CPAT archaeologists Paul Belford and Kate Longley surveying the early benchmarks in the foreground, with the 1948 trig point behind. CREDIT: Clwyd-Powys Archaeological Trust

Archaeologists excavating the Welsh hillfort Beacon Ring (Caer Digoll) made an unexpected discovery relating to the 19th-century Ordnance Survey this summer, which has cast new light on early map-making fieldwork.

Beacon Ring is on the Long Mountain overlooking Welshpool. Since 2008, it has been owned by the Clwyd-Powys Archaeological Trust (CPAT), and for the last few years CPAT has been undertaking a series of excavations, partly funded by Cadw. After a break due to COVID-19 restrictions, the team’s work resumed this past August.

The target for 2020 was a very slight mound in the middle of the site. Rather uncertain antiquarian references hinted that it may have been a Bronze Age burial mound, with some suggestion that cremated bone had been found. The mound itself is located on the highest point inside the hillfort enclosure, which is around 1.8ha in area and surrounded by a single rampart and ditch.

Just off the centre of the mound is an Ordnance Survey Triangulation Pillar (more commonly known as a ‘trig point’). Over 6,000 trig points were erected in the 1930s and 1940s as part of the ‘retriangulation of Great Britain’ – an ambitious project that mapped the whole country to a common datum and the National Grid, which is still used today. The Beacon Ring trig point (number S4718) was erected in 1948, and can only have been in use for a short time, as the forestry on the hillfort today was planted in 1953.

The lower stone slab showing the carved benchmark, and the cast created in the base of the concrete block, here on its side. CREDIT: Clwyd-Powys Archaeological Trust

The ‘retriangulation’ reset the original ‘Principal Triangulation’, which had been undertaken between 1783 and 1853 and provided the basis for all Ordnance Survey mapping until 1936. The excavations at Beacon Ring revealed the remains of an earlier trig point and benchmark, which had been laid out as part of the ‘Principal Triangulation’ in the 19th century.

The older benchmark was in two parts. The lower part was a stone slab, about 900mm square. This had been set into the underlying subsoil, and would have been at or just below the contemporary ground surface. A Board of Ordnance ‘broad arrow’ symbol was carved into the top surface, with a circular hole at the apex marking the precise triangulation point.

The second part was a later concrete block, cast on top of the ‘broad arrow’ benchmark. Its form suggested that it had been created in situ by pouring concrete into a hole cut into the ground – so it seems likely to be a later addition. It is tempting to associate the stone-cut benchmark with the Ordnance Survey’s use of the sea-level at Liverpool as a national datum (from 1844 to 1915), and the later concrete block with the change to the Newlyn Datum, which took place between 1912 and 1921.

As for the mound – the original point of interest – it may have been the location of the ‘beacon’ that gave the hillfort its name. The charcoal-rich deposit above the benchmark was probably the result of bonfires, which we know the local gentry had lit for royal celebrations (notable occasions included Queen Victoria’s Jubilees in 1887 and 1897). Since this is the highest point of the interior of the hillfort, it was probably the location of an earlier beacon as well, first depicted on a 17th-century map.

TEXT: Paul Belford


This news article appears in issue 369 of Current Archaeology. To find out more about subscribing to the magazine, click here.

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