A lower Arenigian limestone bed contains the first Lower Ordovician tephra layer to be found in northern Europe outside of the Caledonian fold-belt. The tephra is unusual insofar as the volcaniclastic fragments are preserved as calcite pseudomorphs surrounded by a thin skin of mixed-layer illite/beidellite. Structures occurring in the limestone beneath the tephra are interpreted as due to percolation of gas. These structures include forcibly injected sedimentary sills and pipes. The pipes functioned as micro-vents after sedimentation of the tephra layer and incipient sedimentation had stopped free percolation of gas through the calcareous sediment. Spar filled cavities contain evidence of early cementation. The succession of events has created structures some of which might incorrectly be regarded as intertidal.