There is increasing evidence that geological features can arise as bacteria interact with purely physical and chemical processes. The ability to distinguish both ancient and modern geological features that are biologically influenced from those that are purely abiotic in origin can potentially advance our understanding of the timing and pattern of evolution, and may even provide a tool with which to identify evidence for life on other planets. Here we report on studies of apparently scale-invariant terrace patterns that generically accompany travertine formation at carbonate hot springs throughout the world. We have characterised the mineral and geochemical environments together with an extensive genetic analysis of the microbial communities throughout the travertine spring drainage system at Mammoth Hot Springs in Yellowstone National Park. We find that the travertine morphology exhibits distinct zones-- facies-- even though many features of the spring water and travertine change smoothly over the course of the drainage path. In addition, we show a surprising, sharp correlation between microbial species and travertine facies which demonstrates that bacteria are sensitive indicators of environmental conditions and may be used for geological facies modeling, constraining abiotic and biotic theories for the origin of the travertine structures. Work performed in collaboration with: G. Bonheyo, J. Frias-Lopez, H. Garcia Martin, J. Veysey, B. Fouke. Work supported by the US National Science Foundation.