Royal Observation Corps Underground Monitoring Post, Llandrindod Wells

In September 2022, we were asked by PAR Homes Ltd and Asbri Planning Ltd to undertake an archaeological field evaluation in fields off Lakeside Avenue, on the southern end of Llandrindod Wells, Powys. In the northernmost field, we uncovered the demolished remains of a Cold War-era nuclear bunker, also known as a Royal Observation Corps (ROC) Underground Monitoring Post (UGMP). Within the vicinity of the bunker we also uncovered the foundations of an aircraft observation station, known as a Type B Orlit post. The presence of the UGMP was known prior to our evaluation, although whether or not it survived underground was not clear. The presence of the Orlit post was a lovely discovery and was not known prior to our investigations.

Aerial photograph of the Royal Observation Corps (ROC) Underground Monitoring Post (UGMP) during excavation

History of the Royal Observations Corps (ROC)

The ROC was established in 1925 as part of the British air defence system which, at this time, was rapidly trying to keep pace with, and potentially respond to, advances in aerial warfare technology brought on by the First World War (WWI). Between 1925 and 1941, the organisation was known simply as the ‘Observer Corps’ (OC). The main role of the OC was to identify hostile aircrafts flying over or within the vicinity of the British Isles. Prior to the establishment of the OC, during WWI, air defence was the responsibility of the Admiralty, to whom information on potential hostile aircrafts was sent via police reports. By 1920, after the end of WWI, British air defences had been rolled back, with most anti-aircraft hardware in the country being decommissioned. However, by 1924 Air Raid Precautions (ARP) guidelines were established, which were designed to inform on the reporting of, and countermeasures against, potential dangers to British civilians from air raids. In addition, ARP guidelines filtered through to urban and industrial planning, engendering a policy of continual aerial defence preparations during peacetime (Page 2016, 119). Within a year, the ARP had developed into a specified methodology for the OC. Prior to the outbreak of the Second World War (WWII), the ROC was plagued with technical and organisational issues, although these were ironed out in 1939 during a series of exercises intended as preparation for potential Luftwaffe raids. During the Battle of Britain in 1941, and the Blitz more generally, the role of the OC was integral to British air defence and aided greatly in allowing the RAF to fight off the Luftwaffe. OC personnel worked tirelessly in tracking enemy aircrafts and passing information on to RAF Fighter Command Groups and Sector Controls. In recognition of its importance to British defence, the OC was granted the title ‘Royal’ by King George VI in April 1941 (therefore becoming the ROC).

An ROC operative conducting air observations during WWII

By 1945, at the end of WWII in Europe, and the cessation of Luftwaffe raids on British territory, the ROC stood down. Yet during that same year, the ROC was once again put on active duty to deal with potential security threats posed by Britain’s involvement in the Cold War. Principally, it was the ongoing expansion of Soviet-style socialism into much of Central Europe, and the perceived threats that this posed to British interests, that led directly to the reformation of the ROC (Clarke 2005, 140). In 1954, the first thermonuclear weapon (or H bomb) was detonated by the USA during Operation Castle Bravo at the Bikini Atoll of the Marshall Islands. The radioactive fallout from this event was both devastating and (perhaps more importantly) extensive, meaning that in the wake of a thermonuclear explosion, large areas of land would be vulnerable to drifting radioactive particles. In 1955, the Strath Committee was established, which comprised a secret group of civil and military experts tasked to assess the potential impacts of thermonuclear weapons on Britain from an environmental, societal, defensive and economic perspective (Hughes 2010). The Strath Committee concluded that the detonation of only a limited number of thermonuclear weapons along the western seaboard of Britain would lead to the release of a devastating amount of nuclear fallout across the entire country (Clarke 2016, 206). As a response to this potential threat, the United Kingdom Warning and Monitoring Organisation (UKWMO) was established in 1957, which was responsible for, among other things, the monitoring of potential nuclear detonations and their ensuing fallouts. During that same year, the UKWMO set up a large network of UGMPs, numbering 1,518 in total, which were operated and maintained by the ROC. The placement of these posts formed a grid-like pattern across the country, with each post being situated in an isolated rural area. Moreover, each post was situated approximately seven to ten miles from its neighbour and were often positioned, for reasons of accessibility, on the periphery of playing grounds or arable fields (Bennett 2018, 215). With the introduction to Britain of UGMPs came the abandonment by the ROC of most of its aircraft observation duties (however, see references to Orlit posts below). Indeed, the principal focus of the ROC between 1957 and the end of the Cold War was nuclear monitoring.

