Labratory analysis of the sediments from the Priest’s Well Basin, Elsrickle
by Richard Tipping, 2 March 2010
A lot of work since November 2009 has shown that the floor of the valley below the Howburn archaeological site contains the sediments of not one, but two lakes. A large pro-glacial lake, so called because it formed in front (pro-) of a glacier, filled the valley as far as the archeological site at one time during deglaciation. This has been christened ‘Lake Howburn’. A second, much smaller lake survived the sudden drainage of ‘Lake Howburn’, as the ice-dam collapsed. This is preserved at the foot of the slope below Elsrickle, and has been called the Priest’s Well Basin. It was the sediment in the Priest’s Well that we sampled on a snowy day in late February 2010.
Since then, Lucy Verrill has been hard at work in the laboratories of the School of Biological & Environmental Sciences. A detailed description of the cores was made. Broadly, there were five different units. In stratigraphic order they are:
- 4.50-4.94m: peat
- 4.94-7.00m: laminated (delicately banded) organic-rich carbonate marl
- 7.00-8.49m: weakly laminated to structureless clay with silt
- 8.49-8.60m: slightly organic silt
- 8.60-8.74m: laminated silts and clays.
In detail, the sediment units are highly complex, and the full description takes up 4 pages of single-spaced A4 paper. Figure 1 is a crude scan of the sediment stratigraphy, but it gives some impression of this complexity.
The first things we needed to define were the physical characteristics of the sediment. This is simpler than it sounds. Lucy recorded the moisture content of sediments by noting the weight lost after oven-drying; the carbon content by noting the weight of the same sediment after burning it at 550ºC; and carbonate content by burning what was left at an even higher temperature, 950 ºC. By measuring carbon and carbonate contents, what remains after furnacing is the silica content. Lucy did these analyses for contiguous 1.0cm samples between 8.74 and 4.54m depth. The results are also shown on Figure 1. The broad divisions of the sediments outlined above are clear. High carbon and carbonate contents indicate warm temperatures. But Figure 1 also indicates the many short-lived fluctuations in sediment composition, particularly above 7.00m depth. Changes in sediment composition are usually caused by significant environmental changes, whether they be in the climate, lake level, soil erosion or the development of ecosystems, either in the catchment or in the lake. Given this, it seems clear that the sediment accumulating in the Priest’s Well Basin were very sensitive to these changes, and is a most exciting record.
To find out how old the sediments are, we used pollen analysis, because the high carbonate content of the lake sediments above 7.00m depth would distort any attempt at radiocarbon dating.
From these, it looks very much as if the organic-rich sediments above 7.00m depth are dated to the earliest part of the present interglacial, the Holocene Epoch. We might expect the change at 7.00m depth to date to around 11700 years ago. In fact, these sediments accumulated very rapidly, because even at 4.50m depth we have not seen much pollen of hazel (Corylus), which migrated to the region around 10 500 years ago (Figure 2).
The boundary at 7.00m depth marks the end of the last glacial phase in Scotland, called the Loch Lomond Stadial. Figure 1 suggests the change from glacial to interglacial climate at 7.00m depth happened very rapidly. In the Greenland ice sheet, this event is known to have happened over a few decades at most.
We could not extract pollen from the clay sediment below 7.00m depth, probably because sediment accumulated so fast that the concentrations of pollen became very diluted, but we think the inorganic clay between 7.00m and 8.49m depth must date to the c. 1200 years of the Loch Lomond Stadial. There would be little enough pollen around the Priest’s Well then, in an arctic climate too harsh for any plants save sedges, grasses and herbs. In this phase, small glaciers developed high in the Moffat Hills, above Carrifrans and Grey Mare’s Tail, but not around Elsrickle, which currently makes the laminations often recorded in this in the clay a bit of a puzzle, because these laminations are often associated with pro-glacial lakes.
At the bottom of our core, between 8.49 and 8.60m depth, we found the pollen of trees like birch (Betula) and tall shrubs like juniper (Juniperus), together with grasses and heathers like crowberry, in a sediment with organic carbon contents of around 7%. This should make this sediment unit belong to a very warm phase before the Loch Lomond Stadial, the Windermere Interstadial, which lasted for just over 1000 years: we have only just realised that we need to sample sediment deeper than 8.74m now, to find out what is beneath this unit. But this warm period was the one when our hunter-gatherers were living at the other end of the valley. At the moment, it seems safest to think that the only open water in the valley at this time was at the ‘wrong’ end of the valley, at Elsrickle. Maybe other camps still lie to be discovered around the Priest’s Well?
But we still have lots of puzzles. Most sediments below 7.00m depth are not only entirely inorganic, but also have no carbonate content. This is a puzzle because carbonate probably came into the basin from the weathered rock and soil around the Priest’s Well as soon as temperatures rose at the start of the Holocene. But if the valley was ice-free in the Windermere Interstadial, carbonate should also appear between 8.49 and 8.74m depth – but it doesn’t. Fortunately, because these sediments are free of carbonate, organic matter should yield good radiocarbon dates. There is precious little organic matter to date, it appears from Figure 1, but one of the most exciting discoveries in the coring programme since November 2009 has been that very delicate, very fragile mats of moss stems, almost certainly in situ and flourishing in summer-warmed shallow water, can be found in these inorganic sediments, even the laminated sediments formed in pro-glacial ‘Lake Howburn’. The next development in our story will come from finding out the ages of these moss stems.
