LINEATIONS, MULLIONS AND RODS
Mullions: cuspate and bulged or lobate forms of material between
contrasting lithologies due to mechanical difference and lack of continuity of strain. The fold hinges preserved in this process are called rods. Generally the latter are more silicic and hence the term quartz rods. See the photograph, for example, of microlithons, very much in this case like quartz rods.
Mullion and rodding structures are both forms of a coarse lineation developed in rocks which have been strongly deformed. In general they are parallel to fold hinges. In the past the two terms were considered more or less synonymous; and Holmes (1928) described mullion structure as recalling the appearance of the clustered columns which support the arches, or divide the lights of mullioned windows, in Gothic churches. The structure is also described as rodding structure, and is typically developed in the Eirebol district, where 'rods' of white quartz, varying in dimensions from those of telegraph poles to those of walking sticks, lie parallel to each other down the dip slope of the Moine Schists. Where minerals of elongated habit like hornblende or biotite are present in the rocks showing mullion or rodding structure the crystals are arranged parallel to each other and to the dip and plunge of the folds. Gilbert Wilson has distinguished between these two types of structure; and have confined the term mullion to the structures which have been formed from the country-rocks themselves; and rods to those which have been developed from quartz or other minerals that have segregated in, or have been introduced into the rocks while movement was in progress.
The first use of the term mullion structure is not known, but it was referred to in print by Hull, Kinahan and Nolan in 1891, who observed it in Donegal, Ireland. They employed the term as if it were one which was already familiar to them, but no earlier reference has been found. Fermor (1909) observed similar structures which he described as 'parallel striated and grooved prisms suggesting logs of wood, in the manganese deposits of India, and named them 'slickensides-grooving'. In 1924 he gave examples illustrating the parallelism of such coarse linear structures to the plunges of the local folds. 'Corduroy structure' was a term suggested for all varieties of lineation which formed ridges or undulations on the rock surface, by Bailey and MacCallien (1937). They included the rippling of schists and microfolding together with mullion structure, under this term. Leith (1923) considered mullions as being a coarse form of slickenside striation, like the Rodadero (Gregory 1914), an idea which was commonly held by early workers in the North-West Highlands of Scotland. This theory was modified by Read in 1926, when he suggested that structures had resulted from the interaction of two deformations the first a compression normal to the lengths of the mullions; and the second, a stretching parallel to their lengths (Read & Phemister 1926). Coles Phillips (1937, p. 597) supported Read after having studied the columns on a vertical plane of stratification, plunging parallel to the local fold axes. The mullions are covered with a thin veneer of polished manganese me, Kandri Mine, Central Provinces, India.
Schmidt (1956, 1957) recognised.the development of mullions in the N.
Eifel district of Germany, and have since made a series of most detailed studies of these occurrences. The structure has also been observed in the Caledonides of Scandinavia. Lindstrom has described examples, and has also commented on Pilger and Schmidt's work in the Eifel (Lindstrom 1958).
Mullions seem to be typically developed in strongly deformed metamorphic rocks, though those of the Eifel lie in a region of relatively lowgrade metamorphism. In Scotland they are found in the Lewisian Gneisses (Peach & Horne 1907, PIs. XXV, XXVI), and in rocks of biotite or garnet grade. Coles Phillips (1937) demonstrated that the lengths of the Oykell Bridge mullions were parallel to b; and I found them to be parallel to the local fold axes, b = B. Since then I have mapped mullions elsewhere in the Moine Series of Scotland and in India, and have found no reason to change this opinion. Nevertheless, Pitcher and Read (1959) have illustrated beautiful mullion structures in the wall-rocks of the Main Donegal Granite, and considered that they are aligned parallel to the direction of movement of the intrusive rock, that is, in a. Lindstrom (1958) has illustrated structures to which the term mullions might well be applied. These examples lie in a region of thrust tectonics; and are considered to be oriented more or less parallel to the principal direction of movement. Kvale also suggested that the mullions of Oykell Bridge, and much of the lineation seen in the Moine Series of Scotland, were likewise aligned in a (Kvale 1953).
