Christchurch Harbour - Geology

Geology

The Harbour is made up of Sandy Gravels, Sands, Muddy Sands, and towards the margins Silty Muds. High and low Salt marsh both exist within the harbour, the latter colonised by Puccinellia and other grasses. Spartina anglica does not exist in significant volume, with no sign that it was previously extensive. The sedimentation of the Harbour has not therefore been substantially affected in the way most other south coast estuaries have by the spread and die back of this species. Any loss of the salt marsh appears to have been offset by the re-colonization of abandoned artificial salt pans. Extensive Phragmites reed beds occupy tidal creek margins and areas of higher marsh, where they front wet grazing meadows. The inflow and outflow of sediment in the Harbour is positive,. Grimbury Marsh, is substantially composed of dredged spoil material. In total the estuarine system is a sediment sink.