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The Last Glaciation of Caithness

The most recent geological period, the Quaternary, began about 2.6 million years ago and is characterised by a series of major ice ages (glaciations) separated by warm periods called inter-glacials. The severity of glaciation varies over time, oscillations in the average temperature result in relatively short colder periods, of several hundred years duration, called stadials and short warmer periods called inter-stadials. We are currently living in the Holocene epoch, an inter-glacial that started about 10,000 years ago.


One of the global affects of an ice age is to transfer water, in the form of snow, from the oceans to the continental land masses; lowering sea level by as much as 100 m. During the last ice age most of the bed of the North Sea was dry land (see the illustrations on the left).


As the thickness of the ice builds up in areas of high land it forms an ice-cap. The weight of the ice-cap causes ice to flow down-slope away from the cap forming glaciers.


As the ice moves it scrapes and grinds away at the rock picking up boulders, gravels and finer materials like sand and silt as it goes. When the ice melts this mixed material, called till, is deposited forming a variety of different land forms, classified according to how they were formed.


The last glaciation to affect Caithness, the final phase of the Devensian Ice Age, reached its maximum extent between 18,000 and 22,000 years ago. Recent studies suggest that, at that time, an ice sheet covered all of Caithness and extended north, covering Orkney and the Shetland Isles. This phase of the glaciation lasted for several thousands of years. As the climate began to warm the ice sheet started to recede, shrinking back from the north towards an ice cap over Cairngorm and the Grampian Mountains. This glacial recession was not continuous; occasional recurrences of cold conditions temporarily halted melting (1).


Evidence from Yarrows and Thrumster

The three diagrams above are meant to illustrate what Thrumster and the Yarrows area might have looked like as the ice melted; the indicated dates are approximate. The determination of the extent and dating of the ice sheet during its different stages is not a precise science. A variety of differing models have been developed over the years; the works of H. P. Sejrup et. al, C. K. Ballantyne and A. M. Hall have been drawn upon to produce this interpretation.


Figure 1 above represents a period when the retreat of the ice is well advanced; a series of low ridges of small boulders, gravels and finer materials like sand and silt, have formed parallel to the edge of the ice. These deposits, called recessional moraines, form when colder conditions temporarily halt ice recession. Meltwater carrying rock debris has cut a broad channel though the locally soft bedrock; this channel still exists as Swartigill Burn.


In figure 2 the ice sheet has melted leaving behind pockets of ice called cirques the meltwater carrying rock debris with it has cut down into the soft bedrock forming a meltwater channel now carrying the Swartigill Burn. On reaching flatter ground the torrent of meltwater slows down and spreads out to form an outwash plain and shallow meltwater lake. The Loch of Hempriggs is all that remains of a once larger body of water. Thrumster Leans mound would have been a small islet in that lake.


In figure 3 the ice has gone, water flow is greatly reduced and the outwash lake has shrunk to form Loch Hempriggs. Thrumster Leans mound is now surrounded by fine silts and clay minerals that settled out from the Meltwater Lake; the larger material having already dropped out from the water flow at the mouth of the meltwater channel to form a delta.


1) Colin K. Ballantyne and Adrian M. Hall The altitude of the last ice sheet in  Caithness and east Sutherland, northern Scotland Scottish Journal of Geology,  October 2008, v. 44:169-181,

Evidence from Yarrows and Thrumster

The three diagrams to the left are meant to illustrate what Thrumster and the Yarrows area might have looked like as the ice melted; the indicated dates are approximate. The determination of the extent and dating of the ice sheet during its different stages is not a precise science. A variety of differing models have been developed over the years; the works of H. P. Sejrup et. al, C. K. Ballantyne and A. M. Hall have been drawn upon to produce this interpretation.


Figure 1 represents a period when the retreat of the ice is well advanced; a series of low ridges of small boulders, gravels and finer materials like sand and silt, have formed parallel to the edge of the ice. These deposits, called recessional moraines, form when colder conditions temporarily halt ice recession. Meltwater carrying rock debris has cut a broad channel though the locally soft bedrock; this channel still exists as Swartigill Burn.


In figure 2 the ice sheet has melted leaving behind pockets of ice called cirques the meltwater carrying rock debris with it has cut down into the soft bedrock forming a meltwater channel now carrying the Swartigill Burn. On reaching flatter ground the torrent of meltwater slows down and spreads out to form an outwash plain and shallow meltwater lake. The Loch of Hempriggs is all that remains of a once larger body of water. Thrumster Leans mound would have been a small islet in that lake.


In figure 3 the ice has gone, water flow is greatly reduced and the outwash lake has shrunk to form Loch Hempriggs. Thrumster Leans mound is now surrounded by fine silts and clay minerals that settled out from the Meltwater Lake; the larger material having already dropped out from the water flow at the mouth of the meltwater channel to form a delta.


1) Colin K. Ballantyne and Adrian M. Hall The altitude of the last ice sheet in  Caithness and east Sutherland, northern Scotland Scottish Journal of Geology,  October 2008, v. 44:169-181,


Thrumster Leans Mound

Thrumster Leans mound (ND 333 462) was an anomalous isolated mound in a flat outwash plane of a glacial melt water channel; it was not clear whether it was a man-made or natural feature. The answer was provided by a small community excavation conducted in 2009; two previous, but very limited, investigations having failed to provide an explanation.


The excavation demonstrated that the mound was a natural feature, but finds of a flint arrow head and flint working debris indicated that it had probably been used during the prehistoric period probably by hunters spotting game. Finds of 19th


century pottery suggests that it was made use of as a picnic site, probably during wild-fouling activities.


The core of the mound is formed from an unsorted deposit of angular and rounded cobbles in a matrix of gravel, sand, silt and clay known as a glacial till. The cap of this deposit appears to have been partially sorted probably by flowing melt-water or wave action.


The mound was formed before the outwash sediments were laid down, and it is oriented across the general line of the glacier. This suggests that the mound is a small section of a recessional moraine; most of the moraine has been buried by the outwash lake sediments.

Figure 1

Figure 2

Figure 3

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