A rod is not listed as a type of triangulation mark. However, it is commonly found on surface and buried blocks where it is used in place of a bolt. Records in the OSGB36 list do not distinguish between bolts and rods.
Typically, the rod protrudes by approx 0.5in above the block surface:
Examples of variations include:
cut on the top surface New Acres square section Saltburn Grange threaded Town End dimpled Whinberry Naze
Damage may occur; either by vandalism or by the OS to indicate that the mark has been 'destroyed' and should not be used:
truncated Bryngates bent West Hile
Figure 7.8 in The History of the Retriangulation of Great Britain  depicts a brass rod with the caption "Used as centre mark in concrete blocks at minor triangulation stations".
- ↑ The History of the Retriangulation of Great Britain 1935-1962, p.225, 'Types of Station Marks'