Wrench Movements in the

Aristarchus Region of the Moon'by GILBERT WILSON

Recei ved 3 February 1970

CONTENTS

page 595598600600601601602606607

I. THE DATA .• •2. G EOLOGICAL DISCUSSION3. STRUCT URAL DISCUSSION . . .

(a) Th e Hills are Anti clinal in Origin(b) The Hill s are the Result of Fissure Eruption

(i) The Feeder Dykes were Injected up Tension Fractures(ii) The Feeder Dykes were Injected up Secondary Shear Fractures . ..

4. C ON CLUSIONSREFERENCES

ABSTRACT: The origin of a cha in of hills en echelon, in the Aristarchus region of theMoon, is discussed. It is considered that the hills largely resulted from dyke extrusionsalong Riedel shear-fractures, and that the chain represents a zone of west-north -west­cast-south-east, right-handed (dextral) wrench mo vement.

1. THE DATA

ARISTARCHUS (N. 24°; W. 47°, astronautical co-ordinates) is a prominentcrater situated roughly between the Oceanus Procellarum and the MareImbrium of the Moon. Its rim, about 40 Ian. in diameter, rises to 2660 m.above the surrounding mare plain, and its floor lies some 3620 m. belowthe rim.

The I: 1,000,000 scale map of the Aristarchus Region , on which Fig. 1is based, was compiled from telescopic data from various observatories.sThis map is purely topographical, and physical features arc shown by hill­shad ing and contours at 300 m. (c. 1000ft.) vertical interval. It is admittedthat heights may contain errors up to 100 m. ; nevertheless the mainfeatures such as hills and ridges stand out clearly. A geological map of thesame region, on the same scale, was comp iled by H. J. Moore (No. 1-465(LAC 39»)and published by the U.S. Geological Survey in 1965. Since thendetails have been greatly amplified by photographs" taken from Lunar

1 Ari starchus of Samos was a Grecian as tr ono mer of the Alexandrian School who anticipatedCo pernicus by suggesting that the Earth turned on its axis and revolved around the Sun. Becauseof thi s o pinion he narrowly escaped being indic ted for disturbing the repose of the G od s. One feelsthat he and Professor Read would have had much in common, as some of the latter's views certainlyd isturbed the peace of mind of many pundits!

• Published for the U .S. Air Force and Nat ional Space Administration by the AeronauticalCh art and Information Center, U.S.A.F. ; 1st Edition. 1963, L.A .C., No. 39.

3 Numbers IV-138 H. ; IV-138 H . ; IV-1441h ; and IV-I44 H a,

595

PROC. G EOL. ASS., VOL. 8 1, P ART 3, 19 70 38

596 GILBERT WILSON

Orbiter IV, which have kindly been made available to me by Dr. GilbertFielder of the University of London Observatory. The maps mentionedabove formed part of the Lunar Exhibit (1969) in the Museum of the Insti­tute of Geological Sciences, London, and I wish here to acknowledge theassistance given me by Dr. P. J. Adams who was responsible for thisexhibit.

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Fig. I. Sketch-map of the south-east portion of the Aristarchus region, based onLAC 39, 1963, and No. 1-465 (LAC 39) 1965. Geology by H. J. Moore, U.S. GeoI.Survey

WRENCH MOVEMENTS ON THE MOON 597

The undulating mare plain within the Aristarchus region is interruptedby long, narrow 'mare ridges' or 'wrinkle ridges' and isolated hills. Theseridges, to the north and south of Aristarchus, run roughly north-south;but to the north-east of the crater they change to north-east and south-east,so forming a reticulate pattern which corresponds to Fielder's A and Bgrid systems (Fielder, 1963). South-east of Aristarchus the regional trend

40

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50Kml.ee

SKETCH MAP OF THE AREA

SOUTH-EAST OF

ARISTARCHUS

1:::::::::::::1 EJECTA BLANKET,ETC. WR1_4:WRINKLE RIDGES

If: HILLS OF FRA MAURO FORMATION

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36

598 GILBERT WILSON

lies between south-east and east-south-east and a well-defined chain oflow elongated hills en echelon has been mapped in this area (Fig. I). It isthis range and its significance that form the main topic of the discussionthat follows.

