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Thanks to funding from the Castle Studies Trust, Historic Environment Scotland are working with researchers Dr Richard Tipping and Dr Eileen Tisdall to understand the chronology and geography of extreme weather events in the high medieval period, and the effects they wrought on archaeological features that led to the abandonment of the old castle built in c.1229 in favour of the new built 200m away in c.1277. Here Richard Tipping gives an update on the fieldwork which too place last Saturday (2 October).

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One of the great puzzles of old Caerlaverock is the so-called harbour (Image 1). It is south of the old castle, two metres lower, and just north of where we think the early medieval coastline was, so it’s difficult to think what else it could have been. But it’s never been independently dated, there are rumours that it was Roman, and there are knotty interpretative problems. One is that the floor of the harbour is at an altitude that would make it tricky to get more than rowing boats in and out in the Middle Ages; this is also a problem if the harbour was older. A second is that there is a fourth side at the southern end, which greatly restricts access. As Richard Tipping explains below:

The harbour has three metres of sand above bedrock. We don’t know how old the sand is. One idea is that it’s all very old – 8000-6000 years old. This would mean that the harbour was never deeper than it is now. Another idea is that the sand is of two periods, with an older bed covered by medieval sand: the same stuff that filled the moat of the Old Castle. And a third is that all three metres of sand are medieval in age. This would imply that the builders created a five metre deep harbour, which was then filled with sand as storm surges drove sediment onshore.

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Dating of the sand is the answer. To this end, a team of eight (Richard Tipping and Eileen Tisdall from Stirling; Tim Kinnaird, Aayush Srivastava, Richard Bates and Laura Bates from St Andrews, and Morvern French and Steve Farrar from Historic Environment Scotland) assembled under gloomy skies and a more intimidating forecast to sample the sediment.

Here Richard Bates explains what they are going to do:

Richard Bates had a vibro-corer, a machine that went through three metres of sand as if it was butter: astonishingly impressive:

The three cores came out and were immediately wrapped in black plastic bags because sunlight, even that under grey clouds, cannot be allowed to shine on the samples. The reason for this bizarre behaviour is in how the sediment will be dated

Tim and Aayush are specialists in optically stimulated luminescence (OSL) and infra-red stimulated luminescence (IRSL) dating. This is a kind-of accumulation clock. Over time, quartz and feldspar sand grains buried by sediment accumulate energy released from surrounding sediments at a constant rate. This energy is released if sunlight hits the sand, or under controlled laboratory conditions, when light at optical or infra-red wavelengths hits the sand. If the energy released can be measured, which is Tim’s and Aayush’s job, we can establish the age of the sand as Tim explains here:

The Castle Studies Trust grant will fund three OSL dates from the base of the sand, the middle, and towards the top. As soon as we learn the results, you will.

And the rain stayed away.

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All images and videos courtesy and copyright of either HES and/or Morvern French of HES

As Drs Richard Tipping and Eileen Tisdall along with Dr Tim Kinnaird of St Andrews return to Caerlaverock to carry out their final piece of field work this Saturday (2 October), Richard Tipping discusses what has happened so far.

Background

Earlier blogs have described the fieldwork, over the summer, at the Old Castle at Caerlaverock in south west Scotland. We have been testing the idea that several very large storm surges impacted the castle, persuading the occupants to re-build, higher up and further inland. That fieldwork involved recording sediments in the moat (Figure 1) and surrounding ditches, confirming that these sediment traps are full of silt derived, we think, from storm surges pushing sediment from the coast, through the harbour and into the moat. We’re checking the origin of the silt from diatom analyses, which can define water salinity. But we hadn’t found evidence that these storm surges were destructive, impacting archaeological features. Now we think we have.  

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Setting

The ‘park pale’ is the name given to a ditched-&-banked enclosure that extends west from the harbour at the old castle for several hundred metres (Figure 1). It was constructed along a small cliff that marked the medieval coastline, separating mid-Holocene estuarine sediment, called ‘carse’ to its north and a series of very broad, parallel east-west trending ridges and basins constructed by medieval storm surges to its south (Tipping and Adams 2007).

In Figure 1, a LiDAR image, Ridges 9 and 10, and Ridges 15 and 18 are seen. Between them, lagoon basins were trapped: Basins 8, 3 and 4 here. Some basins will be radiocarbon dated because peat formed when they were isolated from wave action. 

In detail (Figure 2) the ‘park pale’ is complicated. There is an inner bank to the east, with a crest around a metre higher than the ground to its south. This is lost at a 25m wide gap and cannot be traced further west. Instead, a second, lower outer bank continues north west to the New Castle Burn (Figure 1). The outer bank has, in places, an outer and inner ditch. Another, 4m wide gap may be what Brann (2004) thought was an entrance.

