THE SEDIMENTARY ROCKS OF NORTHTROTTERNISH, ISLE OF SKYE.B y MALCOL~l ~lACGREGOR, ~l.Sc .• Ph.D., D.LC., F .G.S.

[ReCel, ed 15th May, 1934.]

[Read 2nd February, 1934.]

I. INTRODU CTIO N 389II. LITE RATURE AND H IST ORI CA L REV IEW 389

III . D E SCR IPTION OF T HE }<OCKS 39 1(a) The Great E st uar ine Ser ies

(i .) Introduct ion .(ii .) Descr ip t ion of E xposures.

(b) The Marine Sha le Series and Passage Eeds.(i .) Introduct ion .(ii.) Description of Exposures.

(c) Sandstones and Pebb le Be ds (possibly Cretaceous).IV. FAULTING AND RAIS ED B E ACHE S ~02

V . GEOLOGICAL STRUCT U RE AN D P HYSIO GRAP H Y 4 0 2

VI. SUM MARY AND CON CL U SIO N S 404

I. INTRODUCTION.THIS paper incorporates th e results of an inv estigati on of

th e general geology, and, more especially , that of th esedimentary rocks of the northern part of th e Trotternishpeninsula of the Isle of Skye, an area which is represented on th eScottish Ordnance Survey r " Sheet No. go, a small st rip alongthe north of Sheet 80 , and on th e corne rs of Sheets 8r and gI.

The map (Fig. 49) gives a genera l indicati on of th e distri­bution of the rock s. Th e main difficulty encounte red in mappingthe sedimentary rocks is the scarcity of exposures. Inl andthere is a thick coverin g of peat and drift, while the coasta lcliffs are mainly comp osed of igneous rocks. Th e beds are muchdisturbed by numerous intrusive sills, whose relative grea terhardness causes them to form t he prominent features and thusto mask the soft sedimenta ry layers.

II. LITERATURE AND HISTORICAL REVIEW.The first account of th e geology of Skye was given by Mac­

culloch in his" Account of the Western Islands " published in181g. He showed that the sedimentary strata in Skye couldbe correlated with the Lias and Oolites in England. .

Murchison 1121 in the course of his examination of Macculloch' ssections, visited Loch St affin where he collected, apparently fromloose blocks of shelly limestone on the shore , specimens ofCyclas (Cyrena) , Paludina, Ostrea and other mollu scs, some ofwhich he considered identical with fossils of one of th e up perbeds of the Weald Clay in Swanage Bay and th e Isle of Wight.

PROC. G EOL. Assoc ., V OL. X L V . , PART 4 , 19 34 . 2 6

M. MACGREGOR,

He proposed for these the name of the Loch Staffin Beds. Inthe appendix to Murchison's paper Sowerby referred one Cyc!asto the Barton Beds, and compared a Neritina with a WoolwichBeds species.

In 1850 Forbes [3J who was engaged in giving an accountof the palseontology of the Purbeck Beds of the South of Eng­land for the Geological Survey, visited Staffin in order to see theestuarine beds described by Murchison. He gave an account

Stale In MileeQ Tertiary lavas &' Sills

_ Sills in Great Estuarine Series

f

FIG. 49.-MAP OF NORTH TROTTERNISH.

of the beds exposed in the bay, showing that the shelly lime­stone (considered by Murchison to be of Wealden Age) occurredin situ beneath blue clay strata of undoubted Oxford Clay age,and that therefore the Loch Staffin Beds must represent part ofthe Middle Oolites.

Twenty-three years later J. Bryce [2J gave an account ofthe Jurassic rocks of Skye and Raasay, and described both theLoch Staffin Estuarine Beds and the Oxford Clay in the areawhich is the subject of this paper.

J. W. Judd's paper in 1878 [4J summarized the knowledge ofMesozoic rocks in the West of Scotland and referred to the

SEDIMENTARY ROCKS OF NORTH TROTTERNISH. 39I

occurrence of the Staffin Estuarine Beds and the Oxford Clay inNorth Trotternish. For the former and their equivalents else­where he introduced the name Great Estuarine Series, which isnow generally adopted.