In shape, mullions are long cylindrical structures of which the surfaces may be rounded, or they may be irregular, like a sheet of corrugated iron which has been rolled up longitudinally The external surfaces may be polished, covered by a thin veneer of mica or some other mineral, or they may be striated parallel to their lengths. Internally, the rock of which mullions are composed is solid and massive throughout, even though it may be laminated. Commonly they are cut by cross-joints at right angles to their lengths; hence they break away from the exposure like sections of fluted pillars (Wilson 1953, PI. 7, Sander 1948). In size they vary from cylinders or irregular prisms some 2 cm or 3 cm in radius or less, to curving surfaces which have radii of 2 m or more, so that they look like partially buried water mains. Locally, as in Donegal, mullions have been used as fence posts or gate posts; and at Oykell Bridge one stands as a monolith some 3 m in height above the ground.
Mullions can be classified into three varieties:
(a) fold-mullions or bedding mullions; (b) cleavage-mullions;
(c) irregular mullions.
Many are formed by a combination of two or three types.
Fold-mullions possess regular curved cylindrical surfaces which correspond to original bedding or to pre-existing planes of foliation. Commonly they are largely composed of detached or strangled hinges of parasitic folds, and the bedding lamination within the 'mullion accords with the external surface. The 'water pipes' of the River Garry in Scotland are likewise formed from the hinges of sharp folds (Barrow et al. 1905), which are not necessarily detached.
Bedding-mullions are undulations of the bedding plane surfaces which have been smoothed, polished, or striated. Locally they may be formed by a single bed pinching and swelling, elsewhere they may be gentle flexures or large corrugations. In the manganese mines of the Central Provinces, India, such mullions are coated with a highly
polished veneer of manganese oxide, and one fold mullion had a continuous length of 100 m; all were parallel to the plunges of the local fold axes.
Cleavage-mullions are long rock-prisms which may be more or less angular, or partially rounded in cross section (Wilson 1953, p. 126, Pilger & Schmidt 1956, 1957). The prism surfaces are dominantly cleavage planes, and commonly these have been folded by further movement. One or two of the sharp edges of the cleavage slices may be ground away, so that one is left with a cylinder having an approximately oval cross section. Other varieties may show a curved face on one side, while the other surfaces are relatively flat. All, however, are characteristically polished, mica-covered or striated.
Irregular mullions are the most common variety. They are long cylindrical structures, but in cross section they are very irregular, and interlock, each one with its neighbours, like pieces in a jig-saw puzzle. The cylindrical surfaces are grooved, like worn cog-wheels, and striated or covered with a micaceous veneer (Wilson 1953). The internal structure of the mullion may show contorted bedding laminae, and these may locally accord with the external surface but for the most part they are truncated by it, as noted by Coles Phillips (1937).
All these various types of mullions may occur together, in anyone area, and they are strictly parallel. When I have seen them they are cylindrical B-structures (Sander 1948 ), and result from movements normal to their lengths; but they will not declare the sense of the movements, as they are not clearly monoclinic, when seen in the field.
The manner in which these structures are so clearly developed, their very strong lineation, and in some cases the marked mineral orientation which occurs in them (the c-axes of hornblendes, for examples, are parallel to the axes of the cylinders) all suggest that, in addition to a rotational or a compressional deformation, the mullioned rocks have
also suffered a concomitant stretching parallel to their lengths. I thus
disagree with Read's suggestion of two separated deformations; and consider that mullions are examples of Einengung (Sander 1948, Weiss
1954), in which the rocks have suffered a great squeezing normal to their lengths, but under conditions such that stretching parallel to their lengths was possible.
In rocks which have been cylindrically folded, therefore, the axes of
the mullions are parallel to the axes of the folds. Their orientations correspond to those of the hinges of drag-folds, discussed in Chapter 10. Mullions may also form in areas where the folding is more complex as for example in folds which Professor John Sutton has referred to as
inconstant folds (Clifford et 01. 1957, p. 6). There, their orientations are controlled by the local stress distribution. For instance, in the Fannich Mountains of Scotland, where the folding is inconstant, mullions and other linear structures show a steady change in orientation between the converging or diverging regional fold-axes (Sutton & Watson 1954). Steeply-inclined mullions, like drag-folds which show steeply-plunging axes, may also be formed in regions where more or less horizontal movements have taken place along steeply dipping bedding or foliation, and are characteristic steilaxen. Rods are cylindrical bodies of quartz or other minerals which have segregated in, or have been introduced into the country-rock while the tectonic movements were going on. They differ from mullions in that they are not composed of the country-rock itself, but are essentially monomineralic. The most common mineral to form rods is quartz; rarely they may be developed in caicareous rocks and are then composed of calcite; in the Department of Geology, Imperial College, we have a rod of pyrite, from the Rio Tinto mining district of Spain.
