land. Different types of sediment have been deposited. The Canterbury Plains have alternating layers of porous gravels deposited by rivers when the sea was low, and finer impermeable sediment laid down when the sea level was high. These layers trap water and are responsible for the region’s excellent artesian water supply.
Where hilly terrain meets the sea, waves cut into the hillsides, creating cliffs with flat beach platforms at their base. The beach platforms formed during periods when the sea level was higher were left as terraces far above the ocean when the sea level dropped. Other beach platforms were raised by movement of the earth’s crust. These marine terraces are common around rugged sections of the coast.