Middle Hirnantian
At Ullerntangen the bioclastic limestone sequence with its small mud  mounds is overlain by a very much larger, lens-shaped buildup, at least 350 m in length, 100 m in width, and up to 12 m in thickness.  It was originally interpreted as a reef by Kiaer (1897) but Hanken (1974) has argued that it lacks an organic framework, being largely composed of reworked material.
Nevertheless a remarkably diverse biota is recorded from this structure, including the stromatoporoid Pachystylostroma, rugoans Palaeophyllum,
Cyathophylloides, Tryplasma, tabulatans Saffordophyllum,  Rhabdotetradium, Lyopora, Reuschia, Catenipora, Proheliolites and Plasmpora as well as exhibiting pockets with clusters of cystoids and trimerellid brachiopod specimens.  The comparatively random directions of orientation of many of the coral and stromatoporoid specimens on the present exposed, slightly inclined, surface are not by themselves a certain basis for claiming that these organisms, or a substantial number of them, were not part of an in situ framework. Almost half the muddy matrix is composed of skeletal bioclasts - predominantly representatives of pelmatozoans, brachiopods, bryozoans, algae (Vermiporella, Hedstroemia) and the cyanobacterieum Girvanella.  The upper surface of the mound is cut by deep fissures up to 6 m deep, that imply a period of significant erosion possibly under subaerial conditions, and then they were infilled by sandy pelmatozoan-dominated bioclastic limestone in a subsequent depositional phase.  Finally a further sandstone unit overlaps the mound on its northern margin, and exhibits a very rich brachiopod, bryozoan and gastropod fauna near the mound.  This unit represents the uppermost part of the unnamed formation, and appears to be bounded at its top by a significant unconformity (hiatus) to the overlying Silurian Saelabonn Formation.
