Ben Alder

The area around Ben Alder has been central to debates concerning the existence or otherwise of a root zone for the regional nappe complexes of the Central Grampian Highlands. It was believed that the nappes were -squeezed out- on both sides of a zone of steeply dipping rocks called the Geal-charn - Ossian Steep Belt, which is well exposed in the adjacent Aonach Beag and Geal-charn GCR site. This in effect created a mushroom-shaped -fountain- of nappes in which individual nappes were separated by ductile dislocations termed slides. Away from the steep belt to the south-east the primary nappes became highly inclined and all of the structures were subsequently refolded by more-upright folds of at least two generations, resulting in the observed pattern of recumbent structures.
The Ben Alder GCR site is situated on the south-east side of the steep belt, where it has been argued that the effects of superimposed F2 and F3 folds on originally highly inclined, SE-facing F1 folds can be demonstrated clearly in three-dimensional detail. This structural architecture has been crucial to any interpretation of the development of the steep belt and of the nappes between Ben Alder and the Boundary Slide at Blair Atholl. This in turn is fundamental to theories for the origin and evolution of the Grampian nappe complexes in general.
The most recent published work has emphasized the influence of the original depositional framework of the Grampian Group sediments, and in particlar the basin geometry, on the subsequent structural architecture. Hence the Geal-charn - Ossian Steep Belt is no longer regarded as a simple root zone to the major nappes and has been attributed to compression of the sediments against a basement -high- that forms the basin margin. The ongoing debate will continue to draw heavily upon evidence from the Ben Alder area, emphasizing still further the national importance of the Aonach Beag to Geal-charn and Ben Alder GCR sites.