Fig 1. —Blair Atholl-Ben a' Gloe area, western half. Readings give the direct plunge of minor folds or lineation when these alone were recorded, but otherwise refer to the schistosity or lithological banding. Double arrows refer to the positions of readings.

Note. —The small outcrop ⅛ mile E.S.E. of Meall Gruaim summit is Coire Fhiann Group. The western boundary of the narrow band of quartzite ⅔ mile N.E. of Meall Dail-min should be shown as a heavy line (fault).

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  Fig. 3.—Diagrammatic map of the Blair Atholl-Ben a' Gloe area, with generalised sections. The map shows the disposition of the various tectonic units, and the distribution of some index minerals.

that the Banded Group and Coire Fhiann Group occupy corresponding positions between the Quartzite Series on one side and the Grey = Shierglas Limestone on the other. Moreover, although the Coire Fhiann Group contains a number of limestone bands which the Banded Group does not, the two groups are very similar in lithology.

No representative of the Schiehallion “White Limestone” is found in the Ben a' Gloe area. However, the Kirkton of Lude Boulder Bed and the Schiehallion Boulder Bed occur in similar positions in their respective sequences, and are very similar in lithology: there can be little doubt that the two are equivalent, as suggested by Bailey (1925, p. 685).

The succession in Zones II and III of the Ben a' Gloe area may now be compared with the Schiehallion succession, up to the base of the Quartzite Series (Table I).

The absence of the White Limestone in Zones II and III, and the sporadic occurrence of the Boulder Bed, could well be due to original sedimentation, and it is highly probable that the above lithological sequences represent true stratigraphical successions, with no significant interruptions or repetitions of a tectonic nature. Since the Quartzite Series is younger than the Blair Atholl Series, the various divisions of the latter must become progressively older from the Boulder Bed to the Tomnabroilach Schists.

Along the south-eastern boundaries of the Ben a' Gloe area (Zone I), the Tulach Hill area, and the central Schiehallion belt, the Shierglas Limestone comes directly against the Quartzite Series; the upper members of the Blair Atholl Series are completely missing. Bailey interpreted this anomalous junction as a slide, later named the Blair Atholl Slide (McCallien, 1943, p. 45). It is possible, however, that the discrepancy can be explained partly or wholly in terms of variations in the original sedimentary sequence. In those districts where the rocks are highly folded and are involved in sliding it is the commonly accepted view that the most consistent and widespread lithological sequence (i.e., the “normal” sequence) is the one most likely to represent the original stratigraphical succession. Accordingly, the “abnormal” sequences, with omissions or repetitions, are attributed to tectonic causes. In the present case, however, the “abnormal” sequence is very persistent, with a total length of outcrop comparable with that of the “normal” sequence to the north. This suggests that the omission of the upper Blair Atholl Series may be a consequence of original sedimentation.

c. Blair Atholl Series: Monzie Limestone and Monzie Schists. These formations are rather similar in lithology to the White Limestone and Banded Group of the Schiehallion succession, and were identified as such by Bailey and McCallien.

However, outcrops of the Monzie Limestone are always separated by Tomnabroilach Schists from outcrops of the Shierglas Limestone. This indicates that the Tomnabroilach Schists may in fact be younger than the Monzie Limestone. The close association of Monzie Limestone and Monzie Schists further indicates that they are adjacent in the stratigraphical succession, and if the Monzie Limestone is older than the Tomnabroilach Schists, this must also apply to the Monzie Schists. The relative positions of the Monzie Limestone and Monzie Schists in the sequence are more difficult to decide, as both come into contact with the Tomnabroilach

Schists. Either the Monzie Schists or the Monzie Limestone must have been partially cut out by sliding or unconformity, and it is probably the Limestone that has been affected in this way, as this formation appears to be much thinner than the Schists. It is therefore suggested that the Monzie Limestone intervenes stratigraphically between the Monzie and Tomnabroilach Schists, making the succession:

Shierglas Limestone
Tomnabroilach Schists
Monzie Limestone
Monzie Schists

The omission of the Monzie Limestone along most of the south-eastern boundary of the Monzie Schists could easily be due to sliding or to a local unconformity in the original sediments.

