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Mathematical models have recently been used to cast doubt on the biotic origin of stromatolites. Here by contrast we propose a biotic model for stromatolite morphogenesis which considers the relationship between upward growth of a phototropic or phototactic biofilm (v) and mineral accretion normal to the surface (λ). These processes are sufficient to account for the growth and form of many ancient stromatolities. Domical stromatolites form when v is less than or comparable to λ. Coniform structures with thickened apical zones, typical of Conophyton, form when v >> λ. More angular coniform structures, similar to the stromatolites claimed as the oldest macroscopic evidence of life, form when v >>> λ.

Sand volcanoes have been found in three sedimentary associations in the Upper Carboniferous Bude Formation. In two of these associations they formed when the dewatering of fluidized sand beds, deposited from traction carpets, was temporarily inhibited by the deposition of overlying units. In the third association the volcanoes formed during the normal post-depositional compaction of fluidized, muddy, poorly sorted units deposited from subaqueous mudflows. While is seems that the tops of fluidized sand beds were characteristically sheared by a fairly powerful current immediately after being deposited, the waters above the recently deposited mudflow units were commonly stagnant, a11owing sand volcano growth.