The north-west end of the chain begins about 100 km. east-south-eastof the crater of Aristarchus, beyond the limit mapped as 'Crater RimMaterial' and 'Ejecta Blanket'. Its axis runs as far as the southern edge ofthe nameless 'low walled lunar ring' with two prominent deltoids near thesouth-east corner of the map-sheet. Thence the line, slightly offset, iscontinued in a string of craterlets.

The range has an overall length of some 290 km. (about 180 miles). Itstrend in the north-west is N. 66°W., and gradually changes to N. 59° or600W. in the south-east. The width varies from about 5 km. to 10or 12 km.so that the range as a whole forms a linear feature. It consists of fourteenhills which I have lettered IX to ¢ for reference; many of these are sigmoidaland are elongated from north-west to south-east, crossing the axis of therange at an average angle of 210

; two are roughly parallel with the trendof the range. The lengths of the hills vary from 26 km. to 15 km., averaging21 km., and their widths from 7 km. to 3 km. The ratio of lengths to widthsaverages about 5: 1. The outline ofeach individual hill is shown on the mapby one contour line only: it is therefore unlikely that they are much, if atall, over 300 m. high.

Three other hills, 0, a and T, which lie some 80 km. to the south-south­west of the main range, form a discontinuous zone en echelon near thecraters Bessarion A and B. These show orientations and sizes comparableto those already mentioned and are probably related to them in origin.Typical wrinkle ridges (WRl_4) occur to the north of the principal range,between it and the crater Brayley (Fig. I). These are longer and higher thanthe hills of the main chain, but their orientations suggest that they too maybe members of the same family.

Separate groups of isolated hills are shown to the east of the namelesslunar ring. These are formed of the Fra Mauro Formation (If) and aresteeper and higher than the hills ofthe range; at one point they rise to over3000 m. above the surrounding plain.

2. GEOLOGICAV DISCUSSION

The near surface stratigraphy of the Moon is still being investigated,but H. J. Moore summarised that of the Aristarchus District, as it wasknown in 1965, on the map-sheet. The area is mainly covered by MareMaterial, described as: 'Probably volcanic materials: flows, ash falls or

1 The correct term, as the author has pointed out. is undoubtedly selenological, as applied to theMoon; but geological has already passed into common use in that connexion (Editnr).

WRENCH MOVEMENTS ON THE MOON 599

both. Surface may be largely covered by fragmental mate rials. Ranges inthickness from zero to a few thousand meters .' These rocks overlie the FraMauro Formation of 'ejecta or other materials around the Mare ImbriumBasin'. The hills consisting of the Fra Mauro Formation east ofthe name­less crater can therefore be considered to be scattered, irregular inliersprotruding through the blanket of Mare Material. A revised stratigraphicaltable of the Mare Imbrium area is given in Ronca (1969, 373).

Moore ornamented the isolated hills and many of the long windingwrinkle ridges with the common symbol for an anticline with arrow pointsmarking the tapering ends of the features. He interpreted them as: 'Pro­bably underlain by anticline; possibly the site of a volcanic extrusion.'

The hills may therefore be primary: they may be the result of tectonicfolding; they may cons ist of extrusive lava or ejecta which contributed tothe Mare Material; or , alternatively, they may be features of earlier originnow buried by, and reflected (in the sense used by Sidney Powers) througha superficial blanket of later volcanic products. However, in a discussionof the structural control of the mechanism responsible for their formation,it is immaterial whether the elevations originated before , during or afterthe deposition of the Mare Material.

