An unusual type of small-scale diapirism is described from the Ringerike Sandstone Formation (Lower Devonian) of Ringerike, southern Norway. Polygonal mud cracks in shale are injected from below by coarse-grained siltstone which is interbedded with the shale. The possible origin of this structure is discussed, and it is thought probable that vertical and lateral pressures, aided perhaps by earthquake shocks, mobilised waterlogged silt below sun-dried and cracked mud, causing injection of silt into the cracks.