The area of the Troggy Brook has a complex subsurface hydrology, and the surface streams have undergone two periods of radical alteration. These factors make it somewhat difficult to assess the form the hydrology would have taken in the 'Dark Ages'. Here we'll look at where a lake in the Troggy area might have gone, and a possible mechanism behind the wonder: syphoning springs (the following is indebted to information from, and conversations with, John Nettleship and members of the CHT).
First, let's look at the evidence there was a lake/inlet there at all. The geological map of the area* shows the extent of the estuarine alluvium (mud from the Severn) (Map). This reaches all the way back passed the Roman road north-west of Caerwent (probably the Roman Venta Silurum: Info) (See Visit Details: Photo). No work appears to have been done on the age of this alluvium at Caerwent, however, there are a few pointers to it being laid down in the last two millenia. First, the Roman fort of Caerwent has features indicating there was boat access to it1, secondly early building seems to have avoided the area (Map)2, and lastly, work on the alluvium downstream of Caerwent at Caldicot Castle suggests that the area was a broad, tidally influenced, area of water in the Iron Age3.
Although there are no OS point heights in the valley, the Troggy sits well below the 10m contour all the way to just upstream of Caerwent (a rough estimate for the lower Whirly Pool would be 5 to 7m OD). As the tidal range at nearby Chepstow bridge is 50 feet* (15.24m), suggesting a rise of some 7.62m above the average sea level, there is a strong possibility that the current tidal limit would fall around the lower site in the absence of sea walls. The lower half of the area still floods today where it is allowed to4. However it doesn't appear to be tidal and certainly wouldn't qualify as an inlet of the Severn (See Visit Details). Given this, one has to ask why the inlet, like so many "pills" (inlets) along the Severn, has largely filled in. It is not as simple as suggesting the sea has been blocked by walls, as the land height and tidal range has changed over recent millennia.
More on the filling of the Troggy valley...