Severn Estuary -- where was the Neolithic shoreline?

Turner, Allen and Rippon (Archaeology in the Severn Estuary 11 (2000), l-12)

This is a very complicated business, because of the sheer extent of the "levels" all around the estuary. All are agreed that there has been a substantial transgression between the Mesolithic and the present day -- and there are peat beds, submerged forests and archaeological finds to prove it. But there have been short-lived regressions and transgressions as well -- partly due to the complex interplay of eustatic and isostatic factors. The whole scene is also rendered complex by the huge tidal range of up to 15m, and the ongoing process of sediment dumping and redistribution on these vast mud flats. So far as I can see, none of the authors studying the estuary has been able to draw a line on a map and say with conviction: "This is where the Neolithic shoreline (or mid-tide mark) was."