Monument record 1509000000 - North of River Misbourne

Concrete Spigot Mortar Emplacement in very good condition. Identified by UK Pillbox Study Group - EDOBID e35080 (B1).
History:
Following the evacuation of the British Expeditionary Force from Dunkirk in 1940, Britain’s defences were strengthened against the very real threat of a German invasion. Diverse types of defensive structures were built in large numbers across the whole country, some of the most common including road and rail blocks, earthwork gun emplacements, barbed wire entanglements, anti-tank defences and pillboxes. These defensive structures were generally grouped, either at vulnerable or strategically important nodal points, around vital installations such as airfields, or arranged in linear defensive systems called stop lines, which were intended to obstruct the enemy advance.
One particular weakness identified in Britain’s defences was a grave shortage of anti-tank guns; 840 had been left behind in France following the Dunkirk evacuation and only 167 were available in Britain. Besides a lack of weaponry, the required ammunition was in such short supply that regulations prohibited the firing of even a single round for training purposes. A relatively cheap and portable solution to this problem was designed by Lieutenant Colonel Stewart Blacker of the Territorial Army. Unlike conventional mortars, a spigot mortar does not possess a barrel, rather a steel rod, or spigot, is fixed to a base, with the projectile itself housing the propellant charge in its tail. When the mortar is fired, the projectile is pushed down onto the spigot, which explodes the charge and fires the round. The 29mm spigot mortar, or Blacker Bombard, was based on this premise, but with an inclined, swivel-mounted spigot and trigger firing mechanism. It was also fitted with a portable mounting consisting of
four folding legs.
The design for the Blacker Bombard dates back to 1939, although it was not accepted by the War Office until after the events of Dunkirk. The plan was submitted to the head of the Military Intelligence Research department, Major Millis Jefferis, who was receptive to the idea, although other government officials initially opposed. Prime Minister Winston Churchill attended a demonstration of the weapon on 18 August 1940, and on his orders the weapon was put into full production. By mid-December c8000 of an initial order of 16,000 had been issued, and the initial order was increased to 28,000. Eventually nearly 29,000 were made. On 14 September 1941, the Directorate of Fortifications and Works (DFW) published drawing number 55280. This was an important development which introduced the designs for a pedestal mounting for the weapon, replacing the heavy folding legs with a fixed emplacement.
By using these pre-prepared emplacements, the weapon could benefit from enhanced portability and stability, whilst still offering sufficient accuracy and protection for the crew. The type site would comprise a concrete thimble of approximately 1m diameter and 1m in height, with a central steel pin which would engage the underside of the weapon and form a swivel mounting. The thimble was to be centred in a pit c1.2m deep giving access all around, with four concrete ammunition lockers recessed into the walls. A deeper approach trench was also included in the plan, and a separate store building was often included for storage of the weapon and mounting when not in use.
Amersham does not appear to have formed part of any General Headquarters (GHQ) anti-tank line or any other stop line, nor was the town a nodal point or designated strong point. The River Misbourne seems to be key to understanding the context of this emplacement, acting as a natural barrier which was complemented by the mortar emplacement. The Amersham Company of the 5th Buckinghamshire Battalion Home Guard were likely to have been responsible for this emplacement, and in the absence of any firm evidence to the contrary, it is likely that this was an ambush position.
Details:
Principle Elements:
This is an emplacement for a 29mm spigot mortar, also known as the Blacker Bombard, likely dating from 1941-2.
Description:
This example consists of a circular pit finished with a concrete floor and walls. The pit has openings for two square ammunition lockers to the south side, which are set back into the wall of the pit. In the centre of the pit is the large circular concrete thimble, which has a curved top capped by a circular iron plate in the centre. This holds the steel pin or pivot. This design would appear to be a variation on the Directorate of Fortifications and Works plan, adjusted according to its context and the topography of the site.
Selected Sources:
Books and journals
Alexander, C, Ironside's Line: The definitive guide to the General Headquarters Line, (1998)
Dobinson, C S, Twentieth Century Fortifications in England Volume II Anti Invasion Defences of WWII, (1996)
Foot, W, Beaches, Fields, Streets and Hills: the anti-invasion landscape of England 1940, (2006)
Websites
Book of the Blacker Bombard, accessed 29 November 2018 from http://www.staffshomeguard.co.uk/J16-98Blacker2.htm
Defence of Britain survey database, accessed 29 November 2018 from
http://archaeologydataservice.ac.uk/archives/view/dob/ai_q.cfm
Specialist Second World War equipment website, accessed 29 November 2018 from
http://www.wwiiequipment.com/index.php?option=com_content&view=article&id=93:the-blacke
r-bombard&catid=47:british&Itemid=59 (B2).