Birdlings Flat Agates

Birdlings Flat is a wind swept shingle beach on the south side of Banks Peninsula, in Canterbury.  For years it has been a favourite haunt for rock and gem collectors.  A large variety of colourful stones are found there, swept up by the northern travelling ocean currents from the rivers in mid and South Canterbury and beyond.
I have found plentiful numbers of agates of every variety along this beach, while at the same time keeping a wary eye on the waves coming in from the southern ocean.  It is rare that a gemstone collecting trip on the beach ends with dry feet and trousers.

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Most of the agates are swept up by the ocean current from the Rakaia, Ashburton and Rangitata Rivers, and deposited as the current hits the less saline waters at the peninsula.  Fresh water from Lake Forsyth and Lake Ellesmere makes the salt water less dense and aids in deposition of the stones.  However many of the agates are clear and onyx banded; very similar to those found in McQueens Valley, in the south side of the volcanics of the peninsula.  Although similar agates are found in the southern rivers, there seems to be a higher proportion of them here that cannot be simply explained by deposition by ocean currents.  It must be remembered that originally, these volcanoes on Banks Peninsula were at least three times as high as they are today.  So, the question must then be asked: where did those agates and shingle that used to be there go to?  An answer could be that alluvial wash from the volcanoes helped also to form the flat land on the south side of the peninsula, along with the ocean current coming up from the south.  So, McQueens Valley could have been a source for many of the agates found at the Flat.  All this is pure speculation on my part, but could be an interesting research topic for a university geological student.  It should not be too difficult to analyse shingle from various locations, starting from the McQueen andecitic volcanics and moving out towards Birdlings Flat, and analysing the differences.

The pictures below are of small tumble polished agates from my collection.  Click on each picture to see the full size.

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