Stromatolites

Lake Thetis is a shallow lake formed between sand dunes, about one-and-a-half kilometres inland, dating back perhaps three or four thousand years. It is very salty, and is fed by rainfall and groundwater, so its water level rises and falls with the seasons. Although salty, it is full of life, the most obvious being the cyanobacteria which have produced the stromatolites along the south and western sides, and the microbial mats which line the lake all round.

The stromatolites at Lake Thetis show a combination of forms: they may appear clotted in the middle, but have layers of finger-like columns and some rough layering. The mats which surround them also vary according to their location. Different kinds of cyanobacteria produce different types of mats, and there may be a contribution from the tiny siliceous diatoms which live in the lake waters.

The stromatolites end up as solid calcareous structures. Some cyanobacteria actually excrete calcium carbonate, to form a limy layer as they grow. Some others become calcified in a more complex process involving other bacteria. The organic material left as the cyanobacteria grow up to the surface of the stromatolite acts as a base for the deposition of very fine-grained, organically-produced calcium carbonate, often microscopic fibres of the mineral aragonite.

It was a beautiful site to behold.

Best viewed large.