A recent paper by the late G. W. Lee and ]. Pringle [5] dealsgenerally with the Mesozoic rocks of Scotland, while some ofthe igneous rocks of North Trotternish have been described byF. Walker [14].

Three Geological Survey Memoirs dealing with the Mesozoicsediments in the Western Highlands-Eigg and Muck [6J, SouthSkye [7J and Raasay and N.E. Skye [8J-have been useful forcomparison.

III. DESCRIPTION OF THE ROCKS.

The following groups of rocks in descending order are exposedIn North Trotternish :-

(d) Tertiary Basalt flows with many intruded sills of Dolerite.(c) Sandstones and Pebble Beds which are possibly Cretaceous.(b) A Marine Shale Series, with subordinate argillaceous limestones

and sandy passage beds at the base.(a) The Great Estuarine Series, consisting of alternating shales,

limestones, sandstones and mudstones, with many sills.

(a) The Great Estuarine Series.(i) Introduction. The thick estuarine deposits which are

developed in the West of Scotland were determined by Forbesin I8SI to be of Middle Oolite age. They received the nameof the Great Estuarine Series from ] udd in r878. The earliername, Loch Staffin Beds, of Murchison and Forbes was appliedto certain beds in Staffin Bay shown in this paper to have thenature of passage beds, and to be just below the Marine ShaleSeries, under which heading they will be described. Bryce usedthe name Loch Staffin Beds for the whole thickness of the GreatEstuarine Series, but his section south-east of Staffin does not,in fact, include the original Loch Staffin Beds.

(ii) Description of Exposures. An unbroken line of seacliffs, extending from Valtos at the south-east edge of the map,for rather over two miles northwards, affords a good section ofthe Great Estuarine Series. The series also extends to thesouth, where it has been described in the Memoir on Rassayand N.E. Skye [8J, but owing to the general north-westerly dipof the strata, the lowest beds pass below sea-level before reach­ing the area described in this paper. On the other hand, thehigher beds, not seen farther south, are well developed in thearea under consideration.

Unfortunately, detailed examination of this section is amatter of considerable difficulty owing to the precipitous natureof the cliffs, which rise 200 to 300 feet sheer from the sea. Occa-

392 M. MACGREGOR,

Feet.

sionally their bases are concealed by grass-covered scree slopes.The section is very similar to that described by Harker [6, pp.19-26J on the N.E. coast of Eigg, and contains the same beds.

The sedimentary layers are interstratified with intrusivedolerite sills, the most conspicuous of which forms the top of thecliffs, and is remarkable on account of its beautifully columnarcharacter (see Plate 34A). The sediments consist of alter­nating shales, limestones and sandstones, of which the mostconspicuous is a light-coloured sandstone with doggers.

At .a point just before the cliffs retreat from the sea, three­quarters of a mile south-east of Rudha Garbhaig (A. on Mapand Plate 34B) an exposure free from intrusions gives thefollowing section which may be taken as typical :-

Dolerite sill forming summit of cliff.9. Baked Shales and Mudstones. Some layers packed with small

Cyrenas, usually in a fragmentary condition, but occasion­ally with valves still united.

Yellow Sandstone. Fine grained with casts of fossils, in­cluding Unio staffinensis (Forbes), Viviparus [Paludina]sp., and Cyrenas together 10

8. Ostrea Beds. Limestones and shales almost entirely com-posed of Ostrea hebredica (Forbes) .. 8

7. Sandstone. Loosely cemented and rather coarse-grained.Calcareous concretions up to 2 ft. in diameter occurabundantly. Near base calcareous layers with muchcarbonaceous matter and plant fragments 85

6. Calcareous Sandstone with shaly bands becoming sandy up-wards. Some layers packed with Cyrena .. 6

5. Fine-grained Sandstone showing ripple marks 34. Dark Shale with occasional casts of Cyrenas 10

3. Shelly Limestone markedly current bedded .. 2

2. Alternating bands of Limestone and Calcareous Shale. Thecalcareous character depending on the quantity of Cyrenashells present. I" band of fibrous calcite (beef) near thebase 7

I. Baked Limestone made up of commiuuted Cyrena fragmentswith an arenaceous matrix. The lowest bed showscomplete recrystallization due to baking by the underlying~ 3

Total 134

Dolerite sill descending below low-water level.

