The last glaciers in Western Perthshire

The principal aim of the work described in this thesis was to discover the extent of the last stage of glaciation in the western part of Highland Perthshire.

After preparatory stereoscopic study cf aerial photographs, an area of 450 sq. km in Glen Lyon and southern Rannoch Moor was mapped at the 1:10,560 scale and subsequently another 2250 sq. km were mapped at the 1:63,360 scale. Features of special interest, such as outwash terraces and the lake terraces or 'Parallel Roads' of Loch Tulla, were mapped in greater detail and instrumentally levelled.

Particular attention was paid to the character and extent of fresh hummocky drift, described by earlier workers as 'hummocky moraine' and thought to belong to a distinct period of glaciation. From examination of thepatt.ern of this hummocky drift and of the constituent sediments it was concluded that the features are basically kames that are often thickly covered with ablation moraine.

Eight major valley systems were studied. They include Glen Garry - Glen Errochty, the Loch Rannoch - Loch Tummel valley, Glen Lyon, the Loch Tay valley, Glen Almond, Glen Artney, Loch Voil - Loch Earn and the Trossachs - Teith valleys, in addition to part of Rannoch Moor. The evidence of fresh glacial deposits in these valleys indicates that a system of glaciers existed in the area during the last stage of glaciation.

By discussing the valley systems in turn and regarding each as a case that can be justified independently of the others, it is concluded for three reasons that the last glaciers in each area existed during the same lateglacial period. Firstly, there is one clear down -valley limit to the fresh hummocky drift in each valley system. Secondly, with the exception of the isolated Glen Almond area, the spread of moundy drift continues from one valley system to the next via interconnecting valleys. Thirdly, the pattern of glaciers inferred from the evidence appears to be inherently probable.

There are five principal reasons for concluding that these glaciers existed during pollen Zone III. Sediments from present or former lakes just outside the limit of the last Glen Almond glacier and just outside the terminal moraine of the last Teith glacier contain pollen from much of the Lateglacial, including the interstadial preceding Zone III, but the earliest deposits found immediately inside the Teith moraine belong to the Postglacial. Secondly, the Teith terminal moraine occupies a position at the mouth of a Highland valley analogous to those of the neighbouring Menteith and Loch Lomond terminal moraines that were dated by pollen and radiocarbon analyses as having been formed in Zone III. Thirdly, a suite of outwash terraces formed beyond the Teith moraine passes into a buried fan that was largely deposited during a period of low sea -level when the adjacent Menteith moraine was being formed, that is in Zone III. Fourthly, whereas the largest glaciers in the thesis area advanced eastwards from the west Highland watershed area, other major glaciers flowed westwards from this watershed to the western coast. It is generally considered on morphological evidence that the glaciers that terminated at Benderloch, Loch Leven, Loch Linnhe, Loch Shiel and Loch Morar represented the Loch Lomond Readvance in Zone III, whilst radiocarbon dating of organic material proves that the Benderloch glaciers existed during Zone III. Fifthly, it seems entirely logical to expect that the limits in the Highland part of western Perthshire that do not happen to have been independently dated by pollen studies should have been formed in the same period as those that have been dated.

It is concluded that the last valley glaciers in western Perthshire were part of the Loch Lomond Readvance that is correctly correlated with pollen Zone III.