Mushroom cloud resulting from the Castle Bravo thermonuclear explosion

History of Underground Monitoring Posts (UGMPs)

A detailed analysis of ROC UGMPs has previously been carried out by Robert Clarke (2016), where he identified the function and layout of these subterranean structures. A fantastic model of the interior of an ROC UGMP has been illustrated by Bob Marshall (see below). The work of Marshall serves as a useful, visual demonstration of life within, and the workings of, a typical UGMP. Clarke notes that each post was occupied by three or four ROC personnel, whose daily tasks primarily consisted of observation and submitting periodic reports to larger command and control bunkers (Clarke 2016, 76). In total, 31 of these larger bunkers were established across Britain, the construction of which was funded by the Home Office. Once the personnel within the larger bunkers received a report from those operating a given UGMP, the information within the report was then passed on to the military and Home Office (Wood 1992). Depending on the vagaries of local geology, each UGMP was buried to a depth of at least 4m (Clarke 2017, 242). In terms of composition, UGMPs were constructed using materials capable of withstanding the kinds of environments left in the wake of nuclear explosions. Predominantly, these materials included reinforced concrete, steel and tungsten (Clarke 2016, 89).

Digital Reconstruction Cutaway of a Royal Observer Corps Monitoring Post reproduced here with kind permission of Bob Marshall

In effect, each UGMP comprised a subterranean control room measuring approximately 5.8m x 2.6m in area and around 2.3m in height (Clarke 2016, 206). Accompanying the control room was a series of above-ground instruments, which recorded exterior radiation levels and monitored any potential detonations and the amount of energy they released. Initially, communications between UGMPs and elsewhere were often conducted via telegraph wires, which were highly vulnerable and susceptible to damage. In order to rectify this problem, radios were installed in the control rooms of UGMPs in the 1960s, which replaced telegraph communication. All radio equipment was situated within the master post, which comprised a small station positioned against the interior wall of the bunker. Within the master post was a VHF radio, carrier receiver, teletalk device and filter unit. Attached to one of the ventilation stacks above ground was an aerial mast, via which radio communication could be achieved. This mast could be extended into the air via an air pump located within the control room. Also situated above ground was a Ground Zero Indicator (GZI) – an instrument designed to locate the hypocentre of a nuclear explosion. These instruments comprised circular metal boxes with four horizontally mounted pinhole cameras inside, within which sheets of photosensitive paper were placed. If a nuclear blast was set off within or near Britain, the resulting bright flash would burn a mark on the photosensitive paper. The position of this mark on the paper would inform on the height of the nuclear blast, with the position of the blast being determined by triangulating data from GZIs situated at neighbouring UGMPs. The GZI was situated on top of a brick or concrete plinth, with a circular mount fixing it in place. This mount was attached directly to the plinth. Sometimes, the plinth was positioned above ventilation duct of the main entrance stack, as was the case with the Llandrindod Wells UGMP (see below).

