Brecciated dolomitic limestone, Durness Limestone

Brecciated means broken up into angular pieces. This rock lies directly beneath the Moine Thrust, and was crushed as the great mass of overlying rock moved over it.

Outcrop

Brecciated dolomitic limestone, Durness Limestone, Knockan Crag
This photo shows the light-coloured limestone directly underlying the dark-coloured Moine mylonites at Knockan Crag. The limestone has a banding that has been bent into small folds. Carefully studying the structures here tells you the order of events: first the limestone recrystallized and flowed (forming the banding), then it deformed by folding the banding, and finally it became brittle and fractured. The field of view is about 2 metres high.

Hand specimen

Brecciated dolomitic limestone, Durness Limestone, Knockan Crag
The buff-coloured weathered surface of the limestone shows a great many fractures.

Thin section

Brecciated dolomitic limestone, Durness Limestone, Knockan Crag
In thin section we see a fine-grained carbonate rock that has been broken into angular pieces. Between the pieces the rock has been crushed into even finer material that appears dark in colour. Even though the grain size of the crushed material is very small, it is well cemented, and the breccia is as hard as undeformed limestone.

Plane polarized light. Field of view 7 mm across.

D.J. Waters, Department of Earth Sciences, May 2003