Undy Coastal Observation Post (alt name: Portland Grounds, Magor PRN 04292g) forms part of a system of Second World War defensive features, which included pillboxes, anti-tank cubes and coastal batteries, constructed to protect the Severn Estuary / Bristol Channel coastline during the early years of the War. Its purpose was to observe and monitor shipping on the channel.

It is sited on the mud flats of the Estuary at the mouth of the Severn, against and partially cutting into the outer face of the sea wall; this would have provided additional protection and camouflage.  The observation post sits on a platform, raised slightly above the mudflats, defining an area some 80 metres in length and 8.5 metres wide at a point directly in front of the observation post.  The front face of the platform is defined by a boulder stone apron some 5 metres wide.   

Debris around and within the structure indicate that the platform and structure get flooded at high tides.

Description:

The Observation Post was built on site. It is a circular structure of cantilever design, which enabled a 360 degree all-round view. It was constructed of brick, concrete, and steel and measures 2.11 metres in diameter (internally) and stands 2.14 metres high.

The lower half of the structure is red brick, comprising a wall 1.10 metres high and 0.48 metres wide. On top of this is a sloping concrete sill, 0.22 metres high externally, 0.10 metre high internally, and 0.58 metres wide. Evidence of it being cast on site is shown through the wave pattern on the external face, resulting from the use of corrugated iron in the casting frame.  

Set into the concrete sill (and possibly down into brick walls beneath), are 4 steel girders, which hold the roof of the structure thus creating the cantilever design with a 0.36 metre high (externally) embrasure (externally).

The roof is of cast concrete, the imprint of timber planking from the casting frame visible on the sides and underside of the roof.  It measures 3.08 metres in diameter and the casting is 0.5 metres thick externally reducing to 0.35 metres internally. Around the perimeter of the roof are a series of small 5-centimetre square slots, which would have held hooks for camouflage netting.

The 0.61 metre wide entrance is on the northeast side of the structure.  The interior space is dominated by a circular reinforced concrete table, 1.10 metres in diameter and 0.1 metres thick, which rests on three 0.14m diameter metal legs.  Observation instruments would have been mounted on this table.

In construction and form Undy is very similar to that of the Observation Post at Sudbrook Camp (NPRN 800700), also constructed as part of the Bristol Channel defence line and situated on the inner rampart bank of the hillfort, 6.5 kilometres to the northeast. These are the only examples known in Wales.

The structure was visited, photographed and surveyed through UAV photogrammetric survey by RCAHMW in October 2022.  Corrosion of the steel roof supports has made the structure unsafe, and it has therefore been approved for alteration/demolition.

Sketchfab models resulting from the survey are viewable here: Landscape View - https://skfb.ly/pxGvp  Detail View - https://skfb.ly/oWTWW

Louise Barker, RCAHMW October 2022

Sources

Defence of Britain Project (Record Number 3162): https://archaeologydataservice.ac.uk/archives/view/dob/ai_full_r.cfm?refno=3162

Heneb Historic Environment Record: PRN 04292g https://archwilio.org.uk/arch/query/page.php?watprn=GGAT04292g&dbname=ggat&tbname=core