It is possible that the Monzie Schists represent a semipelitic facies of the uppermost Moine: in the Glen Banvie region, the Struan “Flags” are semi-pelitic rather than flaggy.

d. Ben a' Gloe Quartzite and Coire Rainich Phyllites. The Coire Rainich Phyllites show no close resemblance to any undoubted member of the Blair Atholl Series, although they might conceivably represent a fine-grained variety of the eastern Coire Fhiann Group or the Tomnabroilach Schists. Again, these Phyllites appear to be quite devoid of intercalations of limestone or calcareous schists. Furthermore, as mentioned earlier, the Ben a' Gloe Quartzite adjacent to the Blair Atholl Series differs in lithology from that adjoining the Coire Rainich Phyllites. For these reasons there can be little doubt that the Phyllites are separated stratigraphically from the Blair Atholl Series by some if not all of the Quartzite. The Phyllites may well be younger than any part of the Quartzite, but they could also represent a stratigraphical intercalation within the latter. Certain tectonic evidence (infra) is in favour of the second alternative.

The correlation of the Schiehallion and Ben a' Gloe successions above the base of the Quartzite Series is more doubtful than in the case of the Blair Atholl Series. Bailey referred the Ben a' Gloe Quartzite to the Schiehallion Quartzite, and the Coire Rainich Phyllites to the Killiecrankie Schist. Although this correlation implies a certain amount of facies variation in the original sediments, it may well be correct. However, various alternatives are possible.

The lithology of the Coire Rainich Phyllites is very similar to that of the Ben Eagach Black Schist, except for their difference in grain size. It is suggested here that these two formations may be equivalent: if so, the Ben a' Gloe Quartzite must represent the whole Quartzite Series. A comprehensive quartzite of this kind, extending the whole way from the Blair Atholl Series to the Ben Eagach Schist, does occur in the Cairnwell-Glen Shee area (Bailey, 1925, p. 679; 1928, p. 742). On the other hand, if any part of the Phyllites represents a stratigraphical intercalation within the Quartzite, the succession in the Ben a' Gloe range would be closely comparable with the Quartzite Series in the northern belt at Schiehallion. The intercalated phyllites would correspond to the Killiecrankie Schist, the underlying quartzite to the Schiehallion Quartzite, and the overlying quartzite to the Carn Mairg Quartzite.

Along the southern margin of the central belt at Schiehallion the Killiecrankie Schist and the Blair Atholl Series are separated by the Schiehallion Quartzite in the west, but the quartzite wedges out before reaching the Loch Tay Fault, and there is no quartzite between the Blair Atholl Series and the Killiecrankie Schist in the Tulach Hill area or in Zone I.^* Again, the Carn Mairg Quartzite. south-

synform, and that the Monzie Limestone on the west side of Ben Vuirich occupies an antiform. The formations thus appear to be in normal succession at this locality, with the youngest on top.

The axial plunge method also meets with difficulties when applied to the present area. Throughout much of the Ben a' Gloe Quartzite, the minor folds plunge NE or ENE, but in all the other formations they plunge E and SE. Numerically, minor folds plunging E and SE are predominant, but they run at a wide angle to the probable trend of the primary major fold axes. In fact, these minor folds appear to be quite independent of the primary major folds, and were probably formed at a later stage. This also applies to the subordinate minor folds in the Quartzite with a NW-SE strike.

On the other hand, the similarity between the predominant strike of minor folding in the Quartzite and the probable trend of the primary major folds indicates that most of the minor folds in the Quartzite (those plunging to the NE or ENE) are synchronous with the major folds in question. It would be expected, furthermore, that synchronous major and minor folds in a given area would have approximately the same plunge. On this basis, the plunge of the major structures in Zone II may be deduced. In the northern part of the zone the primary major folds probably plunge to the NE or ENE. In the southern part of the zone visible minor folds in the Quartzite are much less abundant than further north, but their general trend indicates that the primary major folds here plunge to the east.

If these deductions are correct, the structure of the alternating belts of Quartzite and Blair Atholl Series in Zone II may be interpreted as in Table II.

The formations appear to be in normal succession, with the youngest uppermost. Structures 1, 2 and 3 will be called the Ben a' Gloe Synform, the Glen Fender Antiform, and the Meall Bhlair Synform respectively. Three generalised sections are shown in Fig. 3. It should be noted that this structural succession is the reverse of the one put forward by Bailey in 1925.