In 1991, the Soviet Union was dissolved, and the Cold War ended. This, in turn, led to a diminishment in nuclear hostilities and, therefore, a reduction in nuclear threats to Britain. Despite the easing of tensions that followed the ending of the Cold War, the ROC was kept in active duty throughout the early–mid-1990s, albeit in a much-reduced capacity. The remaining personnel attached to the ROC were at this point, however, confined to maritime and not terrestrial duties. By the summer of 1990, in the face of radical reforms to the Soviet Union and the Warsaw Pact proposed by Gorbachev, the British government commenced the Options for Change programme, which aimed at reducing military spending. The implementation of this programme ultimately led to the disbandment of all remaining UGMPs in Britain.

Results of the Archaeological Field Evaluation

During our archaeological field evaluation, a trench was excavated across the proposed position of the Llandrindod Wells UGMP in order to determine its presence and state of preservation. As we found out, this was easier said than done! The small size of the bunker, combined with the masses of demolition material spread across the field, made its discovery more difficult than we anticipated. We did have in our possession some photographs of the bunker’s entrance stack before its demolition (see below) which, in terms of their ability to accurately determine the position of the bunker, were limited. After a few attempts, however, we found it. Although no part of the UGMP was found to survive, its broad outline could nonetheless be observed. Unfortunately, due to its previous demolition, the internal arrangement of the bunker was not ascertained, and neither was its structural form. However, the finds collected from within the demolition material of the bunker were able to shed light on some of the activities conducted within and around its confines. The archaeological remains encountered during our field evaluation were recorded photogrammetrically, which allowed them to be preserved as digital 3D models (see below).

Photogrammetric orthomosaic of trench excavated through demolished ROC UGMP (left) and remains of the Type B Orlit post (right)
Photogrammetric orthographic image of east-facing of section of trench excavated during field evaluation through demolished ROC UGMP

The most notable finds retrieved from the demolition material of the UGMP included a Ground Zero Indicator (GZI) mount, a master post carrier receiver, and a master post telecoms filter unit. The GZI mount comprised an upper circular fixture attached to a triangular base via a circular neck (see below). The mount was cast in one piece (in cast iron) and was painted a dark green colour. The GZI (discussed above) was an essential component of the UGMP as it could measure the position and size of any potential nuclear explosion within the UGMP’s vicinity. Amazingly, we had in our possession a photograph of the GZI mount when it was in situ (see below). In this photograph, the mount is shown as being attached to the concrete forming part of the top of the ventilation duct attached to the entrance stack.

Photograph of the Llandrindod Wells ROC UGMP prior to its demolition in the 1990s, showing the entrance stack as well as the GZI mount attached to the ventilation shaft (in red)
The GZI mount collected during the archaeological field evaluation, with part of the concrete once comprising the ventilation shaft of the UGMP’s entrance stack still fixed to its base

The master post carrier receiver comprised a grey plastic box with a speaker grill attached to the front. The find incorporated a front cover, on which the speaker grill was located, which allowed it to be opened for inspection/repair. On this cover was a button and a dial, situated below the speak (microphone) grill. The button, situated on the left-hand side, was for testing the radio and speaker, while the dial, situated on the right-hand side, controlled the volume. A serial number was noted on the right-hand edge of the find, which read ‘EMT 82/21’. In terms of function, the carrier receiver was originally attached to the master post, which comprised a set of equipment mounted to the wall of the bunker that collected readings and messages from derived from the local cluster of UGMPs to which the bunker belonged. The carrier receiver allowed the personnel manning the UGMP to receive the National Attack Warning Red (implying aerial attack), the Fallout Warning (implying a nuclear strike and fallout) or All Clear (implying no immediate danger. Two models of the carrier receiver were produced for use within UGMPs, known as the 1st and 2nd generation. The 1stgeneration model was notably small and comprised a speaker with no buttons or dials. The 2nd generation model was notably larger and comprised a speaker with a test button and volume dial. The carrier receiver recovered during from the demolition was a 2nd generation model.