Towards the south, the intercalation of sills breaks up thesedimentary sequence as seen in the cliff, but the main divisionscan be distinguished as far as the Kilt Rock, where there is athickness of about 100 feet of sediments. South of this pointan increase in number and size of the sills splits up the sedimentsinto small lenticular masses. About 50 yards north of theLoch Mealt waterfall, a pipe of igneous material traverses thebedding of the sediments, at the base of the cliffs (Plate 35).

PROC. GEOr.. Assoc., VOL. XLV. (1934). PLATE 34.

A.-CLIFFS N. OF LAKE MEALT \VATERFALL. Showing sedimentary strata, ofwhich the Sandstone is the most conspicuous, overlain by columnar sill.

Kilt Rock second buttress from left.

B.-SECTION OF THE GREAT ESTUARINE BEDS three-quarters of a mile south­east of Rudha Garbhaig. Sill in foreground overlain by Shales. The

Sandstone above.[To face p. 392.

SEDIMENTARY ROCKS OF NORTH TROTTERNISH. 393

At Loch Mealt waterfall the whole cliff is composed of dolerite,with only a thin baked shelly limestone band at the base. Thehigher beds have been raised to the top of the cliff, and areseen in two small roadside pits north of the bridge spanningthe stream which drains the Loch. They consist of yellowshales and clays with Cyprids, Estheria murchisoni (Jones) andoolitic concretions. The cliff can be descended at a point 250yds. south of the waterfall, and the section is found to be similarto the type-section:-

Feet.3. Dolerite Sill 602. Sandstone with concretions, and subordinate Cyrena shales

and limestones in the lower part 70I. Scree slope to sea-level 70

SL

5.S.!!.

Total 200

N.N.W.

]

, Vertical exaggeration four timesScale in Miles

FIG. 50.-DIAGRAMMATIC SECTION OF CLIFFS FROM VALTOS TO KILT ROCK.

Farther south, towards Valtos, the normal N.W. dip of thebeds brings the sediments again to the summit of the cliff. Thesefeatures are shown on the diagrammatic section along the cliffs(Fig. 50).

The following section is seen in the sea-cliff at Dun Dearg :thicknesses estimated:-

Feet.5. Sill of Dun Dearg 204. Sandstone with some shales, calcareous in parts, rather coarse­

grained towards the top and some layers with small quartzpebbles 80

3. Cyrena Shales and Limestone showing false bedding withcurrent from north. These shales become sandy at the~ ~

2. Sill.. . 30

I. Shales, Limestones and Sandstones, and Sills, mostly grass-covered 60

Total •. 260

This is similar to the type-section, but stops short of theOstrea-beds at the top, and contains a greater thickness ofshales at the base. The succession persists to the south, and the

394 M. MACGREGOR,

white sandstone beneath the shales, which is described in theSurvey Memoir [6, p. 57] does not rise above sea-level untilInvertote, two miles south of Valtos, is reached.

North of the point where the typical section is taken, thehigh sea-cliffs end, and two inland scarps are developed, oneforming Sgeir Bhan and the other ending at An Corran ; oppositeStaffin Island. The sill at the base of the type-section continuesuninterruptedly as far as Rudha Garbhaig, and probably formsmost of Staffin Island, while the sediments have been erodedback to the inland scarps, and form a low grass-covered plat­form. These features are undoubtdly due to faulting, theposition of the faults across the high ground southwards beingmarked by straight shallow valleys nearly a mile long. Thepresent coast and the N.E. side of Staffin Island are probablydetermined by another parallel fault.

