This recent post on the LiDAR and Aerial Archaeology Group of Facebook sparked my interest.
https://www.facebook.com/groups/1508789132952429/?multi_permalinks=2398701490627851
The post describes an apparently circular enclosure revealed in the lidar dtm on the summit of the small wooded island in the centre of Grasmere lake. The author of the post suggested it might be prehistoric, and previously unrecorded.
Enhancing the lidar using a local relief model certainly shows a feature there which I was quick to confirm by that and other enhancements. The author suggested it shows a bank, a ditch, then another bank enclosing a sunken hollow at the top.
I still have doubts (and feel much less certain than 50:50) for reasons I will hopefully explain below. That said, the only way to be sure that this is something or nothing is a ground-truthing visit which perhaps the National Trust archaeologists will manage in the future. My reaction to these things is to explore the topography in as much detail as I can. Visitors to the island are not currently allowed so only remote methods are available.
https://www.facebook.com/groups/1508789132952429/?multi_permalinks=2398701490627851
The post describes an apparently circular enclosure revealed in the lidar dtm on the summit of the small wooded island in the centre of Grasmere lake. The author of the post suggested it might be prehistoric, and previously unrecorded.
Enhancing the lidar using a local relief model certainly shows a feature there which I was quick to confirm by that and other enhancements. The author suggested it shows a bank, a ditch, then another bank enclosing a sunken hollow at the top.
I still have doubts (and feel much less certain than 50:50) for reasons I will hopefully explain below. That said, the only way to be sure that this is something or nothing is a ground-truthing visit which perhaps the National Trust archaeologists will manage in the future. My reaction to these things is to explore the topography in as much detail as I can. Visitors to the island are not currently allowed so only remote methods are available.
I can certainly confirm that there appears to be something shown on lidar images (here I call it a 'doughnut') on the south end of Grasmere Island. It appears to be approximately 50m in diameter, apparently enclosing the summit.
The two images below are my favourite and most used methods of enhancing lidar images for archaeology use.
The two images below are my favourite and most used methods of enhancing lidar images for archaeology use.
Feeling the need to explore the topography of the island (and this feature/anomaly) in more detail I had a look at the 3D visualisations which you can derive from the lidar data in several ways using QGIS. One of my favourite ways is using contours, here overlaid on a multi-directional hillshade.
From the level of the lake (about 61m above sea level), the south end of Grasmere Island rises to a peak (75m ASL) which occupies a broad ridge running NE-SW from a slightly lower second summit on the N part of the island. With a contour interval of 0.5m, the 14m between the water level of the lake and the summit are modelled by 28 contour lines which closely define the shape and altitude of the island and show us where the slopes are steepest.
A second ridge runs steeply down from the main summit SE towards the lake where a building is indicated on the map (locally described as a barn). It is the site of William Wordsworth's poem: Inscription for the House (an Out-house) on the Island at Grasmere.
A second ridge runs steeply down from the main summit SE towards the lake where a building is indicated on the map (locally described as a barn). It is the site of William Wordsworth's poem: Inscription for the House (an Out-house) on the Island at Grasmere.
The question I pose is: does the lidar show ditches and banks encircling a summit (perhaps a small fortified enclosure) or something more natural?
Below using the elevation profile tool showing a section across the southern summit. This suggests a series of steps, about 2m high to the NW of the summit and they certainly don't model a doughnut ring or hollow centre. There are hints of small valley features running along the hillside just below the lower step on the W and NW sides, but these don't continue round the contours on the steep E side.
You can clearly see the two upper steps or ledges in the elevation profile shown below.
Below using the elevation profile tool showing a section across the southern summit. This suggests a series of steps, about 2m high to the NW of the summit and they certainly don't model a doughnut ring or hollow centre. There are hints of small valley features running along the hillside just below the lower step on the W and NW sides, but these don't continue round the contours on the steep E side.
You can clearly see the two upper steps or ledges in the elevation profile shown below.
Elevation Profile on line across the steps and the southern island summit from NW-SE.
Using a contour interval of 0.1m (perhaps pushing the resolution a little too far) I believe we can duplicate all the elements of the island doughnut, apart perhaps for the ditch (if that is what there is) which lies just below the second steep slope NW of the summit ridge. The contours confirm what we see using a slope analysis from lidar. In this representation, the steepest slopes are rendered as black, and flatter areas as white and lighter shades of grey.
The same of course can be demonstrated in a 3D model but is best when you can look yourself rather than on a screen capture from a single angle screenshot as shown below. The interactive model from which this screenshot was taken is freely available on Sketchfab and doesn't need to be downloaded.
3D model of the summit of Grasmere Island from the SE from DTM lidar rendered using CVAT and overlaid with contours at 0.1m. Vertical exaggeration used is x2.
Does any of this prove that the features are natural rather than man-made? Maybe not, but they give me huge doubts that only a ground observation can confirm.
If the feature is of natural origin then how might it be explained?
The contour model suggests to me something called trap topography - a step-like landscape formed by layered volcanic rocks. It occurs when alternating layers of hard, erosion-resistant lava and softer, crumbly volcanic ash weather away at different speeds. The hard lava forms steep cliffs (scarps), while the soft layers form flat shelves (benches), creating a natural staircase. There are certainly hints of this in the neighbouring hills and it is well known in the volcanic hills of the Lakes. I think similar features are visible, for example, in the area around Hardknott Roman Fort.
There is much geological data about this area of the Lake District here:
https://webapps.bgs.ac.uk/memoirs/docs/B01517.html
I can find little information about the topography and geomorphology of Grasmere Island itself, although a volcanic origin does appear likely.
Is what we are seeing in the lidar here a small scale version of trap topography along with some natural drainage? Now I wonder: what are those three small mounds at the N end of the island, NW of the southern summit?
My thanks to Adam Ibbotson for spotting this feature and for an interesting and stimulating post. They happen very rarely on Facebook. The analysis and interpretation here are of course my own.
If the feature is of natural origin then how might it be explained?
The contour model suggests to me something called trap topography - a step-like landscape formed by layered volcanic rocks. It occurs when alternating layers of hard, erosion-resistant lava and softer, crumbly volcanic ash weather away at different speeds. The hard lava forms steep cliffs (scarps), while the soft layers form flat shelves (benches), creating a natural staircase. There are certainly hints of this in the neighbouring hills and it is well known in the volcanic hills of the Lakes. I think similar features are visible, for example, in the area around Hardknott Roman Fort.
There is much geological data about this area of the Lake District here:
https://webapps.bgs.ac.uk/memoirs/docs/B01517.html
I can find little information about the topography and geomorphology of Grasmere Island itself, although a volcanic origin does appear likely.
Is what we are seeing in the lidar here a small scale version of trap topography along with some natural drainage? Now I wonder: what are those three small mounds at the N end of the island, NW of the southern summit?
My thanks to Adam Ibbotson for spotting this feature and for an interesting and stimulating post. They happen very rarely on Facebook. The analysis and interpretation here are of course my own.
© David Martin (cc-by-sa/2.0) geograph.org.uk/p/6277208 Wooded island in Grasmere, taken Tuesday, 17 September, 2019
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