Sea level has apparently risen by something like 1.22 to 1.70m in the area since this time* (with a possible tidal range increase of 0.1m*), suggesting the land is dropping*. This encourages the deposition of materials, as marsh-plants and slack-water conditions generally ensure that marsh-tops stay constant relative to upper tidal heights. However, at the same time, the estuary will have moved upstream under the rise in water-level, the wider section moving upstream at a rate between 0.05 and 1.2m per year* and cutting into older land surfaces. In places, the Roman coast would have been some 350m further out than the current coast*.

The reasons for this sea level change are complex. Modern climate change is raising sea levels by two processes: through melting glaciers and through expanding the earth's water by warming. However, together these mechanisms are responsible for changes two orders of magnitude smaller than those seen here, and will only have occurred in the last fifty years or so. The most likely reason for the change here is the removal of the massive European and British ice sheets at the end of the last glaciation (c.11-18000 years ago). Water from the oceans will have been used to make the ice, but this will have returned relatively rapidly as the ice melted, raising the sea level much earlier than the Romans (indeed, this was probably over by the Mesolithic: Info). However, the pressure of the ice on the land may have pivoted the crust, making a budge in front of the ice. In Britain, for example, Scotland was pushed down by ice, but it seems likely that in return unglaciated southern Britain, including the Severn area, was pivoted up. With the removal of the ice, Scotland has been rising (ie. sea level has been falling in relative terms), whereas the south has been falling (ie. sea level rising in relative terms). More recently, since the 1870's, the pumping of water from the coastal agricultural land and the Severn Tunnel has led to drying and consolidation of the peat and other shallow geologies*.