The low ground is a raised beach, and the scarps mark theposition of the old shore-line, The beds between thetide-marks on the west side of Breun Phort consist of softsandstone underlying typical Ostrea shales and limestones,followed by white calcareous sandstones with casts of Osirea.They therefore represent the upper part of the type-section.Similar beds occur in the next bay-Ob nan Ron-about IS0yards N.W. of the Ferryman's Hut, but their relations areobscured by minor faulting and igneous dykes.

The dip of the beds so far considered is towards the north­west at a small angle, so that the sills forming the high groundmust eventually disappear inland beneath the upper beds of theGreat Estuarine Series. The Kilmartin River, a strike-stream,flows over these beds, but unfortunately no exposures are tobe found in its course, excepting a thin band of calcareous sand­stone and shale, seen in the east bank of the river just south ofthe road-bridge at Staffin Post Office. From this point to themouth the stream flows in a gorge cut in igneous material.

The estuarine rocks on the shore of Staffin Bay are passagebeds which lie directly beneath the Marine Shales, and are con­veniently dealt with later (see p. 398).

The next place round the coast where sedimentary rocks areexposed is at Kilmaluag, and here the upper beds of the GreatEstuarine Series are poorly exposed at intervals in the lowcliffs round the west and north shores of the inner bay, dippingS.W. at angles from IS to 20 degrees. The following beds arerecorded in descending order:-

5. Sandstone with mudstone layers.4. Clay with ealcareous bands, with Cyprids Estheria murchisoni

(Jones) and Paludina,3. Ostrea Limestone.z , Thin Cyrena Shale .I . Dolerite Sill with thin bands of baked shale with casts of Pholo­

domya acut icosta (Sow .),

FROC. GEOL. Assoc., VOL. XLV. (rQ34). PL.\TF 35

DYKE TRAVERSING SEDI"IENTARY STRATA, SHORE 50 Y1)5. "". OF L. "IEALI"

\VATERFALL. ""ate wedging effect on the sediments of the igneous materialon the left.

SEDIMENTARY ROCKS OF NORTH TROTTERNISH. 395

Compact blue mudstones are exposed in the bed of theKilmaluag River near its mouth, just below the stepping-stones,where they rest on a sill and dip W.S.W. at IS degrees. Theycontain a Viviparus [PaludinaJ sp., Unio staffinensis (Forbes),ostracods of the family Cypridre, and Estheria murchisoni(Jones). This last form was collected by Murchison at Mug­stok (see p. 397) and it is also recorded from the Great EstuarineSeries in Ardnamurchan [10, p. 49]. It has been mentioned asoccurring south of Staffin (see p. 393).

The character of these upper beds and their fossil contentindicate that they were laid down under very quiet conditions,and their lithology and fauna resemble Group 9 of the typesection, S.E. of Staffin, and those exposed north of Loch Mealt.There are probably, however, over 100 feet of sedimentarystrata above the Ostrea Limestones at Kilmaluag, whichindicates that higher beds are preserved here than are present inthe section farther south.

The flat marshy ground over which the Kilmaluag Riverflows to the shore is attributed to the presence of the EstuarineSeries and the Marine Shales Series, under which the formersoon must dip. The only exposure is where the river has cutthrough a mass of intrusive dolerite, immediately south of theroad, exposing a few thin bands of much altered calcareous sand­stone and limestone in the bed of the stream. These containCyrena, among which C. cucullata (Tate) and C. brycii (Tate)can be recognised, and bear a close comparison with the lamelli­branch beds which occur just below the Marine Shales in StaffinBay described below. The dip is to the N.E. at a low angle,indicating that a small syncline with a N.W.-S.E. axis is presenton the flat ground north of the road.

No sedimentary rocks appear at the surface anywhere to thenorth of a line joining Kilmaluag Bay with Duntulm Bay, withthe exception of some loose blocks of much altered felspathicsandstone and shale collected at Rhu Hunish. The whole of thisnorthern area is composed of a sill or sills of great thickness andthe cliffs, which form the coast, show sections of columnar doleritefrom top to bottom with a thickness of from 100 ft. to 300 ft.

