Birdlings Flat

BY KATE PEDLEY (UNIVERSITY OF CANTERBURY)
Accessibility: EASY
Birdlings Flat Beach looking west. Photo/ K Pedley
Birdling's Flat is a well-known spot for finding a variety of semi-precious gemstones washed up on the high energy beach.
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.
Fossick amongst the pebbles for hidden treasures! Photo/ K Pedley
The best time to go gemstone hunting is immediately after a storm, when the sea has turned the surface stones over. See what you can find and whether you can identify it as one of the minerals in the list above. Most of the stones on the beach will actually be the grey sandstone and occasional red cherts of the Mesozoic aged Torlesse Supergroup which is the main basement rock for the South Island.
Directions/Advisory

Follow the signposts to Birdling's Flat down Poranui Beach Rd from SH75.

BEWARE this beach can be extremely dangerous! Do not turn your back on the waves! Keep well away from, and out of, the water - the beach is steep and there is a very strong undertow. Because of this it is also best to visit at low tide.

Accessibility: EASY

The carpark is right at the end of the road at the beach edge. To get the best stone hunting areas you need to head across the beach to around the high tide mark.

Features
Sedimentary Volcanic Minerals Active Erosion
Geological Age
Beach deposits less than 10,000 years old. Various ages for the actual stones themselves depending on which geology they have been eroded from!
Zealandia Evolution Sequence
Pākihi Supergoup: 5 million years ago – present
Links
While here, check out the Birdlings Flat Gemstone & Fossil Museum: http://www.cinch.org.nz/categories/1/36/entries/2217 Museum GeoTrip: https://www.geotrips.org.nz/trip.html?id=512