Sub-vertical tubular structures from the lower Nama Group are described and compared with similar examples from the Damara Sequence of Namibia and from the Noonday Dolomite of California. At all localities the country rocks are light grey, peritidal cryptal-gallaminated dolomite. The structures, 0,5 to 2,5 cm in diameter, are circular to oval in cross-section, their length varies between 0,1m and possibly several metres. The pipes were filled either immediately with carbonate mud or small framents of surrounding laminites, or at a later stage with chert and/or coarse sparite; some even remained open. The tubes are orientated vertically to the paleohorizon and flatlying lamination or at an angle to the lamination on the flanks of algal buildups. From their common environment it is inferred that the structures are facies-dependent. The shape of the pipes and composition of their fillings, as well as a comparison with modern, intermittently exposed shallow water sediment, suggest that they are gas- and water-escape structures. Gas is formed by the decay of the algal mats during low tide, while water could playa subsidiary role in generating high pressure.