On the shore of the bay south of Duntulm Castle-CairidhGhlumaig-representatives of the Great Estuarine Series cropout, dipping 5 degrees S.E. They are exposed between tidemarks and in a small cliff at the top of the shore. The bedsconsist of:-

3. Dark unfossiliferous shales.2. Ostrea limestones and Ostrea shales. IS ft.I. Shales, sandstones and limestones. 10 ft. (seen).Division I contains a hard calcareous grit, made up largely

of comminuted shell fragments, and a green glauconitic sandybed with abundant Rhynchonella concinna and M odiola. These

M. MACGREGOR,

beds indicate a marine interlude in the Estuarine Series. TheOstrea-beds--division 2-are similar to those in the. type-sectionS.E. of Staffin.

On the north side of the bay the shales are seen to be baked,and lie against the sill which forms the peninsular of RhuMeanish. They contain Cyrena, Estheria murchisoni andostracods, and are similar to the highest beds at Kilmaluag(see p. 395). They are probably therefore separated from thebeds to the south by a fault with a down throw to the north,and the dyke which crosses the shore is no doubt intruded intothis fault.

The higher members of the Great Estuarine Series are seenin Lon Ostatoin, a burn, one mile south of Duntulm, wherethe following section is recorded:-

Picrite Sill. Ft. ins.Sandstone and Mudstone with Paludina casts I 0Baked Shales and thin Limestone, with Paludina

similar to the bed in Kilmaluag Burn at the stepping-stones .. 10 0

Relatively flat ground (probably shales) 70 0

Gorge without exposures 20 0

Ostrea Limestone 0 6Cyrena Shales 6 0

Ostrea Limestone I 0

Papery Cyrena Shales 6 0

Unexposed . . 18 0

Ostrea Limestone containing also Modiola sp.. . 3 0

Ostrea Shales 6 0

Gap 20 0

Total 161 6

High-water mark.

The Ostrea-beds persist here through a thickness of at least40 ft., which is much greater than in other sections. Thisfact, and the presence of the marine intercalation at Duntulm,suggest that conditions were more marine in this locality. Thedeposits above the Ostrea-beds, of which there is a thickness ofabout roo ft., resemble those at Kilmaluag, and the mudstones atthe top contain Paludina.

There are a few other small exposures of the Great EstuarineSeries on the west side of the peninsula. At the mouth of LonSgapail, I mile E. of Ru Bornaskitaig, about 25 ft. of shales andmudstones with abundant ostracods were seen, dipping to theN.E. under a sill which is the continuation of the picrite sill notedin the previous section.

In a road-cutting at Peingown, north of Heribusta, a sectionof about I2 ft. of much disturbed baked shales with some sandybeds was observed. It is probable that these also belong to theGreat Estuarine Series, but are separated from the beds at LonSgapail by the picrite sill.

SEDIMENTARY ROCKS OF NORTH TROTTERNISH. 397

Just east of the jetty at Camas Mor about 6 ft. of Osirea­Limestone with Cyrena-shales are exposed on the shore. Theydip S.W. at about 5 degrees beneath a thin fine-grained doleritesill.

Finally, Murchison recorded beds at Mugstok (Monkstadt),which were exposed in a canal cut about 100 years ago to drainLoch Chaluim [11, p. 3Ill He described them as blue shalescontaining" Am. koenigi, Ostracea in masses.tmany belemnitesflattened Tellinse (?) etc." The ammonites and belemnitessignify the presence of the marine shales, but the " Ostracea inmasses" and the" flattened Tellinse," shown by Jones to beEstheria murchisoni (Jones), suggest that the Ostrea-beds and theupper members of the Great Estuarine Series are present. Un­fortunately there are now no exposures in this district.

Considering the Great Estuarine Series as a whole, thegeneral sequence previously established in South Skye andRaasay holds good for the area under consideration. There is aclearly marked division into :-

(c) Fine grained mudstones and shales with Paludina,Estheria, etc.

(b) Ostrea-beds.(a) Cyrena-shales, limestones and sandstones.