The internal structure of the hills is unknown ; but their apparentsimilarity, though on a smaller scale, to the wrinkle ridges is suggestive ofa common origin for both features. Fielder's descriptions of the surfaces ofthe ridges militate against their having been formed by compressionalfolding of Mare Material. They 'are apparently long, sinuous elevations . . . 'the tops of which 'display longitudinal rifts like those in mid-oceanicridges and features like dyke outcrops'. He considers they are 'igneous inorigin and that, if they are extrusion phenomena, the lavas generallysolidified before they travelled more than about 10 km. across the Moon'ssurface' (Fielder, 1963,81). They would thus appear to be linear volcanicaccumulations, analogous to Mount Hek1a in Iceland, but on a vastlyelongated scale, lying above the tops of dykes that had broken surface .This similarity to terrestrial vulcanism is further supported by the way inwhich some extensive lunar lava flows have been extruded as fissureeruptions from the wrinkle ridges, and now cover parts of the Moon'ssurface in a manner reminiscent of the outpourings from the Laki Fissure(Fielder & Fielder, 1968, 1 and 19).

No matter whether the east-south-easterly chain of hills is made up of aseries of anticlinal hills, or of extrusions or intrusions fed by dykes, therange as a whole is an excellent example of a monoclinic structure. Thenearly straight line axis of the range, the orientations of the hills, theirfairly regular spacing and apparently un iform heights all indicate that theirformation was structurally controlled. They are the outward and visiblesign of some inward wrench movement concentrated along the line of the

600 GILBERT WILSON

range. Determination of the sense of this movement depends on themanner in which the near surface lunar rocks responded to the shearingstresses imposed on them. One can envisage three possibilities:

(a) The individual hills are anticlinal in origin,(b) The hills are the result of fissure eruptions:

(i) The feeder dykes were injected up tension fractures.(ii) The feeder dykes travelled up secondary zones of shearing fractures.

3. STRUCTURAL DISCUSSION

(a) The Hills are Anticlinal in Origin

If the hills represent anticlines, the range as a whole corresponds toCampbell's (1958) zig-zag pattern of left-handed folds en echelon.

The relationship between folds en echelon and major wrench-faultmovements has been recognised on the Earth, and examples have beenillustrated by Moody & Hill (1965, figs. 7, 9, 10 and 22). Other workershave found similar associations: Lees (1952), Quennell (1958, 1959), andFreund (1965) in the Jordan Valley and eastern Israel, and Pavoni (1961a,b) in Asia Minor. The average of thirty-eight angular measurements be­tween important terrestrial wrench-faults and the folds closely related tothem is approximately 22°, a figure that is remarkably close to the 21°average between the axes of the lunar hills and the trend of the zone inwhich they lie.

If this analogy between lunar and terrestrial structures be correct, theorientations of the hills would indicate a zone of left-handed (sinistral)shear in the lunar crust.

All the terrestrial examples of folds en echelon mentioned above areclosely associated with prominent, large-scale wrench-faults; such struc­tures, however, do not show in the vicinity of the chain hills,of neither onthe Aristarchus Map Sheet, nor on the Orbiter IV photographs. Moreover,

Fig. 2. A. The orientation of folds en echelon and the direction of the principal com­pressive stress. Such folds would be developed along a zone of left-handed simple shearin the early stages of movement.B. The orientation of tension fractures and the direction of relative tensional stressalong a zone of right-handed simple shear in the early stages of movement

WRENCH MOVEMENTS ON THE MOON 601

resolution of the forces developed during simple shear into principalstresses, and experiments with models using layered materials show that,in the early stages of deformation, before visible wrench-faulting hasoccurred, the folds develop at 40 to 45° to the strike of the zone of shear,Fig. 2, A, and Plate 19,A. These folds may also tend to fan outward as theybecome more remote from the axis of the zone (Pavoni, 1961a, b; Freund,1965, fig. 1).

Not only is it considered very doubtful whether the near surface rocks ofthe Moon can be tectonically folded in the way we see on Earth, but alsothe physiography of the hills as seen in the photographs does not supporttheir formation by folding. They appear to be flat-topped plateaux withsteep sides which resemble the edges of lava sheets elsewhere on the Moon.Hence, the hypothesis that these hills represent fold-structures formed bylocal compression seems very dubious.

