Covered Bolts

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Covered bolt at Redscar
Covered bolt at Fochno
Covered bolt at Therfield Heath

Lake District

A very small number of bolts have been found covered with a metal plate approx 12" square with the warning sign 'ORDNANCE / SURVEY / DO NOT / DISTURB' embossed on a raised circular disc at centre of the plate. To date (February 2023) all of these have been located within the southern portion of the Lake District National Park.

'Covered Bolt' is a trig baggers term; the corresponding OS term for the cover is 'Detector Plate'. This is mentioned, for example, in the Office File sketches for Ponsonby Fell and Kentmere Hall. In T:UK and OSGB36 the trig mark itself is listed as either a Bolt or Buried Block. As with Concrete Rings, the purpose of the cover is to aid in the detection of the trig when using a metal detector.

Trig Stations with a Covered Bolt
T:UK Station name Order Computed Block
TP17120 Gilpin Park 3 1950 SD47
TP8079 Lowick 2 1950 SD47
TP16492 Ponsonby Fell 2 1952 NY11
TP8081 Redscar 3 1950 SD47
TP13003 Top o' Selside 2 1950 SD47
TP7820 Wansfell Pike 2 1950 SD47
TP17121 Kentmere Hall 3 1950 SD47

Trig Stations where a Covered Bolt is suspected but not confirmed
T:UK Station name Order Computed Block Notes
TP16495 Blake Fell 3 1952 NY11 Detector Plate 1'4" x 1'4" over Block


A single instance of a cover with a different style has been found at Fochno in Dyfed. This in turn resembles the passive station sketch for Pegler's Tump. An unmarked iron plate covers the bolt at Therfield Heath.