What is silt?
Silt, sand, and clay are terms for specific sizes of the rock particles that make up soil – see figure 1.
Silt is left on flatter areas after flooding because the water currents are too slow to carry sand, and clay is so small and light it stays in suspension. Silt is also used as a general term for finer materials left behind by floods.
The east coast of the North Island has been particularly badly affected by flooding from Cyclone Gabrielle because many of its rocks are siltstones and mudstones. They are predominantly made of silt and clay particles, and are highly erodible, so large amounts were carried by the floodwaters.
Silts left behind by flooding (and also deposited by wind), is how many of Aotearoa New Zealand’s extremely fertile soils began, then vegetation built up the organic matter, biology, and structure to form soil. While flood silt is
