Dr Ridd expressed the view that one of the remarkable features of the Carboniferous deltas over the region discussed by Dr Ramsbottom is that they apparently built out into bodies of fresh water rather than into the sea. Among other things this raises a nice problem of the definitions of transgression and regression.
Sedimentological work on the rocks has allowed many of the individual deltas to be reconstructed, with their delta-fronts prograding into deep fresh water. The shoreline at such a time was the boundary between the delta-top and the delta-front and, Dr Ridd suggested, it is movement of this shoreline which defines transgressions and regressions.Goniatite-bearing beds blanket truly vast regions, as Dr Ramsbottom pointed out. A possible cause is a eustatic rise of the sea level and an incursion of marine conditions over a distant basin rim. But whereas marine incursion affected basin and delta alike, the extent of the associated transgression (by the definition above) was much more limited. The transgression affected only that area which had previously been landward of the shoreline and was therefore confined to the delta-top.
Dr. W. A. Read stated that, as a sedimentologist who had worked extensively on the late Viséan, Namurian, and Westphalian A successions of central Scotland, he was happy to report that in these rocks the sedimentological evidence is fully in agreement with Dr Ramsbottom's concept of a series of eustatic rises in sea level. However, evidence of equally large-scale eustatic falls is very rare indeed. In the