Formation of the Parallel Roads - Glaciers and Lakes
An icefield developed to the west of Glen Roy and the Great Glen during the Loch Lomond Stadial, with a further ice centre to the south, over Rannoch Moor. Glaciers flowed eastwards along the glens from the icefield in the Western Highlands. One tongue of ice blocked the entrance to Glen Gloy, while another extended eastwards to block lower Glen Roy. Here it met a glacier that had extended into the middle section of Glen Spean from the Loch Treig valley. The blocking of Glen Spean led to a lake being impounded with a surface level of about 260 metres above sea level. This lake overflowed eastwards through the Loch Laggan valley, to drain eventually into the River Spey (Diagram A). As the glacier advanced up lower Glen Roy, it cut off a lake in this valley and the rising water eventually found its way across a pass at 325 metres above sea level (Diagram B).
A further advance of the glacier up Glen Roy resulted in the blocking of this exit for the water, and the lake rose to 350 metres, to drain across the low ground at the head of the Roy and Spey glens (Diagram C). A lake at a level of 355 metres above sea level impounded in Glen Gloy drained through the head of this glen across a pass into Glen Roy. As the ice retreated, the overflows were unblocked in the reverse order; finally the ice dam broke near Spean Bridge and the 260 metres lake drained away under the glacier very suddenly and rapidly towards the Great Glen. The sequence of diagrams shows the progressive advance of glaciers into Glen Roy and adjacent glens, producing ice-dammed lakes at successively higher levels of A 260 metres above sea level; B 325 metres above sea level; and C 350 metres above sea level.