Tr 3 (ii) is the line of a 50m long sediment-stratigraphic transect of 24 hand-sunk boreholes from the ‘carse’ in the north, which the inner bank rests on, south across Basin 3 to the canalised Old Castle Burn. The transect was designed to test the idea that the 25m gap in the ‘pale’ is an erosional feature from storm surge impacts.

Stratigraphy

What Tr (ii) shows is that the low ground of Basin 3 is floored by bedrock less than a metre down. This is covered by well-sorted sand and then by poorly sorted coarse to very coarse sand and grit with common pebbles.This fills Basin 3, thickening shoreward. This is interpreted as a storm surge deposit, deposited in a high-energy marine environment. Boreholes on the inner bank, a metre higher than Basin 3, and 25m inland, also recorded thick gravelly sand, impenetrable at 90cm depth on the inner bank. This thins north but is still found 20m inland from the medieval coastline.

Narrative

There were at least two storm surge ridges formed before the events recorded in Basin 3. Basin 8 (Figure 1) was formed by the second storm surge, as yet undated. This created Ridges 9 and 10. These broad gravel ridges in turn protected the western ‘park pale’ from subsequent marine erosion (Figure 2). What is now the outer bank, with its associated ditches, probably represents the original boundary of the ‘pale’. The ‘pale’ pre-dates the storm surge event described here. It is shown by the sediment stratigraphy to be, broadly, medieval in age and, broadly, contemporary with the old castle.

Eastward, Ridges 9 and 10 merge with the ‘carse’ of the medieval coastline, leaving this part of the coast vulnerable to later storm surges. At this point, the outer bank of the ‘pale’ is lost. In its place are storm surge sediments. The ‘pale’ was eroded by the storm surge. This storm surge pushed at least 90cm of gravelly sand north onto the surface of the ‘carse’. The cliff may have been formed after this storm surge, by later erosional wave action. Gravelly sand was pushed or thrown 20m beyond the cliff. Waves also scoured the easily eroded ‘carse’, lowering the surface by around 0.5m, up to 100m inland.

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References

Tipping, R. and Adams, J. 2007. Structure, composition and significance of medieval storm beach ridges at Caerlaverock, Dumfries & Galloway. Scottish Journal of Geology 43, 115-123.

Project lead, Dr Richard Tipping, looks back at the second week of fieldwork he and Dr Eileen Tisdall are undertaking to try establish whether climate change did impact the re-positioning of Caerlaverock Castle.

Home again – after one of the most torrid week’s fieldwork I can recall: whilst Northern Ireland had the extreme weather warnings just to the east, Dumfries had day-time temperatures exceeding 26⁰C all week. And you’ll recall that I was inside a wood, where breezes never penetrate.

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At Caerlaverock on the Solway coast, we are trying to work out the natural and human impact of huge medieval storm surges on the 13th century old castle. Last time I was looking at how the moat, and possibly the castle itself, was inundated with seawater and mud driven shoreward by storms. This week I focused on the coastline itself, away from the castle. Before the storms hit, the old castle was on the coast: it had a harbour. You can still walk along the degraded cliff to the west of the castle and imagine the seascape. After the storms, the coast lay some 250 metres south of the castle. A series of large gravel ridges, tens of metres wide, grew southward, beautifully revealed in LiDAR images that peel away the nearly impenetrable woodland.

Each of these ridges created shallow basins protected from tides. The harbour became pointless as a result. But all we knew from earlier work was that the youngest basin was formed around AD1400. How old were the earlier ones? Could we find sediment dateable by radiocarbon?

The fieldwork, I knew, would be hard. I’d looked for dateable sediment before, simply by coring the sediments in the basins, and found only one site; and now there was the woodland to contend with. All you can do is remain patient and optimistic: unglamorous, muddy and unspectacular work. The woodland meant that some basins couldn’t now be found, others where I couldn’t core systematically, and still others where systematic coring failed to find anything to excite. But I think we can date three of the eight basins and, usefully, these include the earliest basin and the basin that blocked the harbour. Stay tuned.

But it was the ditches around the old castle that were the highlight of a difficult week. Sense was made of the moat. The builders formed one of the four moat sides from an existing stream channel: the moat here is a lot deeper because of this. But then the castle mound was not made parallel with this moat side. It does seem to be a very slap-dash construction. The attempted buttressing of the castle by stonework in the moat is shown here.

And we can now show that the mud of the storm surges penetrated beyond the castle, not by a great distance but enough that the old castle would have been surrounded in these extreme events, laid siege by nature while the younger new castle was laid siege by Edward I and the English in AD1300.

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Feature image courtesy of Historic Environment Scotland