The Cairngorms provided the headwaters for the Dee and Don systems prior to the Quaternary. Both river systems appear to have operated along W-E alignments in the early Tertiary and fed sediment to the Gannet fan in the North Sea. Drainage incision in response to regional uplift brought the deepening of a series of major basins along their courses east of the mountains.
The preglacial course of the Don can be followed into the Avon Embayment (Linton, 1954). The former course of the Dee can be traced well to the west of its current position by the broad valley benches that occur above the Geldie. It is unclear how far west the headwaters lay but there is no major topographic barrier between upper Glen Feshie and the Gaick Forest.
The Spey is also following today an ancient drainage route. This valley may have been initiated in Devonian times and later infilled with sediment - certainly, the parallel valley of Glen Rinnes retains patches of these rocks on its floor. The Spey also has a series of topographic basis along its course, of which Glen More in one. These basins reflect long periods of differential weathering and erosion in the Tertiary, followed by final phases of glacial excavation of their floors.