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,