(b) The Hills are the Result of Fissure Eruptions

(i) The Feeder Dykes were Injected up Tension Fractures

The emplacement of dykes and dyke-swarms along fractures normal tothe direction of relative tension (Anderson, 1951) in the terrestrial crustis now widely recognised; and that tension fractures en echelon can developalong zones subjected to simple shear is well known (Fig. 2, B). The forma­tion of such fractures was demonstrated experimentally by Riedel (1929);calcite-filled veinlets en echelon along incipient shear-zones in limestonewere described by Shainin (1950); tensional and other fractures occurringin unconsolidated material above and along the line of the San AndreasFault have been illustrated by Gilbert (1928), and zones up to 125 miles(78 km.) long of short normal faults en echelon in north-central Oklahomaare considered to have resulted from wrench movements in the basement(Fath, 1920).The mechanics of this type of fracture have been discussed byWilson (1952, 1960, 1961).

The intrusion ofdykes en echelon oriented at right-angles to the directionof relative tension and the accumulation of their extruded products on theMoon's surface is probably the simplest mechanism to visualise as respon­sible for the formation of these lunar hills. A parental intrusion at depthworking its way up a major shear-zone would tend to be deflected into thepreferred paths which would be opened to it when local tensional stressinduced by the wrench movement and magmatic pressure acting togetherovercame the tensile strength of the surrounding rock. The resulting formof the intrusion would be very similar to that illustrated by Anderson(1951, fig. 18, 56) though the mechanism considered here is not the sameas he suggested. An example of small tension gashes filled with quartz andbranching off a single vein is shown is Plate 19, B.

602

A

GILBERT WILSON

B

Pc-Principal Compressive Stress. R&R'- Riedel Shears.D- Displacement Shears. T - Tension Fractures.

Fig. 3. A. The relationships of the fracture patterns to the orientations of the principalstresses in a wrench-zone developed as the result of right-handed simple shear.B. Generalised plan-view of the fracture patterns that can develop above, within,or in the vicinity of a zone of right-handed wrench movement

It must, however, be realised that tension fractures in a shear-zone areinitiated at an angle of about 45° to the line of the zone (Fig. 2, B). Withfurther movement this angle increases; and if the fractures continue togrow, their central parts may be rotated to over 900 to the zone while thedistal ends continue to extend at 45°. The fractures thus become sigmoidalin section, with the S-shape facing, as it were, against the sense ofthe wrenchmovement, which here would then be right-handed or dextral (Riedel,1929; Wilson, 1960). Hence, if the feeder dykes had been injected up suchtension fractures , the surface linear extrusions above them would also beexpected to be elongated at a large angle to the length of the chain , andthey might also show more markedly sigmoidal forms than those which wenow see in the individual hills.

However, the shape of the cratered wrinkle ridge (WR4), south of Bray­Iey-B, is strongly suggestive of extrusion from a dyke that followed asigmoidal tension fracture, the orientations of which would accord with aroughly east-west right-handed deformation.

(ii) The Feeder Dykes were Injected up Secondary Shear FracturesThe patterns formed by secondary shear fractures developed during

movement along a major zone of shearing have been described by Riedel(1929), Ernst Cloos (1955), McKinstry (1953), Wilson (1960; 1961),Skempton (1966), Einarsson (1967), Tchalenko & Ambraseys (1970), andTchalenko (1970): they are diagrammatically illustrated in Fig. 3.

If the main shear zone breaks surface as a wrench-fault, the boundingDisplacement Shear Surfaces (D) will form the dominant feature, but thefault may well be accompanied by Riedel shears (R and R') of which R lies

PROC. GEOL. ASS., VOL. 81 (1970) PLATE 19

A. Model folds en echelon produced in wet tissue paper lying on two boards, and formedby sliding one board laterally relative to the other

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5:

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emB. Obverse and reverse sides of a hand-specimen in which a single vein let is continuedin a zone of tension gashes en echelon. Loose fragment of Culm Measures, NorthCornwall

[To face p, 602

To .1'(1( '( ' n , 603]

Trace of Da. ht-c Bayez fault. Iran

PROC. GEOL. ASS., VOL. 81 (1970) PLATE 20

Opposite: Aerial photograph of the trace of the Dasht-e Bayez fault (Iran) which wasresponsible for the 31 August J968 earthquake. The movement was left-handed as shownby the local displacement of streams and walls. Riedel R shears can be seen south ofthe main fault trace, making an acute angle with it. The lines of crater-like featurestrending north-west to south-east are old shafts sunk during the construction ofsubterranean irrigation conduits (qanats)