One anomaly requires explanation. The elongated pebbles in the Quartzite presumably owe their shape to deformation associated with the primary major folding, as no other fold system of comparable intensity seems to have affected the Quartzite. It would therefore be expected that the direction of elongation would be at right angles to the major fold axis. Instead, the long axes of the pebbles xtend to run ESE at about 60° to the probable axis of major folding. This anomalous orientation suggests that the area as a whole has been affected by a sinistral distortion, directed along a NE-SW axis. Such a distortion would scarcely alter the orientation of the minor folds in the Quartzite, but would rotate the a-lineation in an anticlockwise sense. It is probable that the a-lineation was originally directed NW-SE, but has been rotated about 30° in the way described above. The sinistral distortion may have accompanied the formation of the Loch Tay Fault System, in which sinistral transcurrent displacements are predominant. The lineations and

minor folds in formations other than the Quartzite probably also ran NW-SE, but were rotated into their present position by the sinistral distortion.

Over most of Zone II, the interformational boundaries show numerous irregularities and indentations, on scales ranging from a few tens of yards to several hundreds of yards. Although many of these undoubtedly correspond to primary major folds, others represent secondary major folds formed at a later stage (infra), and in many cases it is impossible to decide which type of folding is responsible.

The form of most of the outcrops in Zone II can be explained in terms of comparatively simple folds, but this does not apply to the outcrop of Coire Rainich Phyllites in the centre of the Ben a' Gloe range. All minor folds in the vicinity plunge steadily to the NE, indicating that the outcrop represents a closed body, synclinal in form, with quartzite both underlying and overlying a folded lens of phyllite. If the Coire Rainich Phyllites are in fact wholly younger than the Ben a' Gloe Quartzite, the contact between the phyllite lens and the overlying quartzite must be tectonic: the quartzite may have been thrust over the phyllite during an early stage of deformation. The lens may, however, be a stratigraphical intercalation between two layers of quartzite, and its upper contact an unconformity.

Zone I contains no formation with minor folds running consistently in the NE and SW quadrants, but it seems likely that the order of superposition deduced for the Crochton fold persists over the whole zone. According to this view, the general structural succession in Zone I must be:

The various fold cores are more numerous and on a smaller scale than those in Zone II, and will not therefore be given individual names.

If the above succession is correct, the plunge of the major folds must apparently vary from one part of Zone I to another. For example, the Shierglas Limestone cores on Sron na h-Innearach and Meall Breac cannot share the steep SW plunge of the Crochton fold if they are indeed synforms: the lithological dips observed at the western end of the Sron na h-Innearach core indicate that it plunges to the NE, and this is probably also true of the Meall Breac cores. On the other hand, the Shierglas Limestone cores south of Loch Moraig must plunge to the SW if they are antiforms. The inferred plunge of the major folds in Zone I is shown on the diagrammatic map (Fig. 3).

In Zone III, the plunge of the major folds can nowhere be determined directly, but the various lithological outcrops can be matched very closely with others in Zone II, and are probably to be correlated, as shown in Table III.

No major fold cores are visible in Zone IV: the Glen Banvie Series and the nearby Struan Flags represent a single fold limb.

(b) Secondary Major Folds. The primary major folds are themselves affected by a series of slightly later folds, which are on a similar scale and will be called the secondary major folds. The most important of these may be divided into three types:

(i) Folds on axes with a trend roughly conforming to that of the primary major folds.

(ii) Folds on axes running at a wide angle to the primary major folds. Most of these plunge towards the E or SE.

(iii) Torsional movements about vertical axes.

These types are distinguished for the sake of convenience, although they grade into one another, and folds of different types can produce similar geometrical effects. In particular, folds of types (ii) and (iii) may both cause a swing in the strike of a primary major fold core, and it is sometimes impossible to refer a given structure to one type or the other.

Folds belonging to type (i) are not very common in the area. The only important example is the open syncline separating the Meall Bhlair and Meall Gruaim quartzite outcrops. This fold is slightly asymmetrical, the NW limb being the steeper of the two, but neither limb is overturned.

Folds belonging to type (ii), on the other hand, are moderately common. They may be either open or compressed; the majority plunge to the E or SE, and appear to be the largest members of a fold system whose main representatives are the minor folds in the schists and limestones. Medium-scale folds of this type, with axes plunging to the E and SE, are responsible for the form of the most westerly outcrop of quartzite around Allt na Caillich. An exceptionally good example is visible in the deep gully of Allt na Caillich itself, where exposures are virtually three-dimensional. This fold can be traced along the contact of the Quartzite and the Blair Atholl Series: it is essentially a monocline, overturned towards the NE on an axis running 30° S of E. Most of the minor irregularities in the Shierglas Limestone-Killiecrankie Schist boundary around the headwaters of Allt Glen Loch are due to medium-scale folds plunging to the ESE, and a large open fold plunging to the SE is responsible for the V form of the outcrops between Coire Fhiann and Allt Crom-alltan.

