BIOSEDIMENTS AND BIOFILMS 291 non-biological organic carbonaceous material, which can form by a number of different processes, and has been shown experimentally to adhere to abiological fossil-like structures, lending them additional biological character Figure 10 Conical stromatolite from the 3.42 Ga Strelley Pool Chert, Pilbara region, Western Australia The layering comprises alternating fine chert and carbonate laminations These are rela tively flat, except in the stromatolite itself, where the laminae form a distinct cone This kind of structure may be attributed to the activity of microbes The oldest microfossils The microfossils that were, for a long time, cited as the earliest evidence for life come from the Apex Chert in the Warrawoona Group (Pilbara region, Western Australia) This discovery comprised septate, filamentous structures (Figure 13) in what was thought to be a sedimentary chert layer However, this chert layer was later shown to be a black chert dyke, which led to the contention that the putative microfossils could be hydrothermal artefacts or biomorphs (see above) Other carbonaceous structures interpreted as microfossils have been found in hydrothermal black chert deposits in the 3.49 Ga Dresser Formation (Warrawoona Group) Spheroidal structures of similar age, but more poorly preserved, have been reported from the Onverwacht Group in the Swaziland Sequence of South Africa Although a biological origin for these discoveries is plausible, every report of microfossils from rocks of Early Archaean age has been contested and alternative abiotic explanations have been offered for their formation Less contentious filamentous structures, interpreted as fossilized bacteria, have been found in the Late Archaean (2.7 Ga) Tumbiana Formation (Fortescue Group, Pilbara), but the oldest microfossils of widely accepted microbial origin come Figure 11 Columnar stromatolites, exposed in cross section, from the 2.7 Ga Tumbiana Formation (Fortescue Group) in the Pilbara region, Western Australia Hammer is approximately 40 cm in length