Photograph reproduced by permission of the Geological Society of America

at a small acute angle-between 10and 30°-to the principal plane ofmove­ment, the less common R' appears at 80° to 60' to it. Rand R' are conju­gate shear fractures and, like tension fractures, are inclined against thesense of the main movement; theoretically they form equal angles of lessthan 45° with the principal compressive stress. Einarsson (1967) hasillustrated examples of R-shears cutting the lavas of Iceland. Others ofboth Rand R' types were developed during the recent left-handed wrenchmovements on the fault which was responsible for the Dasht-e Bayazearthquake in east-central Iran on 31 August 1968(Tchalenko & Ambraseys,1970). An aerial photograph of a section of this fault is shown in Plate 20in which the main fault-trace and subsidiary R-shears are plainly visible.

The relationship of these fracture patterns to the stresses responsible isillustrated in Fig. 3, A, together with the direction in which tension frac­tures might also be expected to form. Intersections of different sets offractures can lead to the development of zig-zag breaks which would followfirst one direction and then another (Fig. 3, B). Riedel shears and tensionfractures are not mutually exclusive and may develop along the same zone,but Tchalenko (personal communication) has observed in experiments andmud flows that, if they are combined, the R-shears develop first and thendegenerate into tension gashes as the strain increases. Combination ofthe two results in the formation of fissures in which the ends of tensionfractures are continued as R-shear fractures (Wilson, 1960). [f the move­ment continues beyond the stage when the initial fractures are formed,tensional fractures will tend to gape and become sigmoidal, but shearfractures remain tight and more or less straight, though there will be move­ment along them.

It is widely recognised on the Earth that the orientations of dykes arecommonly controlled by the directions ofrelative tension in the crust, butintrusion along secondary shear planes is not so well established (Anderson,1951,23). Recently, however, intrusion of magma up Riedel R-shears hasbeen suggested to account for 'the direction of the 5-km.-Iong volcanicHekla fracture and of the steep wall of Grimsfhall at the foot of which theGrimsvotn volcano is situated' (Einarsson, 1967, 135). The principle isfurther discussed in Einarsson (1968).

Another probable example is to be found in Northumberland (Fig. 4).Here, two zones of Tertiary dykes en echelon are known on either side of

604 GILBERT WILSON

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Fig. 4. The Tertiary dyke echelons north and south of the Cheviot Hills, after Holmes& Harwood, 1929. Based on Crown Copyright Geological Survey Maps. By permissionof the Director, Institute of Geological Sciences, London

the Cheviot Hills. These zones trend more or less east-west: the morenortherly consists of a right-handed echelon of dykes, the more southerlyof a left-handed echelon. Hence the dykes of the two echelons tend toconverge westward. Holmes & Harwood (1928) likened their orientationsto the marginal crevasses of glaciers, and so considered that the interveningblock of the Cheviot Hills had moved eastward relative to the ground to thenorth and south. Anderson (1951, 41-2) rightly pointed out that thisanalogy was incorrect, because the dykes of the two echelons do not lieat roughly 90° to each other, as do marginal crevasses. The orientations ofthe dykes of the northern, or Holy Island, echelon lie between 5 and 27°(average 15°) to the trend of the zone that contains them. Those of themore southerly or High Green echelon make angles of 0 to 32° (average17°) with the line of their zone. The dykes cannot therefore have been

WRENCH MOVEMENTS ON THE MOON 605

intruded up tension fractures or they would lie at 45° or over to the trendsof their respect ive echelons; but their angular relationships to those trendsdo accord with their intrusion along Riedel R-shears. However, as bothtension fractures and Riedel R-shears in a zone of wrench movement areoriented in the same quadrant, the regional eastward translat ion of theCheviot Hills suggested by Holmes & Harwood still seems probable.

Recently Chadwick (1969), working in Greenland, has mapped numer­ous dykes which occupy shear fractures ; these latter include R- and D­shears, and possibly R'-shear fractures.

