Ironstone from Erfoud, Morocco.

Stromatolites are a form of sedimentary rock that is often highly stratified and contains a lot of calcium. It is caused by cyanobacteria sediment capture and hold. This then forming layer upon layer in a manner that is similar to the manner in which coral reefs. Stromatolites are the oldest fossils, and are found from about 3.5 billion years ago. In the Precambrian they were common, but can be found nowadays, they still formed only in a few special environments, in particular in places where the water is very salt or alkaline. The reason for this is that in normal environments stromatolite a much faster by animals 'grazed' would be then it can be built up. A known location of stromatolites is Shark Bay in Western Australia.