A large fold with an unusual strike occurs near the centre of the Ben a' Gloe range, on the east side of Kil 'ic-ille Chaluim. A belt of quartzite with a westerly dip (exceptional in the Ben a' Gloe range) forms the common limb of an antiform to the east and a synform to the west. The strike of the fold axes vanes from NNE-SSW opposite the summit of Kil 'ic-ille Chaluim to N-S around Allt Coire Chruinn-Bhalgain, the plunge meanwhile decreasing from about 30° SSW to nil. Strictly speaking, this fold belongs to type (ii), but it is definitely non-typical.

In Zone I the dominant strike swings from NE to NNW between Stac nam Bodach and Loch Loch. It does not appear necessary to assume torsion about a vertical axis, as there is no evidence that the primary fold axes have rotated. The change in strike is probably due to a broad, comparatively gentle fold with a south-easterly plunge which has tilted the primary major folds towards the north-east.

The most important example of type (iii) is found on the west side of Glen Girnaig. The pronounced swing of the primary major fold axes in this sector appears to be due to an extensive dextral “twist”, affecting the rocks of Zone I. These axes swing round much farther than can be explained by tilting on SE-plunging axes. Moreover, the general dip west of the Girnaig differs little from that found in other parts of Zone I. This would be expected if torsion about a

vertical axis had taken place. If, on the other hand, the swing in strike had been due to tilting about an axis plunging to the south-east, the general dip west of the Girnaig should have been considerably greater than elsewhere: it is likely that the primary major fold cores originally possessed a comparatively uniform dip to the south-east, and the tilting mentioned above would only have served to increase the dip in the sector affected.

There is evidence that the three types of secondary major folding occurred at different stages. Folds of type (i) affect the minor folds in the schists and limestones, but those of type (iii) do not. This indicates that folds of type (i) are younger and folds of type (iii) are older than the minor folds in question. Furthermore, those members of type (ii) which plunge to the E and SE probably belong to the same fold system as the minor folds m the schists and limestones.

If the a-lineation in the Quartzite and the minor folds in the schists and limestones have in fact been rotated by regional sinistral distortion (supra, Section IV, 1a), the secondary major folds have almost certainly been affected by the same process.

The strike of the primary major folds indicates that the main deformation of the area was due to a phase of crustal compression in which the axes of maximum and minimum pressure were directed in a vertical plane, striking NW-SE. In all probability the axis of maximum pressure was approximately horizontal and the axis of minimum pressure vertical. As this system of forces produced minor folding in the Quartzite with the same trend as the primary major folds, it almost certainly produced similar minor folding in the schists and limestones, although this has been obscured by some later tectonic process, for instance the formation of the slides or the development of the E- and SE-plunging minor folds which predominate in these rock-types.

This system of forces does not account for the orientation of any other set of folds in the area. Although the stress system which produced the secondary major folds of type (1) was probably very similar, it must have been developed at a much later stage. It is clear that the secondary major folds of types (ii) and (iii), and the minor folds in the schists and limestones, were all associated with tectonic displacements to the NE and SW in which the horizontal component was predominant. These displacements cannot be explained in terms of a stress system with the axes of maximum and minimum pressure both lying in a vertical plane with a NW-SE strike.

The prevalence of north-easterly and south-westerly movements during this stage of deformation suggests that either extension or compression was taking place on a regional scale, parallel to the main axis of the orogen. On the whole, the evidence is m favour of extension. If the rocks of the area had been compressed along a NE-SW line, the Ben a' Gloe Quartzite could scarcely have failed to show a considerable amount of secondary folding on axes oriented within the south-east and north-west quadrants, but such folds are by no means abundant. An explanation for this fact can be suggested on the assumption that extension, not compression, has taken place. The process of extension would probably not affect the comparatively resistant Quartzite in the same way as the more plastic rock-types. Instead of resulting in the complex penetrating fold movements that are characteristic of the schists and limestones, the process would be more likely to split up the primary fold cores of Quartzite into a series of discrete masses, separated by dislocations running NW-SE (at right angles to the direction of regional extension).

Assuming that general extension along a sub-horizontal NE-SW line accompanied the formation of the minor folds in the schists and limestones and the secondary major folds of types (ii) and (iii), there can be little doubt that the prevailing axis of minimum pressure was parallel to the line of extension. The corresponding axis of maximum pressure must then have lain within a vertical plane,