Boulder conglomerate at Delnabo
Whereas the bed of the Avon here contains abundant granite boulders, this 400 million year old river deposit contains none.
Old Red Sandstone of the Tomintoul outlier
The Tomintoul basin contains alluvial fan conglomerates that pass northwards into sandstones. The sediments accumulated in Lower Old Red Sandstones times towards the close of the Caledonian mountain building period, around 410 million years ago.
The existence of this ORS outlier indicates that the current
land surface remains close to its level in the Devonian. The main watershed then
lay close to its present position and the Tomintoul basin was surrounded by
mountains. The hills of the sub-Devonian terrain can be seen west of the Ailnack
gorge, where steep-sided quartzite tops are emerging due to the erosion of the
overlying ORS sediment. The absence of granite clasts from the conglomerates indicates
that Cairngorm Granite was not yet exposed, although the presence of aplites may
suggest that its roof zone was undergoing erosion.