The master post carrier receiver recovered during the archaeological field evaluation

The master post telecoms filter unit comprised a small black box, rectangular in profile, with a detachable cover on the front held in place with screws. The find had bevelled (or rounded) edges, while on the interior was a circuit board a wires. On the front of the find, in white print, were the words ‘TELECOM’, ‘FILTER UNIT’ and the model number ‘WB1410’. In terms of function, the filter unit selectively sorted incoming telecommunications signals through a specified (and desired) range, while suppressing others. More specifically, the WB 1410 filter separated the carrier from the speech and power received by the teletalk, which represented the main communication tool within the UGMP bunker.

The master post telecoms filter unit recovered during the archaeological field evaluation

Finally, one of the more unexpected discoveries of our archaeological field evaluation were the remains of the Type B Orlit post. As mentioned above, the presence of this feature was not known prior to our archaeological field evaluation. The remains of this feature were uncovered to the east of our trench, during initial attempts to locate the approximate position of the UGMP. Our discovery of the Orlit post, therefore, occurred almost by chance! The surviving remains of the Orlit post comprised four concrete foundation pads, which together formed a broadly square shaped footprint. The southeastern foundation pad had remnants of brickworks attached to it, indicating that it once held a column above it. Not one the photographs we had in our possession of the UGMP showed the existence of an Orlit post within the area. This indicated that the post was likely demolished during the use of the UGMP. Clarke, in his research of Orlit posts, describes it as “an overground observation point for aircraft reporting….constructed using pre-cast concrete panels manufactured to a design by Messrs Orlit, Bedfordshire” (2016, 14). ). It is clear that Orlit posts ranged quite significantly in form and size, with two types being defined by Clarke. The first, Type A, was constructed over a concrete base at ground level. The second, Type B, was constructed on four precast concrete legs (Clarke 2016, 162). It appears that the feature we discovered can be identified as a Type B Orlit post. From this finding, we now know that within the Llandrindod Wells UGMP the above-ground aircraft reporting was, for a time at least, being conducted in conjunction with underground nuclear monitoring. In form, the Type B Orlit post was effectively a raised concrete box accessed via a wooden door, within which communications board, telephone and various other equipment were located (see below). On the right-hand side of the entrance, a sliding door was often incorporated to provide access to a small observation platform, which was open to the elements albeit for a moveable, corrugated tin cover. On top of the Orlit post, an instrument was mounted, which plotted the bearing and altitude of unidentified or potentially hostile aircrafts. Despite the importance of the Orlit post to the UGMP and the intricacy of the activities conducted within, the structure was markedly rudimentary, and when viewed from the exterior it had the appearance of a simple store shed.

Diagrammatic section and plan of an ROC Type B Orlit post (after Wood 1992)

Selected Bibliography

Bennett, L. 2018. ‘Cold War Ruralism: Civil Defense Planning, Country Ways, and the Founding of the UK’s Royal Observer Corps’ Fallout Monitoring Posts’. Journal of Planning History 17 (3), pp. 205–25.

Clarke, R. 2005. Four Minute Warning: Britain’s Cold War. Stroud: Tempus.

Clarke, R. 2016. Landscape, Memory and Secrecy: The Cold War Archaeology of the Royal Observer Corps. Unpublished PhD Thesis: University of Exeter.

Clarke, R. 2017. ‘The Secret Taskscape: Implications for the Study of Cold War Activities’. In U. Rajala and P. Mills (eds), Forms of Dwelling: 20 Years of Taskscapes in Archaeology, pp. 236–52. Oxford: Oxbow.

Hughes, J. 2010. ‘The Strath Report: Britain Confronts the H-Bomb, 1954–1955’. History and Technology 19 (3), pp. 257–75.

Page, A. 2016. ‘Planning Permanent Air Raid Precautions: Architecture, Air War and the Changing Perceptions of British Cities in the Late 1930s’. Urban History 43, pp. 117–34.

Wood, D. 1992. Attack Warning Red: The Royal Observer Corps and the Defence of Britain 1925 to 1992. Portsmouth: Carmichael and Sweet.


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