Nedern Brook Wetlands

The site comprises flat low-lying flood-meadow in the shallow valley of the Nedern Brook, which flows from the north-west near Caerwent to the Severn estuary between Caldicot and Portskewett. ==Geology and hydrology==

Soils on the site comprise heavy clay with some peat, superimposed on glacial and estuarine gravel and mud deposits. In turn, these overlay Triassic dolomitic conglomerate in the west and Carboniferous Limestone in the east. A hydrological study by the British Geological Survey, published in 2016, ==Castrogi Brook==

The watercourse commences as the Castrogi or Cas-Troggy Brook from multiple springs in the vicinity of Cas Troggy on the northwest side of Wentwood. It flows first northeast and then southeast through a series of entrenched meanders to the east of Wentwood. The brook then enters the broad vale in which Caerwent sits and, passing under the A48 road at Cas-Troggy Bridge, it assumes the name Nedern Brook. ==History==

In Roman times, it is likely that trading vessels sailed up the Nedern Brook to Caerwent. The waterway later silted up. This may have been the result of natural processes, possibly accompanied by active land reclamation in the mediaeval period, and certainly exacerbated by work relating to the excavation of the Severn rail tunnel in the 1870s. In 1879, work on the tunnel was temporarily stopped when it was inundated by fresh water from what was known as the "Great Spring". Improved pumping was introduced, the spring was sealed, and the water diverted away from the tunnel. The hydrological survey in 2016 stated that "the concrete lined channel installed by Victorian engineers, in an attempt to reduce water inflow into the Severn Tunnel, is reported to be in poor condition and ineffective in retaining water in the brook". The holes were blocked with clay as part of the Severn Tunnel work. ==References==