1. Flint flakes, burnt stones and fragments of hand-made pottery of Iron Age type were found in `ancient dwellings' on Grassholm before 1951. The first modern archaeological survey on the island was carried out by D.B. Hague, then of the RCAHMW, in 1972. Hague initially visited the island in 1963 with the naturalist R.M. Lockley, who had observed during a visit in 1956 that structures were being revealed by the caustic effect of seabird guano, which was killing the dense mattress of Festuca rubra on the western side of the island and exposing the light soil beneath. The visit was brief and only a rough plan of the settlement was made, but Hague returned to the island in 1972 and, during a five day stay, was able to survey and carefully excavate the exposed remains.

Hague's settlement was comprised of three conjoined structures. In the centre was a rectangular shaped house, 9.5 metres by 8 metres, constructed of mixed orthostats and coursed masonry, and standing c. 0.3 metres high. This was abutted to the south by a slightly larger building, 9.5 metres by 11.5 metres, and to the north by a rounded enclosure, 13 metres in diameter. Field walls were traced radiating out from this cluster of buildings to the north and east.

New aerial photography by the RCAHMW in 2001 and 2011, has revealed even more structures on the island. Nestled between rock outcrops on the eastern side of the island are the remains of walls and rectangular structures, covered by tussocky grass. In the lee of a rock outcrop in the centre of the island is a circular building that is likely to be a roundhouse. The prehistoric pottery and roundhouse shows potential for very early settlement on the island, but equally tantalising is the possibility that the main rectangular structures date from Viking, or Early Christian times. The island seems to have been used as a target for bombing practice by the United States Air Force during the Second World War, leaving bomb craters and fragments.

O Davis, RCAHMW, 30 January 2012

2. The island has been visited for the purposes of archaeological fieldwork and reconnaissance on 13/10/2012 and 14/10/2016 in the company of the RSPB.

T. Driver & L. Barker, RCAHMW, 17/10/2016

3. Grassholm is a study site within the EU-funded CHERISH Project 2017-2021.

New LiDAR survey of Grassholm Island. In February 2017, Bluesky, a company specialising in the acquisition of aerial survey data, was commissioned by the CHERISH Project to collect 0.25m ?leaves-off? (winter conditions with low vegetation and bare trees) LiDAR (airborne laser scanning) data of six Welsh islands at low tide. This included a new survey of Grassholm, allowing its archaeology to be analysed and mapped to a high degree of accuracy. Bluesky flew from its East Midlands base and collected the data using its Teledyne Optech Galaxy LiDAR system on 24 February. The processed data is archived with the Royal Commission (Driver and Hunt 2018).

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