Rocks & Soils
Banks Peninsula, including the Port Hills, emerged from the ocean as two volcanoes, forming an island off the Canterbury coast. The plains had not built up (with shingle eroded off the mountains) as much as it is today and in a warm period the sea level would have been much higher. See Education for more detail. The first volcano, centred on Lyttelton Harbour, first erupted 10 million years ago. This volcano produced several lava flows, resulting in the present solid and hard basalt rock outcrops visible in various places in the Reserve, including the two waterfalls. It also produced many layers of softer crumbly, stony rock which weathers to produce volcanic soil.
There were several Ice Ages about 2 million years ago, each one producing glaciers out of the mountain valleys at least as far as the main river gorges and the sea level would have been about 150m lower than today. Between the ice ages, the warmer climate made the glaciers retreat and the sea level rise. Winds blew silt left where the glaciers had been, onto the plains as well as the island volcanoes (Banks Peninsula), resulting in the loess (clay) deposits which still exist on hillsides. The soils and loess on the hillsides has eroded faster in recent times, where forest has been removed, land has been excavated or stormwater has been mismanaged, resulting in more silt in waterways and coastal waters. The various types of soils within the Reserve are referred to under “Plants > Ecosystems” and are described as: Cashmere, Takahe, Clifton, Evans Steepland, Scarborough Hill, Kiwi Hill, Heathcote and Horotane.
We hope to reduce soil erosion, especially through tunnel gully processes, by identifying and rectifying inappropriate stormwater discharges as covered under the subject Water.
Monitoring of silt levels will be done for the stream water as covered under the subject Water. Check for soil erosion activity.