Conglomerate; Stoer Group, Clachtoll,
Lochinver, NW Scotland
This is the type of rock commonly called a basal
conglomerate, laid down on an old land surface as the first
unit of a new sequence of sedimentary layers. Like many basal
conglomerates it consists mainly of broken fragments and
pebbles of the same rock types as the underlying, older material.
In this case the pebbles are of
metamorphic rocks (gneisses of various kinds) that
are unlikely to have travelled far from their source. They
are slightly rounded, and of very different sizes. The sandy
matrix (the fine material in between the pebbles)
is dark purplish brown from the oxidation of its iron
content by the atmosphere, and the brown staining has
affected parts of the light-coloured gneiss fragments. This
material would have accumulated in valleys and hollows in
the land surface, to be swept along and re-deposited by
occasional floods. It takes a strong current of water to
move pebbles this size.