Some sources regard the Mounth as extending as far west as Drumochter Pass (A9), but it is now generally agreed to start at the Cairnwell Pass (A93 - highest main road pass in Britain, Glen Shee ski centre). Here, a high undulating plateau invaded by deep glacial troughs (Glen Isla, Glen Callater, Glen Muick, Glen Clova) culminates in Glas Maol (1068m /3504') on the main watershed, with the outlying granite Lochnagar (1155m/3789') and its surrounding White Mounth. To the east the plateau broadens into a lower moorland incised by river valleys, notably Glen Esk and Glen Tanar, descending gently east to North Sea coastal cliffs between Stonehaven and Aberdeen. This is the best-preserved expanse of the ancient Highland pre-glacial upland surface.