The metasedimentary rocks of the area have had a
complex structural and metamorphic history. The structural
analysis indicates that the rocks have suffered four periods
of folding, and by study of the major and minor structures
of each generation the age relationships of the fold movements can be established. The history of the fold episodes
may be summarised:
1. first period of isoclinal folding. No major
isoclinal folds have been recognized, but first minor
structures are widespread and indicate that the whole area
has been isoclinally folded. This first period of folding
is probably the structural equivalent to the first movement
episode recognized in Morar by W.Q. Kennedy (1955) - the
formation of the Morar nappe.
2. period of tight, asymmetric folding during which
all the rocks of the area were again folded. The second
folds are Similar in style and their orientation has been
considerably modified by the third fold movement. The
evidence suggests that after the second period of folding
had taken place the rocks were vertically oriented.
3. period of open asymmetric folding about a N.N.E -S.S .W.
axial plane trend. The third folds are partly Similar
and partly Concentric in style. The Diollaid Bheag
Antiform, a fold some two miles wide, formed during this
period of folding.
4. period of open asymmetric folding about a N.W. -S.r.
axial plane trend. The fourth folds are of local
occurrence, and their affect upon the orientation of the
rocks and structures of the earlier fold movements is
It has been found that the stratigraphical succession
established by Richey and Kennedy (1939) in the Lioìne
rocks of Morar, could not be extended into the whole of
the area mapped. The structural evidence indicates that
the rocks have been isoclinally folded, and it is concluded
that the stratigraphical succession has been repeated.
The few sedimentary structures that have been found
indicate that the pelitic rocks of An Stac and Rois-Bheinn
are stratigraphically younger than the psammitic rocks
which have been correlated with the Upper Psammitic Group
of Morar (Richey, 1938). It is suggested that the original
stratigraphical succession may have consisted of four main
lithological groups. In the area mapped all the psammitic
and pelitic bands contain thin calc- silicate ribs.
Richey and Kennedy (1939) stated that these calc- silicate
ribs are absent in the Lower Psammitic Group and Lower
Striped Schists of the Llorar succession. It may be
suggested therefore that the lower part of the Ilorar
stratigraphical succession is not represented in the
The rocks of the central and eastern parts of the
area have been strongly migmatised, the migmatisation
taking place before and during the second period of
folding. In the area of migmatisation the rocks are
coarsely crystalline gneisses and the pelitic rocks have
been transformed into oligoclase -biotite- gneiss and locally,
granite gneiss. Thick discordant pegmatite veins are
It is concluded that during migmatisation the rocks
were at sillimanite grade of metamorphism, and associated
with this period of high grade metamorphism the anorthitehornblende and anorthite -pyroxene metamorphic zones of
the calc-silicate rocks were developed.
Recrystallisation again occurred during or after
the third and fourth fold movements, but it is probable
that regrowth of garnet did not take place. Throughout
the area the biotites, and occasionally the garnets, show
alteration to chlorite, and these features of retrogressive
activity must have taken place during or after these late
phases of recrystallisation.
During the four periods of folding it is evident
that the rocks responded to deformation in a very plastic
manner, no cataclastic, or brittle structures found in
the area can be definitely correlated with the four fold
movements. The style of the fold periods is such that
the intensity of deformation decreases, from isoclinal
folding during the first fold movement, to very open
asymmetric folding during the fourth movement episode.
It is interesting to note that the metamorphic history
shows a similar decrease in intensity. The evidence
suggests that at approximately the same time as the first
and second fold movements the grade of metamorphism of the
rocks was extremely high, but during the later periods of
open folding, the rocks only suffered low grade metamorphism. (fig. 27)
The suggested age relationship between metamorphism
and movement is only tentative. Because the rocks have
undergone recrystallisation several times during their
movement history, and that recrystallisation appears to
have taken place during or after the final fold movement,
an accurate correlation between the metamorphic and
structural histories is difficult to establish. The
several phases of folding provide important 'marker horizons
in the structural and metamorphic history, and only by
comparison and correlation with the structure and meta-morphism of adjacent areas will the complete age relationship between metamorphism and movement be determined.