The thickness of the beds is similar to that in S.E. Skyeand Raasay, but there is a greater thickness of rocks belowand less above the Ostrea-beds in Eigg than in North Trotternish.The development in North Trotternish is more arenaceous, likethat of Eigg rather than that of Strathaird which lies betweenthem geographically. In age the Great Estuarine Series cannotbe later than Bathonian owing to the discovery above it ofCornbrash in Raasay [8, p. 58J and Kellaways Rock in Strathaird[7, pp. 128-9J; (see also [5, p. 200J).

(b) The Marine Shale Series and Passage Beds.(i) Introduction. Although the dark shaly beds at Staffin

Bay were first observed by Macculloch, and considered byhim to be the highest secondary strata in this area, it was notuntil 1851 that Forbes [3, p. I07J showed that they representedthe Oxford Clay of England.

The Marine Shales, which are at least 200 ft. thick, are darkblue or grey in colour. They are soft, and become plastic whensaturated with water. At certain levels the shales are calcareousand contain a few bands of concretionary limestone; but, unlikethe Great Estuarine Series, the homogeneity of the depositseems to have rendered it resistant to the injection of sills. Dykes,however, cut the formation fairly freely. The thickness of theestuarine passage beds at the base is unknown as they pass belowlow-water mark.

398 M. MACGREGOR,

(ii) Description of the Exposures . Owing to their naturethe beds are badly exposed, and the series is best seen in StaffinBay, where it crops out at intervals from beneath the shingleand boulders of the shore from a point east of Digg to a pointeast of Flodigarry House. The exposures are liable to constantchange, as the storms of even one winter may alter considerablythe distribution of the shore material. The beds usually dipsteeply t o the west , but are much disturbed.

The following section was taken on the shore below Point C(see Map and Plate 36A), and is probably not far from theexposure recorded 80 years ago by Forbes , to which it bears aclose resemblance. The lower divisions can only be reached atlow-wat er during spring tid es.

F eet.I I . Da rk Shale s much clea ved . Seen in sma ll cl iff at t op of the

sho re a t Poi nt C and on shore 1 0 0 yds. farther north . . 3010. Dark Sha les with many a mm oni tes .. 309. S hales with few fossils . . . . . . 158. Two bands of concretionary lime stone with dark shale between 37. Shales. calcareo us with few foss ils . . . . . . . . 206. Green Sandy Bed I

5. Pebble B ed with sandy lay ers . . I

4. Grey laminated sands . . . . 33. Hard blue calcareous Sandstone with lamellibranchs . . 32 . Grey laminated sands . . 1 2

I . Calcareous Sands tone with lamell ib ranchs I

Total II9

The dip is to the west , varying from about 45° in the lowerbeds to 2 0 ° in the upper beds.

The beds below the Pebble B ed (5) are those describedby Forbes as the Loch Staffin Estuarine Beds. They are in thenature of passage beds from the Great Estuarine Series to theMarine Shales (see p. 391);

The Sandy Beds (2) and (4) are soft and finely bedded, thelamination being accentuated by the presence of much car­bonace ous material. They are composed mainly of ratherangular quartz grains and some felspar and contain a few lamelli­branchs, particularly Cyrena. Th e Hard Bands of CalcareousSandstone (I) and (3) contain abundant shells, man y of whichare in a fragmen tary condit ion, showing that considerablecurrents were in action during deposition. The material other­wise is t he same as the soft beds, but there is a matrix of crystal­line calcite. The fauna includes T rigonia tripartita (Forbes),Ostrea hebredica (Forbes), forms recorded as ? Potomya by Forbesbut probably Pleuromya , P erna murchisoni (Forbes), Pseudo­monotis sp ., and many Cyrenids among which C. j amesoni(Forbes), C. cucullaia (Tat e), and C. maccullochi (Forbes) are themost plentiful. This faun a and the lithology indicate the

PROC. GEOL. Assoc., VOL. XLV. (IQ34).

A.-PART OF THE EXPOSURE AT "POI~T C," showing Marine Shales dippingwestwards with two bands of concretionary limestone (light).

