Granite erratic on the Beinn a' Bhuird plateau deposited at 15.1 ± 1.2 ka
Glacially-transported tor block
Erratic train of large granite blocks, Da Dhruim Loin
Granite erratic on the Kincardine Hills, probably Clach an-iurnaich
Clach Bàrraig, below the ski car park
Definition: An erratic is a boulder transported and deposited by a glacier having a lithology different than the bedrock upon which it is sitting. Erratics are useful indicators of patterns of former ice flow.
The distribution of erratics in the Cairngorms indicates that the core area of the mountains supported an independent ice dome during the Quaternary. Metamorphic erratics are absent from these areas and only glacially-transported granite blocks occur. The picture is complicated slightly by the septum of schist, quartzite and diorite and extends N into Glen Derry: these rocks have been transported both NE and SW of the former ice shed in this area.
In the Northern Cairngorms, there is a significant carry of metamorphic erratics on to the margins of the granite. These erratics reach a maximum elevation of 800 m OD above Coire na Ciste. Some of these erratics were carried by the late readvance of ice from Strathspey at around 14 ka BP but others may have been deposited prior to this time.
Metamorphic erratics also occur on the lower, northern flanks of Ben Avon. They record a phase of ice movement in the Late Devensian when ice was able to cross the An Lurg ridge and to move SE, at a time when the ice cover on Ben Avon must have been restricted.
Although perhaps not true erratics, large glacially-transported granite blocks occur widely in and around the mountains. These impressive blocks provide valuable information about ice flow at high levels in former ice sheets. There are two broad types of block: those derived from tors and those from cliffs. Blocks derived from tors can be identified by their weathered and rounded appearance and by the presence of weathering pits. In the illustration, the weathering pits are in the shaded area on the bottom left of the block, showing that the block has been tilted during glacial transport. Block derived from cliffs are more widespread, especially on valley floors. High-level blocks form boulder trains away from source areas in several locations:
Erratics of Cairngorm erratics also occur beyond the granite outcrop and indicate the flow of ice out from the mountains. Cairngorm Granite erratics are reported from the Hills of Cromdale north of the mountains and Morven to the east. Granite erratics also occur at up to 800 m to the SW of the Cairngorms: they appear to relate to an early expansion of Cairngorm ice prior to the Last Glacial Maximum. A few massive blocks have been carried from tors, especially on the northern slopes of Ben Avon. Cosmogenic exposure ages indicate at least two periods of glacial transport were involved, one during the Last Glacial Maximum and also an earlier phase.
Naming the glacial erratics of the Cairngorms