Dr M. F. Ridd urged caution in equating transgressions with high sea level and regressions with low sea level. He went on to say that in considering transgressions and regressions, the relevant factors could perhaps best be expressed, not on a triangular diagram as one speaker had suggested, but on a square diagram. Eustatic sea level changes and isostatic adjustment of the crust are both important but, Dr Ridd suggested, the third factor, tectonism, is better considered in two parts. A tectonic depression of the area of deposition will give rise to a transgression (all other things being equal), and uplift will give a regression. However, it is possibly tectonism on land, in the provenance area, which is the most important factor of all. Tectonic uplift there results in an increased sediment supply and a regression of the shoreline. It must be said, however, that in practice it will often be difficult to distinguish such tectonic uplift of the land area from more regional isostatic uplift.
Dr Julia A. E. B. Hubbard writes: The authors should clarify the evidence upon which their ‘deep’ facies have been reconstructed in the light of the hydraulic evidence available from oceanographic work that the parameters the geologist records are those of the sediment-water and biota-water interface and not of depth per se.