There has been a considerable and almost continuous buildup of silt along the Severn since the Roman period*. However, the deposition has been complex. Land in this area is (very) slowly sinking, and the relative sea level rise has built up some sediments, but cut into others1. In addition to this, complex current systems and channels will have moved large banks of mud around. Even without human alteration, this would have produced a complex pattern of silting that varied pill by pill. However, on top of this, humans have variously altered the natural patterns by building a series of coastal defenses and reclaiming land. This, combined with the variation in sea level, has cause a variety of land surfaces to come and go. These are summarised in this Diagram. What is clear from work in the area is that the pills themselves have been silting up since the Mesolithic* (Info), but that at times, for example in the Iron Age (Info), the sea level rose to turn the pills into tidal inlets*.
The conditions in the Troggy's pill in the Dark Ages are unknown, but were likely to be those of falling tidal inundations and silting-up as the Iron Age high in tidal influences diminished2. If we assume the stream was wide enough for flat-boat navigation to Caerwent during the Roman period (possibly through Roman dredging), and largely filled with sediment up to Caldicot by the end of the Medieval period, the Dark Ages could have seen tidal water as high as the position of the Whirlpools. This may be backed up by the position of the parish boundary. The boundary was delimited c.700 CE when the local King, Brockwael, gave the land to the church. A copy of the transfer and boundary from c.895 CE* preserved in the Book of Llan Dâv (Other Works) doesn't mention the whirlpools or a lake, but describes the boundary as following the Troggy. However a Map of the boundary from the 1775 sale of Caldicot lordship shows the parish boundary leaving the Troggy to take in the eastern-most whirlpool. It then traverses a square area, which is the current area of flooding. If the boundary is ancient (and the Book of Llan Dâv shows parish boundaries in this area are pretty resilient), it may suggest the area was flooded and counted as part of the Troggy c.700. The area certainly matches the Wonder description very nicely (see Visit Details).
Which brings us to the features themeselves... More