The classic locality showing rods in Britain is on the mountain of Ben Hutig (Beinn Thutaig on Sheet 114, Geological Survey of Scotland, and in the early reports) in north Sutherland, Scotland (Peach & Horne 1907, p. 603; Wilson 1953, pp. 131-8). Here one can see rods of vein quartz which vary from about 60 cm to about 1 cm in diameter. They'. . . are composed of irregular grains of quartz with some flakes of white mica. The rods are associated with mica-schists of Moine type, which have been denuded into hollows, these siliceous ribs project and form conspicuous features' (Peach & Horne 1907, p. 603). Some of the dip surfaces of exposures look as if they were covered with a layer of parallel white bamboo canes; a few of these may be as thick as a man's arm. Nearly all the bigger rods are striated parallel to their lengths. The more impressive of the Ben Hutig rods lie in the hinge-zones of parasitic folds. The quartz has segregated in these zones of relatively low pressure; probably as Hallimond suggested, as miniature saddle-reefs (in discussion of Wilson 1953, p. 146). Then, as the movement continued the segregations were rolled up at right angles to their lengths . Other rods have originated from the deformation of quartz veinlets which were parallel to the foliation or stratification of the rocks, and which were then folded with the material surrounding them . The resistant quartz hinges now form long projecting ribs which one sees on the weathered rock surfaces. The deformation of early tension gashes which were oblique to the planes of movement may likewise result in the formation of isolated rods of quartz (Wilson 1953). fwm seggahon quaetz in the hinge zones of parasitic folds (after Wilson 1953). In transverse profile, the rods of Ben Hutig vary from oval or nearly circular, to very irregular cylinders or lenses. The extent of the rounding depends on the original shape of the quartz body, and on the amount of movement which the segregation quartz has suffered. More recent work in this area has shown that many of the thinner rods deformed conglomerate pebbles of quartzite and vein-quartz. These have suffered from two successive, but continuous phases of deformation. In the first, the pebbles were flattened and drawn out into thin elongates lenses; the elongation is parallel to b. In the second phase, these lenses were folded with the rocks that contained them or they were, themselves, rolled into siliceous rods by slip. In both cases the axes of rotation, B, were parallel or nearly parallel to the original b-direction. Some of the larger pebbles which have been deformed in this manner are twisted obliquely to their lengths, which suggests that locally there may have been slight divergencies in direction between the two phases of movement. It is obvious that the rods formed by the extension and rolling of conglomerate pebbles do not conform to the definition of rods given on p. 91, which applies to those derived from segregation quartz. Nevertheless, the final products of the two may be so difficult to separate in the field, that until the original parentage of the structure can be established, the term rod, which is descriptive and has no genetic significance, may with reason be applied to both. Later it can be qualified if necessary. Rods are thus, like mullions, parallel to the axes of the local folds, and lie at right angles to the maximum compression. They vary from
orthorhombic to monoclinic structures, in which b = B. A similar relationship has been observed by Coles Phillips in the crystalline schists of Start Point, South Devon, where 'quartz-rods are in places well developed and accord in direction with other lineated features in these rocks' (in discussion of Wilson 1953, p. 147). Mullion and rodding structures are both forms of a coarse lineation developed in rocks which have been strongly deformed. In general they are parallel to fold hinges. In the past the two terms were considered more or less synonymous; and Holmes (1928, p. 16) described mullion structure as recalling
the appearance of the clustered columns which support the arches, or divide the lights of mullioned windows, in Gothic churches. The structure is also des_ribed as rodding structure, and is typically developed in the Eirebol district, where 'rods' of white quartz, varying in dimensions from those of telegraph poles to those of walking sticks, lie parallel to each other down the dip slope of the Moine Schists. Where minerals of elongated habit like hornblende or biotite are present in the rocks showing mullion or rodding structure the crystals are arranged parallel to each other and to the dip and plunge of the folds.