Developments in understanding the ice age lake sediments from Loch Howburn
by Richard Tipping, 31 July 2010
FIG 1. Coring in the Priest's Well basin, looking up to Elsrickle, in February 2010
In the last report (Sediment Core Analysis 1), I described the work Lucy Verrill and I were doing in trying to understand the record of environmental change contemporary with the hunter-gatherer camp above the valley floor some 14 000 calendar years ago. We had christened this ‘Lake Howburn’, but with due deference to Tam’s sensibilities, we are rightly calling it ‘Loch Howburn’.
Loch Howburn originally extended from the narrow gorge of the Candy Burn where it falls to Candy Mill to the north east to fill the valley floor directly beneath the archaeological site. There are in fact two lost lochs, because a deeper but smaller loch lies at the foot of the long slope from Elsrickle (Figure 1), which we have called the Priest’s Well basin: the Priest’s Well is a chalybeate spring from which ‘magically’ emerges orange-red water. To complicate matters, both lochs were originally joined – see my last report.
The extent of Loch Howburn
Fig 2. The valley floor that was Loch Howburn and the three possible outflows
By probing beneath the soil of the valley floor with special corers we can record where the sediments of these former lochs were. This is why we know Loch Howburn stretched up-valley to Howburn Farm. Lake sediments on the valley floor are found up to around 45m above sea level (asl). Lake level must have been higher. Lake level is controlled by the altitude of the outflow or the height of something like a glacier blocking the lowest outflow. North east of Howburn Farm the watershed, the col, is around 255m asl but colleagues in the Biggar Museum Trust showed that to the west there are two cols at around 250m asl. These therefore controlled lake level. One outflow passed what is now Cocklaw Farm; the other was in the gap in the hills between Biggarshiels Mains and Greenwood. We have now probed the sediments in both outflows. Lake sediments at the Cocklaw farm outflow have been found up to around 250m asl, showing the lake spread this far west. But no lake sediments have as yet been found in the Biggarshiels gap. They may be absent because the glacier blocked this outflow as well as in the adjacent Candy Burn.
FIG 3. Banded grey sediment in a 1.0m long Russian corer: February 2010
What do the lake sediments look like?
Sediment cores are taken with hand-operated but heavy equipment: it is a team effort. We can distinguish ice age lake sediments from younger ones by several characteristics. They are pale grey rather than mid-brown because there was effectively no biological activity that might produce peat-coloured organic matter. They are high in a mineral called silica from rocks elsewhere in southern Scotland, not local to the lake catchment. They are also often delicately banded because there were marked seasonal contrasts in the supply of sediment from glaciers and hillslopes (Figure 2).
FIG 4. Millimetre and narrower laminae in the oldest lake sediment to have formed in the Priest's Well basin
In the laboratory these bands can be seen to have even more detail. Figure 3, for example, shows the thin laminae that formed when the glacier delivered sediment to the lake. The pale brown laminae are of silt-sized particles delivered in the summer months when meltwater streams were bigger; the pale grey laminae are clay-dominated and represent the slow settling-out of mud in winter months when meltwater was frozen. So each pair of grey-brown laminae represents a year in the ‘life’ of this lake. Looking closer, some laminae can be seen to be occasionally thicker: these probably record the variability of weather from year to year. And some laminae are made of sand-rich mud, perhaps pushed into the lake in particularly warm summers when meltwater release was greater. In autumn 2010 an undergraduate at Stirling University, Matt Bradley, will begin the task of measuring the numbers, thicknesses and particle sizes of these laminae to reveal longer-term climatic patterns at the end of the last major ice age, from high-resolution photos, from microscope slides where even more detail is revealed and from X-rays (Figure 4) that might tell us about hidden details.
FIG 5. X-raying sediment cores at the National Museum of Scotland laboratory
The lake sediment in the Priest’s Well basin holds much more information, though. In the warm period we live in, called the Holocene Epoch, the sediment in the lake was no longer supplied by glaciers. Soils on the volcanic rock around the basin supplied calcium carbonate to the lake in a process called leaching. This is a form of chalk, and it’s a creamy-white colour. Soils also began to accumulate organic matter from decaying plants, and in the lake itself plants and animals grew and died to form dark brown layers. All these processes have led to lake sediment that is also laminated, probably also controlled by the seasons (Figure 5). We have a lot of work to do over the next few months in fully understanding these sediments.
So how old is Loch Howburn?
The evidence for Loch Howburn being formed by a glacier is impressive: we have a lake chemistry dominated by silica sediment from the region; we have great thicknesses of sediment from a source rich in mud; we have laminated sediment that looks at present to be glacier-fed. Loch Howburn also seems to have drained quickly, and the lake vanished with this emptying. It was at this time that the Priest’s Well basin emerged as the only open water. But when was the glacier in the Candy Burn, and the Biggarshiels gap? Was Loch Howburn open water when hunter-gatherers watched for reindeer and wild horse? We have always assumed not because deglaciation is thought to have been complete long before 14 000 calendar years ago, maybe 16 to 17 000 calendar years ago.
But we have submitted samples from sediment that formed in Loch Howburn towards the end of its ‘life’. The material dated is unusual – rather than date the mud itself, which is fraught with difficulties, we concentrated tiny moss stems preserved in the mud. Figure 6 shows these scattered throughout lake mud but they also form distinct mats. These stems still have tiny leaves attached, so they are thought to have been preserved where they grew, with mats of stems perhaps representing growth in particularly warm summers. Two samples tell us that Loch Howburn was still filling the valley floor between 12 and 13 000 calendar years ago, after the hunter-gatherer camp. This is a major surprise, one that would baffle scientists researching deglaciation, and our first instinct is to think the radiocarbon dates don’t make sense. However, it is not yet clear why this should be given the careful selection of moss stems.
To sort this puzzle out, scientists from Royal Holloway College in London will look for traces of ash-falls from volcanic eruptions in Iceland and the continent, and we’ll ask for more radiocarbon dates. Its still too early to run away with the idea that Loch Howburn was actually as young as it appears.