A Feno Marker is a type of survey marker, developed by the French company Faynot, which is quick to install and well anchored . 15 instances occur in the OSGB36 list  of which 10 are 13th order - GPS. All were installed between 1987 and 1989.
6 of these marks have been recorded as destroyed:
- Cattlelaith Lane (2 marks),
- Kings Standard Hill (1987),
- Spitalgap Lane (2 marks of which one destroyed, the other not found),
- Gibbet Hill
5 are thought to be missing:
2 recorded as not found:
2 Fenomarks have been found:
The installation of a Fenomark has been described as follows: "Apart from the obvious yellow, white or whatever colour block at the top, the mark itself is a hollow rod of variable length. Within the rod are three strands of galvanised rod about 5mms in diameter. After the rod is driven onto the ground a tool is driven down the centre of the tube which drives the 3 rods out the bottom. These are designed to twist and provide a corkscrew anchor into the soil. Consequently ... very difficult to extract. The system was originally developed in France for anchors for grape vine wires." 
Fenomarks in the UK are not exclusive to the OS. For example, they can be found along the S bank of the Humber estuary, E of the bridge, where they may have been placed by the National Rivers Authority (NRA).
- emails from Ian Wilson and Graham Pennington to the Trigonomy list, 14/02/2013