TABLE I. The Relationships between the Shapes of Individual Hills, theProbable Orientations of the Dykes Responsible for the Hills with Referenceto the Axis of the Range, and the Types of Fractures that Controlllld theStructures.

HILLNW CENTRE SEEND END

ex 32 0 26R D R

~45 55 27

T T R

r 51 14 30T R R

8 10 33 12R R R

E a 0 0D D D

~a a a0 D D

11.a 20 0D R D

112 -- 65 --TorR'

e 5 16 31R R R

HILLNW SE

END CENTRE END

t 0 15 15D R R

K -- 27 -R

A 23 -- 0R D

JJ 25 30 27R R R

v 10 66 --R T

~66 14-- T R

0 20-- R -

'IT -- 28 --R

P - 25 --R

28 IWR : 0 to 15 IR 3 D and R__......:.--:~...:.:._I_W_~__.:.:...-I-__....::;,...-.:..:.;:..:.:..._

IWR : 45 to 101 T and R

WR=Wrinkle RidgeR&~=Riedel Shears

D..Displacement ShearT =Tension Fracture

606 GILBERT WILSON

4. CONCLUSIONS

The evidence outlined above indicates that, in addition to tension frac­tures, we can have three directions of shearing up which dyke intrusion ispossible-D-shears, Rand R' shears--either alone or in combination. Allof these, except the D-shears, will be oriented in the same general sense inrespect to the main wrench-zone (Fig. 3).

Returning now to the problem of our lunar hills, we find that the averageangle they make with the axis of the chain, 21°, is well within the limits ofthe theoretical Riedel R-shear orientations. If these shears are also associa­ted with tension fractures, the wedge-like penetration and intrusion ofmagma along them will locally be facilitated, as discussed by Anderson(1951,24), and may be comparable with the intrusion of sills along planesof "'dding described by Tweto (1951, 525-8).

It is therefore concluded that the hills were formed, as Fielder (1963)suggested for the wrinkle ridges, by the piling up of lava flows around andabove dykes which broke surface; that the elongation of each hill reflectsthe plan view or orientation of the dyke responsible, and that theseorientations were structurally controlled. The dominant orientationaccords with that in which one would expect Riedel R-shears to lie; but,where the hills are curved, other shear or tensional fractures may havebeen responsible. This relationship is summarised in Table I.

If these conclusions are correct, the chain of hills represents a wrench­zone on which the movement was right-handed (dextral). The same appliesto the three hills near Bessarion A, and the same structural control can berecognised in the wrinkle ridges (WRl-4) between the range of hills andthe crater Brayley. Similar groups of hills en echelon occur elsewhere onthe Moon's surface and from their orientations it should be possible todetermine the directions and senses of wrench movements in areas wheredisplacement of crater rims or other features (Fielder, 1964) cannot berecognised.

'Don't have any preconceived notions-go out and map the area as if itwere part of the Moon' was the advice given to more than one researchstudent by Professor Read when he was at Imperial College. I trust thatthis complete reversal of the instructions he used to give, in that I haveendeavoured to interpret a piece of lunar geology as if it were on Earth,will in no way be taken as indicating any diminution of my respect andpast friendship toward him.

WRENCH MOVEMENTS ON THE MOON

REFERENCES

607

ANDERSON, E. M. 1951. The Dynamics of Faulting and Dyke Formation with Applica­tions to Britain. 2nd Edit.• Oliver & Boyd, Edinburgh and London.

CAMPBELL, J. D. 1958. En echelon Folding. Econ. Geol., 53, 448-72.CHADWICK. BRIAN. 1969. Patterns of Fracture and Dyke Intrusion near Frederikshab,

South-west Greenland. Tectonophysics. 8, 247-64. Amsterdam.CLOOS, E. 1955. Experimental Analysis of Fracture Patterns. Bull. geol, Soc. Am., 66

241-56.EINARSSON, T. 1967. The Icelandic Fracture System and the Inferred Causal Stress

System. In S. Bjornsson (Edit.), Iceland and Mid-Oceanic Ridges. Sympos.•Reykjavik, Soc. Sci. Islandica, 38, 128-41.

