Stalagmites with a crayfish-like profile are found in two natural tunnels in new South Wales in windy, moderately-lit sites; some of them were noted tourist attractions 100 years ago. The morphology and orientation of these stalagmites has been examined in relation to the light intensity falling on them . The internal structure, mineralogy and microbiology of one stalagmite and a related flowstone have been studied, resulting in a hypothesis for the mode of formation of the crayfish-like stalagmites. The stalagmites are compared with other speleothems and similar surface concretions. It is concluded that these stalagmites can be classified as stromatolites.