Author: Rachel and Chris
This week we are going to introduce some of the mounds that we surveyed in September and explain why we were interested in them and how that shaped the kind of work we did. Archallagan is a plantation in the south of the island – it is a very popular spot with walkers, dog owners, bicycle riders and families. The 1860s OS map of the Isle of Man shows 14 cairns (check out last week’s blog for an explanation of what a cairn is) marked in the area of the plantation. Half of the cairns fall inside the area now covered by the plantation and half fall outside in some agricultural land to the east of the access road that runs along the west edge of the plantation.
There is some dispute about these cairns – nobody is sure exactly how many there are, and whether they are all burial mounds. Some of the cairns in this area were ‘opened’ in 1872 by the land owner who found a number of inverted urns associated with bones (check out last week’s blog for more about inverted urns!). The Reverend A.E. Clarke offers us an account (with an illustration) of the opening of the mounds some years later 1912. P.M.C. Kermode counted eleven mounds visible in the plantation in 1899 and seven identifiable to the east of the road. Ordnance Survey mappers in 1957 identified at least eight within the plantation and noted that they were covered by “heather, tree stumps and newly planted trees”.
We are interested in how many there are and whether or not they are all burial mounds. Some of the mounds could potentially be shielings (see our first blog post for more details on that). The linear mound in the ploughed field to the east of the plantation could potentially be a large damaged cairn, two abutting cairns, or the remains of an old field boundary. From a heritage preservation perspective it would help to know whether they are burial mounds or shielings and, secondarily what kind of condition they are in. The mounds in the plantation are protected as they are not currently subject to ploughing or development. By contrast tree roots can cause a lot of damage to archaeological sites as can the process of clearing harvested tree stumps for further planting. Research like this can help take stock of the current condition and nature of the archaeological sites so all land users can act appropriately.
From walking around the plantation and down the road by the side of the plantation we were able to identify several of these mounds on the ground. We also consulted the aerial photographs to see if we could identify the mounds discussed by various antiquarians and archaeologists. Aerial photographs can be great for identifying sites but they are very limited in areas with a lot of vegetation growth and some archaeological features are only visible under special conditions (e.g. extreme damp or drought).
To overcome this we have also been using LiDAR data. LiDAR stands for Light Detection and Ranging and effectively involves scanning the land surface with an aerial laser source. This technique is able to detect very small changes in the land surface, measuring the amount that the ground slopes at any point. On the LiDAR image below the darker areas are those with the most slope, and the lighter areas have least. While LiDAR can penetrate thin vegetation, such as scrub, gorse and some trees, it struggles to ‘see through’ very dense forestry, as at Archallagan Plantation. The odd patterning on the image are where the LiDAR ‘extrapolates’ slopes between the points where the laser is unable to penetrate any further.
As we can see in these images, LiDAR is excellent at detecting the mounds in the agricultural fields around the plantation. These are very hard to see on the ground and barely show up on the aerial photography. The LiDAR imagery quite clearly identifies some mounds in the fields that are pretty much in line with what we can see on the 1860s map – so the remains of these mounds are still there! The LiDAR for the Isle of Man is also detailed enough that a mound would appear differently than a round walled building such as a shieling if those walls were well preserved. However, if shieling walls collapse and the site is ploughed it is possible it would look more like a round mound.
The next step is geophysical survey, which provides an assessment not of the land surface (the topography) but of what is immediately underneath the topsoil. We carried out two kinds of geophysical surveys in the field to the east of the road in September. Firstly, a magnetometry survey and secondly, a ground penetrating radar survey. Magnetometry is a geophysical technique that measures differences in the magnetism of the soil. The magnetism of soil varies and changes as a result of human activity, particularly burning. We can use the changes in magnetism to detect archaeological features. Ground Penetrating Radar (referred to as GPR) detects differences in the density of the material under the topsoil at different depths. GPR can sometimes allow us to detect different layers of soil and stone under the ground. We will explore both of these techniques in more detail in future blog posts.
Geophysical surveys can be hard to carry out in plantations as the technique involves walking a specific grid pattern while the machines collect data. In a plantation with trees, roots and other vegetation dotted all over the place walking an even and regular grid becomes very difficult. Walking such a grid in a field is straight forward. So, with the very kind permission of the landowners we carried out a geophysical survey in one of the fields, targeting some of the known locations of the mounds.
At present our geophysicists Kate and Alex (check them out on our Team page) are processing the data that we collected in September. We hope that using the two forms of survey together we might be able to decide whether these features are round mounds or not. We will blog about what we have learned from the survey in the coming weeks. So watch this space! And if you are lucky enough to live on the Isle of Man enjoy a round of ‘spot the mound’ during your next walk/bicycle ride/horse trek in Archallagan [apologies in advance to any children forced to spend their weekend spotting mounds as a result of this post!]!