----. 1968. Submarine Ridges as an Effect of Stress Fields. I. geophys, Res., 73,7561-76.

FATH, A. E. 1920. The Origin of Faults, Anticlines, and Buried 'Granite Ridge' of theNorthern Part of the Mid-Continent Oil and Gas Field. U.S. Geol. Surv.,Prof. Paper No. 128C, 75-84.

FIELDER, G. 1963. Lunar Tectonics. Q. II geol. Soc. Lond., 119, 65-94.----. 1964. Strike-Slip Faulting in the Vaporum Region of the Moon. Q. II geol.

Soc. Lond., 120,275-81.---- & J. FIELDER. 1968. Lava Flows in Mare Imbrium. Geo-Astrophys. Lab.;

Boeing Sci. Res. Lab. Document DI-82-0749, Aug. 1968, 36 pp., Seattle,Wash.

FREUND, R. 1965. A Model of the Structural Development of Israel and AdjacentAreas since Upper Cretaceous Times. Geol. Mag., 102, 189-205.

GILBERT, G. K. 1928. Studies of the Basin Range Structure. U.S. Geol. Surv., Prof.Paper No. 153. 92 pp.

HOLMES, A. & H. F. HARWOOD. 1928. The Age and Composition of the Whin Sill andRelated Dikes of the North of England. Mineralog: Mag., 21, 493-542.

LEES, G. M. 1952. Foreland Folding. Q. JI geol. Soc. Lond., 108,1-34.McKINSTRY, H. E. 1953. Shears of the Second Order. Am. J. Sci., 251, 401-14.MOODY, J. D. & M. J. HILL. 1956. Wrench-Fault Tectonics. Bull. geol. Soc. Am., 67,

1207-46.PAVONI, N. 1961a. Die nordanatolische Horizontalverschiebung. Geol. Rdsch., 51,

122-39.----. 1961b. Faltung durch Horizontalverschiebung. Eclog: geol. Helv., 54,

515-34.QUENNELL, A. M. 1958. Structure and Geomorphic Evolution of the Dead Sea Drift.

Q. JI geo/. Soc. Lond., 114, 1-24.---. 1959. Tectonics of the Dead Sea Rift. Int. geol. Cong., 20 (Mexico). 1956.

Ass. Servo Geol. Africa, 385-405.RIEDEL, W. 1929. Zur Mechanik geologischer Brucherscheinungen. Zentbl. Miner.

Geol. Paldont., 1929B, 354-68.RONCA, L. B. 1969. Recent Advances in Lunar Geology. Proc, Geol. Ass., 80, 365-78.SHAININ, V. E. 1950. Conjugate Sets of en echelon Tension Fractures in the Athens

Limestone at Riverton, Virginia. Bull. geol. Soc. Am., 61, 509-17.SKEMPTON, A. W. 1966. Some Observations on Tectonic Shear Zones. Proc, l st Congo

Int. Soc. Rock Mech. (Lisbon), I, 329-35.TCHALENKO, J. S. 1970. Similarities Between Shear Zones of Different Magnitudes.

Bull. geol, Soc. Am., 81, 1625-40.---- & N. N. AMBRASEYS. 1970. Structural Analysis of the Dasht-e Bayaz Earth­

quake Fractures. Bull. geol. Soc. Am., 81, 41-60.TWETO, O. 1951. Form and Structure of Sills near Pando, Colorado. Bull. geol. Soc.

Am., 62, 507-32.

608 GILBERT WILSON

WILSON, GILBERT. 1952. A Quartz-Vein System in the Moine Series near Melness,A'Mhoine, North Sutherland. Geol. Mag., 89,141-4.

----. 1960. The Tectonics of the 'Great Ice Chasm' Filchner Ice Shelf, Antarctica.Proc. Geol. Ass., 71, 130-8.

----. 1961. The Tectonic Significance of Small-scale Structures, and their Import­ance to the Geologist in the Field. Annls, Soc. geol, Belg., 84, 423-548.

Gilbert WilsonDepartment of GeologyImperial CollegePrince Consort Road,London S.W.7