Gribun Unconformity

The Gribun area of Mull contains a fascinating variety of geology. Mesozoic sediments can be found overlain by Palaeocene lavas. On the shore at Balmeanach and further to the SW are psammites of Moine age, over which the Mesozoic sediments lie unconformably.  Dykes and sills cut the area as well. The coastal scenery is dramatic, esp the cliffs which the road winds round. Further to the SW lies the Wilderness, an area of broken coast, sea stacks, natural arches and caves. Lots to see! This page deals with the famous unconformity which is best seen on the shore near Balmeanach farm.

 

Moine psammites dipping at about 30 degrees underlie Triassic conglomerates

 

The unconformity here is striking and is a text-book example of what is known as an angular unconformity. There is a large age gap between the dipping psammites of Moine age (1000 million years ago) and the Triassic conglomerates which are almost horizontally bedded on top. (250 million years ago)

 

Unconformity seen from the shore, looking to the cliffs

 

The Triassic conglomerates lie below Jurassic and Cretaceous rocks. These can be found further up, before the whole sequence is overlain by basaltic lavas of Palaeocene (early Tertiary) age. These igneous rocks form the great cliffs that tower above Gribun.

 

Looking towards the basalt lava cliffs (Palaeocene) above Balmeanach

 

Close examination of the conglomerate reveals that the clasts in the rock are mainly of Moine psammite, clearly derived from the underlying rocks. The conglomerate and psammite is also very clearly seen in the vicinity of Mackinnons Cave (qv). 

 

Other photos of the location:

Click on an image to open a larger picture.

Gribun unconformity

Gribun unconformity
Gribun unconformity

Gribun unconformity
Unconformity seen from shore

Unconformity seen from shore
Gribun Unconformity

Gribun Unconformity
View SW to basalt lava cliffs

View SW to basalt lava cliffs
Unconformity seen from shore

Unconformity seen from shore
Conglomerate

Conglomerate
Psammite in foreground, conglomerate beyond

Psammite in foreground,
conglomerate beyond
Gribun unconformity. Psammite in lower LHS, conglomerate, basalt

Gribun unconformity. Psammite in
lower LHS, conglomerate, basalt cliffs
in distance.

 
 

Similar or related Geological Sites:

  • Mackinnons Cave (nearby)
  • Bloody Bay Quarry
  •  

References:

Bailey, E.B., Clough, C.T., Wright, W.B. et al. (1924) Tertiary and Post-Tertiary Geology of Mull, Loch Aline and Oban. Memoir of the Geological Survey of Great Britain, HMSO, Edinburgh.