A composite cross-sectional model of a Mid-Dinantian Waulsortian buildup and its lateral equivalents is assembled from 10 measured sections. Five of these are in the Furfooz 'reef southeast of Dinant and the others, well correlated, are in 'non-reef facies in the Dinant and Namur synclinoria (ranging up to an original 100 km or more from the area of the buildups).This model provides the framework for a petrographic study (over 700 samples) in which 25components, mainly dochems, were recorded. The simple A assemblage typifies the classic Waulsortian facies-and many of its Iaterai equivalents.The others have increasing compositional diversity, with B and C occurring in the Waulsortian buildup facies and assemblage D mainly limited to %a& deposits. Many components of these assemblages (calcareous algae, micritization, plurilocular foraminifera, etc.) have distribution patterns showing marked diachronism downstope on the flank of the buildup. This is interpreted as representing a lowering of critical depth thresholds for individual components due to a major drop in sea level late in Tournaisian times. Different components shift downwards 'in step' at slightly different stratigraphic levels and this allows construction of a relative depth scale.Taking the lower limits of the calcareous algae as a reference level (Om), other lower limits are, approximately: micritization and cryptalgal coating (-90 m), plurilocular foraminifera (-120 m). hyalasteliid sponges . This relative scale is tentatively converted into a water depth scale using as criteria the distribution of the calcareous algae, the limit of micritization and cryptalgal coating, and evidence indicating that the upper parts of the buildup were in the shallows at the end of Tournaisian times when the buildup had a sea floor relief of about 220 m.Changes in the depositional environments throughout the formation of the buildup are then traced, from initiation of growth in subphotic depths, probably exceeding 300 m, through shallowing phases possibly leading to emergence. indicated by a dotted line, as in A. The area of interest in rhe present study lies between 60 and 170 km on the distance scale.