Defence against potential invasion by German forces after the withdrawal from Dunkirk became the main concern of the early years of the Second World War. The first level of ground defence was based on forming a "coastal crust" to resist the initial invasion (If you are interested, several other caches in the East Lothian at War series deal with elements of the coastal crust in the area.)
Inland, fixed defences were put in place to resist the advance of any enemy force that got ashore. In some areas these were arranged along natural obstacles such as rivers or canals and set up as “Stop Lines” to slow and hold advancing forces. McKryton has set up a series of caches in Fife to mark elements of the Fife Stop Line which gives an excellent picture of how these defences would be managed. A Bookmark list is here. East Lothian had no designated Stop Line. Instead the interior lines of defence concentrated principally on positions which could cover roads and other transport routes and so impede any enemy advance.
This cache is at the site of a Flamme Fougasse installation on the Danskine to Carfrae road. The Flamme Fougasse was a weapon designed to produce a "lake of fire" often across a road at a choke point such as a hill or sharp bend where vehicles would have to slow down. A large container, 50 gallons or more in capacity, was built into a slope or positioned in trees to provide a head of pressure. The container was filled with a mixture of petrol, oil and rubber. There would be a command post hidden beside the road where the operators could keep the road under observation and set off the weapon. A pipe led from the container through the control point where the flow of inflammable fluid could be turned on; and then to the roadside where an explosive charge would set off the "lake of fire" fed by the contents of the storage tank.
In this example, the storage container was a concrete tank built into a brick tower among the trees on the brae above the bend in the road. There is plenty of room to pull off the road at the point of the bend close to the cache site. The tank and tower are still there, as is the first section of the feed pipe. There is now a hole in the back of the tower and, with a torch, you can see into the tank. The cache is concealed in what is left of the command post.
The construction is an impressive bit of building for what would have been a "one-shot" weapon. Tactically, the site is an excellent one for the Fougasse; the sharp bend and the slope would certainly have caused a convoy of military vehicles to be slowed down to a crawl at this point. But strategically it is not at all clear why this very minor road required protection or why it was expected that any worthwhile targets would come this way!
About 2,000 Flamme Fougasse installations were recorded in Scotland. It seems to have been a particularly popular device with the Home Guard units in East Lothian. Another is recorded nearby. There was also one at the station in Longniddry which was tested and which successfully set fire not only to the road but also to the fence running along the railway line. One was built in Aberlady which had a large storage tank built into a barn near the corner of the Haddington Road. This fed a perforated pipe that ran down the slight hill and for nearly a quarter of a mile along the side of the road towards Aberlady Bay. Another on the harbourside at Dunbar fed into pipes that would spray the flaming liquid over the water at the entrance to Victoria Harbour. When this was tested it was found to be so effective that any boats at moorings had to be hastily moved into the inner harbour to prevent them being burned out.