Dig Diary Special – Wednesday, August 3, 2016

Day Twenty-Three

Back in the lab with geoarchaeologist Jo

Jo McKenzie with a near-perfect sample through the floors of Structure Eight – carefully balanced on the cake-slice used to extract it.

Hi everyone. It’s Jo McKenzie here – as seen in some of the blog posts from the first two weeks of this season; ten hectic days of talking, planning, and of course, taking samples!

I’m now back at the desk, working on, among other things, some of the Ness data.

So here’s a view from “off site” at some of the analysis under way.

I’m an Honorary Research Fellow at the University of Bradford, and one of several specialists privileged to be part of the Ness research team.

Sampling in Structure One, 2013, at the north side of the central hearth. Two upper sample tins are in place through the then partially excavated Phase 2 deposits, with the sampling sondage showing the depth of the Phase 1 sequence below.

I’m a geoarchaeologist – I use a range of archaeological science techniques to look at soils and sediments on site.

At the Ness, my focus is soil micromorphology. Small, undisturbed blocks of the “dirt”, representing layer upon layer of activity within the structures, are carefully extracted and a microscope slide made, showing a complete “thin section” through the deposits.

Micromorphology is a powerful technique for understanding areas of intensive use, such as hearths and floor surfaces.

Down the microscope, features too small to be seen during excavation can be identified and interpreted in context, such as fuels, bone, botanics and a whole range of environmental pointers to both human activity and past conditions on site.

Processing these samples isn’t cheap, and I, and the Ness team, are very grateful to the Society of Antiquaries of Scotland  for awarding grant funding for a pilot analysis of some of the samples taken so far.

These are targeted to investigate a crucial part of the Ness archaeology – the hard, stony, yellow and grey clayey “floors” found within every building.

Figure 2: Sampling in Structure One: the sequence up close.

This material provided levelled, hard-wearing surfaces crafted by the builders (and maintainers) of the Ness structures through the time they were in use.

Under the microscope, we can characterise these different clay and stone mixtures, and possibly even see where some of these materials may have come from – a process which will work closely with site geologist Martha’s research into the presence of “foreign stone” on site.

In 2013, sampling in Structure One revealed an amazing sequence of floor surfaces, sandwiched between darker deposits representing human activity.

The close-up of the sample tins in place (Fig 2) shows a Phase Two surface of yellow stony clay (2, 3), possibly showing some resurfacing or maintenance (1), above earlier Phase One surfaces, ranging from fine grey clay with fewer, but larger, iron-rich sandstone fragments (4) to thicker, stonier, yellow deposits (5, 6).

I’ll be comparing these to a number of samples spread through Structure Eight – from the area around the big southern hearth, to this particularly nice sequence through the floors at the north end of the structure, sampled – and featured in the diary – in 2015 (7).

Sampling at the north end of Structure Eight. Located through a surface relating to possible primary flagging at this end of the structure, these deposits are potentially significant for understanding the early development of the building.

Micromorphology allows us to investigate these surfaces in minute detail – not only their mineral character but the size and shape of rock fragments, the microstructure of the clay/silt matrix, and features indicating trampling and compaction above, such as clay movement.

Of the surfaces shown below, 2 (Structure One) and 7 (Structure Eight) appear most similar in composition, with the grey clay of 4 and the far less stony 5 (Structure One) quite different. Did the people of Ness have particular “recipes” for constructing these surfaces, choosing their materials and techniques as carefully as they did for the fine stonework of the walls?

‘Yellow clay’ surfaces in Structures One and Eight. Image width:10mm.

Thanks go here to the very skilled Julie Boreham, of Earthslides, who made these beautiful slides! You can take a look at the slide manufacturing process for the Ness and many other projects at the website.

Individual microscopic features, and their relationship to these surfaces, can help us build a more detailed picture of how the people of the Ness used these buildings through time.

A network of samples through each structure – near hearths, within recesses, at entrances, and along likely “heavy footfall” areas – can provide micro-archaeological evidence for different types or intensities of activity.

Below, we can see that the dark lens between (1) and (2) in Structure One is made up of finely crushed charred material – most likely burnt peat.

Micro-archaeology in Structure One: Phase 2 shows dense lenses of charred materials; with larger fragments of bone, ash and char seen nearer to the hearth. Below these, ‘calcium-iron-phosphate’ features, indicative of degraded bone, are seen in both Phases 1 and 2. Image width: upper – 2 mm; lower – 1 mm.

The char is denser and more compacted to the base of the deposit, trampled into the clear horizontal upper surface of the yellow clay.

Nearer to the hearth, in the second sample through this part of the sequence at (3), similar lenses of crushed char are mixed with much larger fragments of unburnt bone, bigger char fragments, and unburnt peat and ash, in less compacted deposits. We wouldn’t be able to see these differences during excavation – these images are only two millimetres wide.

Under the microscope, features that can’t be seen at all during excavation are also a key source of information.

The lower two images above show two of the Structure One, Phase 1 deposits. Although no bone fragments are seen, both deposits show groups of yellow-orange features with a distinctive “radial” pattern. These are “calcium-iron-phosphate” features, a by-product of bone decomposition.

Features such as these, visible only under the microscope, help us understand archaeological activity within the Ness buildings – centuries of wet Orcadian weather and environmental processes notwithstanding!

And on that note – perhaps sometimes it isn’t such a bad thing to be stuck “in the lab” – although I look forward to coming back up to the Ness in a couple of weeks, rain or shine.

Meanwhile, as you can see, I’ve only scratched the surface when it comes to this exciting set of samples, and so work here will continue, contributing to the story of what happened, and where, within Structures One and Eight.