Sediment Speaks: A Day at the Kaims – Part 2

This post is a continuation of “A Day at the Kaims – Part 1“, and focuses on both the technical and experiential aspects of environmental coring at the Bradford Kaims wetland site.

Archaeological coring: how and why?

Matt Ross teaching BRP students how to describe core samples

The following has been written by Matt Ross, a graduate student currently researching at the Bradford Kaims. He’s also generously been teaching those interested and willing to brave the knee-high muddy bog water how to core and describe sediment samples in the field.

Anne, Matt, and Eva following a day of coring

Throughout the season a team of sediment corers have been braving the wet and mud that is the Bradford Kaims, to record the sediment that lies beneath.

Using a 6m long auger, as demonstrated by Richard Tipping in our earlier blog post, it is possible to extract sediment samples and compile a vertical stratigraphy. Repeating this along a transect, a cross-profile of the landscape can be constructed. By examining changes in sediment type, colour, composition and organic content (i.e. wood fragments, plant stems and calcareous shells) both vertically and laterally throughout the profile we can then piece together the history of environmental change across the site.

Matt and Anne with 5 metres of auger

Such changes occur over time in response to natural and/or anthropogenic forcing: examples include climate change or forest clearance for agriculture. As sediments are composed of material (and organisms) within the catchment, they accumulate vertically and, unless disturbed, will remain in chronological order. The rate of change is also indicated in the sediment profile.

Measuring the depths of various sediment layers–the empty sections mark the areas where we’ve removed sections to feel and analyze

For example, a unit of coarse sand with a sharp upper boundary may mark a rapid flood erosion event, whereas the accumulation of several metres of peat reflects relative climate stability over thousands of years. Sediment coring can therefore provide a rapid assessment of palaeoenvironmental conditions over vast areas. But why does it matter?

Establishing the past climatic conditions and landscape history can provide important context to understand prehistoric settlement at the site. For instance, if we know there was an open body of water during the time of occupation, we can assume that it may have been exploited.

Site exploitation: our burnt mound. Evidence of occupation and long-term site use, the mound is the likely the discarded build-up of shattered stones used for heating water. Click the photo for more information.

Coring at the Bradford Kaims has focused on the low-lying Embleton’s bog, where we have identified two open bodies of water, either side of the promontory, on which Trench 42 is located. These lakes appear to originate in the lateglacial, or the early Holocene (10,000 years ago) and have experienced fluctuating water levels, as indicated by bands of Marl – a lacustrine deposit of calcium carbonate rich mudstone. The lakes were then quickly succeeded by wetland conditions as the climate ameliorated.

Pollen analysis, currently being carried out at Stirling University, will provide detailed reconstruction of vegetation cover at the time of settlement.

The Kaims wetlands regressing into a lake state

Learning to Core 

After first break, Anne (a fellow BRP-er) and I joined Matt for a day of coring.  Knee-high grass lined the foot-flattened path to the most recent transect line where we would be coring. After hearing stories of thigh-high muddy water and the need to bring a complete change of clothes, I was pleasantly surprised to discover that the bog water only rose halfway up my knee-high wellies (though it did get deeper as we moved further south).

Matt and Anne in a slightly damper area

Our first core of the day–Core 4–was marked with a bamboo shaft sticking a few inches above a mud and water filled hole in a circle of flattened grass. It was decided that, to start, I would help Matt with the auger while Anne recorded the sediment changes for each metre core, since she had cored previously. Because we were continuing with a core sample from a previous day’s work, albeit at a greater depth, we had to “clean out” the infill of sediment and water in the hole, by coring to the same depth reached before.

Matt and Anne with 5m of auger

Let me tell you, finding the right hole amongst a half-dozen similar holes filled with muddy water and grass, and trying to fit the auger into it without creating a bigger hole, is no mean feat. I think we gave up on the old hole and made a new one after 3o minutes of struggle. We also never managed to get past the 4 meter depth at that particular spot. While we struggled though, Matt did give me a very nice introduction to the coring process and the geography of the Kaims.

Later in the day, Anne and I switched off and I got an opportunity to record the data. Richard Tipping joined us later in the afternoon and showed us the benefit of years of coring experience–he was almost quicker at setting up and adjusting the auger by himself, than Anne, Matt, and I were together. His equally quick assessment of the core samples was also impressive. 

Using a knife to clear away excess sediment and expose the core

I was surprised to learn how similar the process of coring is to the environmental archaeology and flotation I do here at the castle. It was vindicating that my description of the soil matrices, colours, and inclusions of the BRP environmental samples have been semi-accurate and that I’ve developed a somewhat transferable skill. 

By the end of the work day, we had completed an additional 3 cores to a depth of 6m each. The cores themselves showed a variety of sediment layers including peats, peaty clays, sandy clays, sandy silts, silty sands, and marl. The marl was a creamy to light grey very soft clay. Never having heard of it or encountered it before, I looked it up. Apparently, marl is a “calcium carbonate or lime-rich mud containing large amounts of clays or silts”, and is formed under freshwater conditions.

An example of marl from Urswick Tarn, similar to what we found at the Bradford Kaims

It’s interesting, not only because it was the most markedly different in appearance, but because its presence indicates a period of fairly rapid climate or landscape change–warming or deforestation–and rise in water levels.

Other things we noted in the core samples included the presence and relative quantities–very rare, rare, common, abundant–of wiry and fleshy stems, wood fragments (incl. size), whole shells and shell fragments. I found it interesting that there was significantly more variation in the cores to the north, even at less depth, than there were to the south. For instance, between 3 and 4 metres at Core 4, we encountered maybe 6 or 7 distinct changes in matrices. At the same level in Core 6, the sample was almost entirely a mid-reddish brown silty peat.  Despite this, Matt suggested that all the core results were fairly consistent with what they expected (and hoped) to find.

Last, but not least, for your and my enjoyment:

       

I would love to see a final report of the results, to see how my work was used. I would also like to thank Matt for his wonderful demonstration of coring methods and techniques as well as an illuminating explanation of some of the recent environmental findings. 

— Megan Taylor

“I personally thought it was a great (if not slightly wet) and informative experience, and even though I will probably be sore for the next few days, I can definitely recommend splashing around in the wetlands for a day!”

–Anne Hartog

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