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Wednesday 21 December 2011

Recent Work On Skomer Island, Pembrokeshire

Skomer Island, located off the south-western coast of Pembrokeshire, is a National Nature Reserve, a Marine Nature Reserve and a Scheduled Ancient Monument, famed for its bird life and puffins, and its relict prehistoric agricultural landscape is among the best preserved anywhere in the British Isles.

Two major archaeological surveys have examined the island in the twentieth century, in the 1940s and 1980s. The first was by Professor W.F. Grimes who produced an archaeological map, based on a transcription from a set of Ordnance Survey aerial photographs, checked and augmented by detailed survey on the ground. Forty years later Professor J.G. Evans complemented and built upon this earlier work by recording in detail small enclosures and habitation sites from ground survey. He also expanded the archaeological map from transcription of vertical aerial photographs supplied by Cambridge University and oblique aerial photographs taken by Terry James, then of the Dyfed Archaeological Trust. Evans argued for a simple prehistory of Skomer since in his opinion the fields and farms showed little complexity and may have been built, settled and entirely abandoned in no more than a century.

However, aerial survey in March 2008 by Toby Driver of the Royal Commission in low light, and with compacted vegetation after a heavy frost, yielded a new collection of images that revealed discrepancies with the mapped detail. Mapped field shapes were generalised, altered, wrongly depicted or had details ‘smoothed out’ when compared to the new aerial images. In other places the excellent conditions in which the new photographs had been taken picked out extremely denuded lynchets and boundaries showing clear phasing among overlapping field systems. This suggested a more complex history of the island and provided the stimulus for new fieldwork and survey by the Royal Commission to examine the archaeology and historic land-use of Skomer.
Field systems in the south of Skomer show up spectacularly in low light during March 2008.
Crown Copyright: RCAHMW: AP_2008_0306

New survey and fieldwork

Skomer has no large grazing animals and so the vegetation is dominated by coarse tussocky grass. A field visit in August 2010 revealed that this formed a kind of fescue mattress effectively obscuring much of the archaeology and making it difficult to map features from aerial photographs. Therefore, in February 2011 the Royal Commission commissioned a LiDAR survey of the island in order to identify the full extent of the surviving earthworks. Light Detection and Ranging (LiDAR) is a method of remote sensing that obtains three-dimensional data points by using an airborne laser, mounted on an aircraft.

The LiDAR sensor is mounted below an aircraft where it emits short infrared laser pulses towards the earth’s surface, fan-shaped across the flight path. Each pulse will result in multiple echoes or ‘returns’. The first return will usually be received from the tops of trees and vegetation, but as the laser penetrates the canopy, further returns are received from branches and understorey. Typically, the last return is received from the ground surface. As the aeroplane moves forward the position of each return, or point, can be calculated using a satellite navigation system, while the pitch, roll and yaw of the aircraft is recorded by an inertial measurement unit to increase accuracy. Each point therefore has a set of x, y, and z coordinates to reflect its position and elevation. These points can then be processed to create a highly detailed terrain model of the ground surface.

Digital surface model generated LiDAR image of Skomer (top) and interpretation (bottom). 
Copyright Reserved, Environment Agency Geomatics Group; hillshade DSM view generated by RCAHMW. Map based upon Ordnance Survey mapping: Crown Copyright and database right 2009. All rights reserved. Ordnance Survey Licence number 100022206

The results of the survey have been staggering, revealing hitherto unknown complexity to the field systems and clear phasing of boundaries, particularly within the interior of the island . A first season of fieldwork in April 2011 began the process of ground-truthing these new discoveries, but also undertaking a more nuanced ground survey of the surviving remains. A typological study for the settlement in the north of the island confirmed that Skomer’s field systems are complex and long-lived. The variety of boundary types is notable and there are many subdivisions. The low banks of stone and earth have many different forms and characteristics, and individual boundaries do not have a consistent character throughout their length. What appears to be a single coherent boundary from the air may on the ground comprise a composite of varying build and type, and suggests a succession of phases of expansion, contraction and abandonment.

Skomer has only one prominent megalithic monument, the Harold Stone, which is a 1.7 m high monolith of local stone sited towards the eastern edge of a block of fields, looking out over the maritime approaches to the island. Aside from the Harold Stone, a variety of less obvious monuments of potential Early Bronze Age date are recorded on Skomer. These include the barrow near the Wick along with a considerable number of cairns. In re-visiting some of these cairn cemeteries, and during the re-survey of other settlement areas, the recent survey showed new evidence for megalithic structures of Neolithic or Early Bronze Age date. Among these are at least three unrecorded standing stone pairs, clearly identifiable as unusual paired stones in isolation, or incorporated in later boundaries with stone in-filling, precluding their use as gateposts. A further possible stone pair is closely associated with an earthfast natural slab, prominent on the skyline on the north-eastern part of Skomer Island (SM 7266 0998). There is the strong likelihood that this represents a megalithic or sub-megalithic monument and it lies just to the north of a cluster of cairns.

Probable megalithic site on northeast Skomer, with two orthostats found in close association with a large, earthfast rectangular slab of natural origin, seen from the south. The twin orthostats break the skyline when seen from all southern approaches and look out across St Brides Bay to the peaks of St Davids Head in the north .
Crown Copyright: RCAHMW: DS2011_448_001

Future work

Further ground survey is planned for 2012 to complete the typological study of the island’s field systems and investigate the megalithic site in more detail. Skomer still has many archaeological secrets to be revealed!

Further reading:

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