A 'Block' is a concrete block approx 1 cu ft in dimension emplaced such that the top surface is at ground level. A bolt, with head diameters measured to date between 3/4" and 2 3/8", is fixed at the centre of the upper face of the block. Over time surface soil/vegetation etc has built up rendering the blocks often now several inches below ground level.
For a general description of a 'Buried Block' see 'Block' above. 'Buried Blocks' were designed to be buried below ground level (0.6m is an indication of the depth), however, they can be found much closer to the surface. At some point the OS experimented with introducing a ferrous substance (iron filings?) into the concrete mix of some buried blocks in order to use a metal detector to aid in locating these trigs. Apparently it wasn't very successful. See also 'Concrete Ring' below, for a variation on the standard concrete cube.
'Concrete Ring' is a trig bagger's term denoting a distinctive type of Buried Block seen in Lakes/Northern Pennines and elsewhere. Typically, the block comprises a central slightly conical bolt of 3" diameter. This is surrounded by a concrete ring of approx 15" internal and 21" external diameter. The ring may bear the inscription "Ordnance Survey Trigonometrical Station" although this may be eroded.
The OS records do not distinguish Concrete Rings from other types of Block hence it is not known how many exist. Until 2014, 10 Concrete Rings had been located, all in Northern England (squares NY and SD). However, they have subsequently been located more widely, e.g. SO, SS, ST and SU with 26 having been identified by late 2019.
A concrete ring as described above is distinct from an 'emplacement ring'. The latter is a term used by the OS to describe a metal ring (possibly with a concrete or similar casing) placed loosely on a buried triangulation mark. With the use of a metal detector the emplacement ring can assist in locating a trig mark, but is not in itself a mark.
The photograph to the right is the Saddleback buried block located at the summit of Blencathra, Cumbria. It was reported missing in 2018 and was replaced in early 2019.
The term 'Surface Block' is derived from the OS Passive Stations Database. A number of OSGB36 Blocks were reused as Passive Stations, and some new Blocks were constructed. In practice, Surface Blocks are physically indistinguishable from Blocks.