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Stories of Loss: The promontory fort & climate change
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Dinas Dinlle
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Airborne Laser Scanning (ALS)
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Airborne Laser Scanning
Airborne laser scanning (ALS) (also known as LiDAR) is an active remote sensing technique that is used to create accurate and precise 3D models and visualisations of landscapes. As with most innovative technologies, ALS has its origins in the military where it was first developed to carry out underwater scanning to identify submarines. In the UK it was widely adopted during the 90s where it was used initially by the Environment Agency to create terrain maps to assess flood risk. However, it was not until the turn of the millennium that the potential of ALS for archaeological survey began to be recognised.
In practice, a ALS survey consists of the transmission of an active laser beam from a fixed-wing aircraft towards the ground. The reflection of the beams transmitted back to the aircraft are then measured to give distance values which are used to create a 3D Digital Elevation Model (DEM) of the landscape below. The intensity of the returning beam can also give an indication of the type of material that the beam was reflected from. This coupled with the height data can be used to identify and remove vegetation from a DEM, which in turn offers a view of hidden features and landscapes that may be obscured by vegetation. Global Navigation Satellite Systems (GNSS), or Global Positioning Systems (GPS) as it is more commonly known is also used during a survey to ensure that the 3D model is geo-located on the earth.
Using ALS on the CHERISH Project
ALS has been used by CHERISH as a way of investigating and recording the archaeology of some of the more remote areas around our coastlines. Beyond the project it will also be extremely helpful for monitoring of environmental changes such as coastal erosion and sea-level rise.
The first CHERISH ALS surveys were flown in 2017 for six islands in Wales (Puffin Island, Skerries, Bardsey Island, St Tudwal’s, Ramsey Island and Grassholm Island). The 0.25cm resolution data has been used to accurately map archaeological features to produce new maps of all upstanding archaeology on each island. In Ireland, an ALS survey has been commissioned for the area surrounding Dublin bay. ALS data has also been used alongside aerial photography from which several cropmarks have been discovered across the islands.
Publicly accessible LiDAR data
LiDAR data is becoming increasingly available and can often be viewed and downloaded free of charge. National LiDAR datasets can be downloaded from the following sites:
- Wales: Lle Geoportal
- Ireland: Open Topographic Data Viewer
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13. Castlemartin to Stackpole Warren
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Introduction
The coastline between Castlemartin to Stackpole Warren area is well known for its impressive high limestone cliffs upon which many prehistoric promontory forts were built. Many of these sites have suffered in the past from coastal erosion, with some sites now almost entirely lost to the sea. The sand dunes at Stackpole Warren are also extremely interesting and contain valuable information relating to how weather patterns have changed in the past and how they shaped the coastal landscape we see today. The sand dunes have also covered and preserved important Bronze and Iron Age archaeological sites.
Castlemartin Promontory Forts
This part of Pembrokeshire is blessed with an abundance of promontory forts which range from single ditch enclosures to monumental structures. Many of these sites have benefitted from being located within the boundaries of the Castlemartin practice firing area where they are now effectively ‘defended’ by the army. However, whilst they are well protected from human interference, they are not so protected from the forces of nature which have had noticeable impacts upon the archaeology.
Within this area CHERISH are monitoring and researching five promontory forts: Linney Head, Flimston Bay, Crocksydam, Crickmail Down, and Buckspool Down. Each is unique in the nature of the landscape it occupies and the ways in which they were constructed. Through using a combination of UAV and analytical earthwork surveys CHERISH is not only monitoring recent erosion but also researching some of the many outstanding archaeological questions previously not addressed.
There is also the site of Castlemartin Corse which is a valley wetland that has been strongly influenced by coastal weather systems and a large, mobile dune complex which is gradually moving inland. CHERISH has cored the wetland and recovered a core of sediment that dates back about 5000 years. Through using this technique, it is hoped that patterns of storm frequency and intensity in the bio and geochemical record can be identified to increase the understanding of present and future climatic change.
Stackpole Warren
In the east of the project area is the extensive system of sand dunes known as Stackpole Warren. Unusually, the dunes here are situated on top of high limestone cliffs which rise to around 20m above sea-level. The Warren is rich in archaeological evidence of occupation from the Mesolithic to the Roman Period. Excavations have indicated that the sand was mobilised in at least two distinct phases, the first in the Late Bronze age and then intermittent sand movement during the Iron Age to Romano British Period.
CHERISH has recovered cores from three areas on Stackpole Warren, which will be dated using Optical Stimulated Luminescence (OSL) to improve our understanding of when these sand movement events took place. It may also be possible to determine over what sort of timescales sand deposition took place. Through this work CHERISH hopes to stimulate a wider appreciation of the lives of the early inhabitants in this area, and the climatic challenges they encountered.
Why are we working here?
Coastal erosion has had a noticeable impact upon the archaeology in this region where large proportions of sites haven fallen into the sea. CHERISH is working in this area to provide baseline data for sites that have seen little attention from archaeologists in the past. Archaeological and paleoenvironmental research will also draw wider conclusions about regional patterns of climatic variability in the past as well as identify the main processes causing the erosion.
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