The "corrosion surfaces" or "discontinuity surfaces" which characterize Middle Ordovician limestones in Iowa, Minnesota, Illinois and Wisconsin are reinterpreted as hardgrounds. Before lithification, many of these surfaces were burrowed by organisms, which characteristically produced two sorts of open dwelling burrow. The sediment around these burrows became lithified whilst the burrows remained open. Later, erosion of the upper surface of the hardgrounds truncated the burrows. One of the burrow systems is previously undescribed, and provides the type ichnospecies of Scalaridomus ichnog