Basalt lava flows in the cliffs at the east end of Birdlings Flat. Photo/ K Pedley
This coast (called the Canterbury Bight) is exposed to powerful southerly waves. The combination of loose sands and gravels and high energy wave action has caused rapid coastal erosion and strong net northward transport of the sediments over the last 10,000 years. This has been building up the Kaitorete Barrier (which extends to the west) over time, and forming Te Waihora/Lake Ellesmere behind it, less than 5000 years ago.
Most of the agates are transported down-river primarily from along the lengths of the Rakaia, Ashburton and Rangitata Rivers. They source the minerals from the various bedrock geology, including the silica rich volcanics of Mt Somers. They are then swept up by the ocean currents, and deposited as the current hits Banks Peninsula where the fresh water from Lake Forsyth and Lake Ellesmere makes the salt water less dense and aids in deposition of the stones. Coloured quartz of many types is the most common, particularly chalcedony (micro-crystalline) varieties i.e. agates (mostly clear or grey or white with curved bands) sardonyx (red parallel banded) or onyx (white or black parallel banded), carnelian, jasper, prase (yellow/green coloured), and plasma (dark green). Other quartz forms, quartzite, volcanic pebbles, occasionally petrified wood of various colours, and rhodonite have also been found here.
The basalt lava flows exposed at the eastern end of the beach are of the Miocene aged (around 8-10 million years) Mt Herbert Volcanic Group associated with the extinct Akaroa Volcano.