On the way back from Death Valley we stopped at an outcrop of rocks from the Crystal Springs formation. These rocks are Precambrian in age, over a billion years old! They are mostly marble.
Looking for a sample to take home
Embedded in them are lovely examples of Stromatolites. I've never seen any like them before. Stromatolites are dome - shaped fossil structures made by cyanobacteria. The cyanobacteria grew as a thin mat on an underwater surface, and the filaments of the bacterial mat trap grains of sediment. A new layer grows over the older, sediment-choked layer, and the layers build up into a dome.
Nowdays, cyanaobacteria do not form these dome structures. They get too much competition from green algae, and too much grazing by animals like snails. In the Precambrian, there were no green algae or snails. There's one place I know of, Shark Bay in Australia, that has living stromatolites today. The bay is too salty for other life.
The dark band at the top represents a layer of sediments that covered up and killed these stromatolites about two billion years ago
Nobody knows exactly why they build up such regular-looking domes, but they do.
You can see the fine layers - each is a millimeter or less thick
After days of examining modern cyanobacteria, it felt wonderful to meet the fossil traces of 1,000,000,000 + year old cyanobacteria. It gave me a new respect for these amazing organisms.
Each red or white bar is one centimeter long