B.-SANDSTONES AND PEBBLE BEDS (CRETACEOc;S) U~DERLYIN(j SILL AT

DUN SKUDlBURGH.[To face p. 390.

SEDIMENTARY ROCKS OF NORTH TROTTERNISH. 399

continuance of the shallow water conditions of the GreatEstuarine Series, but the presence of many marine forms suggestsa salt water environment.

The material of the Pebble Bed (5) is the same as that of thebeds below, but is much coarser. Rounded white quartz pebblesup to tIT in diameter are present in abundance, set in a matrixof small grains of quartz and felspar. The same lamellibranchfauna persists in this bed-Pseudomonotis being particularlyabundant-but it is accompanied by the guards of small belem­nites of " oweni" type. The latter must either have lived invery shallow water or their remains have been washed up andincorporated in a shore deposit.

As the beds are followed upwards in the succession, thepebbles and the lamellibranchs vanish and in the Green SandyBed (6), which is strongly calcareous and glauconitic, belemnitesare the only fossils found. This bed is considered to have beendeposited not far from low-water mark. It gradually passesin its turn to the Shales (7) typical of the Marine Shale Series,which were probably deposited in water of considerable depth.

There is no evidence of unconformity or non-sequence in thesuccession, since a gradual upward transition occurs both offauna and lithology from typical estuarine deposits (r)-(4),through shore deposits (5), and shallow water deposits (6), todeeper water muds (7). This change of environment is com­pleted in a thickness of little more than ro feet. It is explainedby assuming a depression of the whole area, which would havethe effect of cutting off the supply of terrigenous material at thesame time as deepening the sea. In other words, there is a marinetransgression.

The lower shales (7) and (9) are generally calcareous; andbands of concretionary limestone are occasionally developed(e.g.) in division (8). Fossils are not plentiful; except belemnites(principally B. sulcatus), but there are a few ammonites. Thepresence of Quenstedocerates of the vertumnum and maria:groups near the base, indicates that little, if any, of the MarineShales Series has a greater age than Upper Divesian.

The shales (ro) above, are notable for the enormous numberof Cardioceratids which they contain, especially in the upperlayers. There appears to be a gradation of forms from thosewith coarse ribs and very pronounced chevron ornament, such asCardioceras (Vertebriceras) sp, to almost smooth forms, with asharp forward bend of the ribs at the ventre and the developmentof a keel. Along with these are Peltoceratids whose stronglytuberculate outer whorls are to be seen standing out on thesmooth water-worn surface of the shales.

This fauna continues into the base of division (II), whichcontains many small Cardioceras, along with a few smalllamelli­branchs-Cucullcea and Nucula-together with Serpula, Tere-

400 M. MACGREGOR,

10

IZ

3IZ1710

Feet.15

Z0-30

bratula? ouoides (SOW) and nodules of phosphate and pyrites.Higher layers contain many large crushed Cardioceras withGryphcea dilatata and small belemnites.

It is convenient to take the base of the Marine Shale Seriesat the Pebble Bed (5) as it contains the first remains of cephalo­pods. The Marine Shale in this section appears, therefore, to re­present part of the Upper Divesian (Oxford Clay) and the baseofthe Argovian (or Corallian). The passage beds below division(5) of the section must represent part of the time interval be­tween the Marine Shales of Upper Divesian age and the GreatEstuarine Series which are not younger than Bathonian (see p.397). Perhaps they are to be correlated with the sandstoneblocks of Kellaways Rock age found in Strathaird [7, p. 128]and with the Brora Roof-bed of the same age, which contains asimilar fauna to that found at Staffin [9, p. 79]; see also [5, p.203].

In the bay at Kildorais between the tide-marks there is anexposure of about 100 ft. of shales which are almost vertical, witha strike approximately north and south.

The following is the section observed:-

8. Dark Shales with few fossils ..7. Dark Shales with abundant large belemnites6. Dark Shales with many large smooth ammonites

? Deuisosphinctes5. Dark Shales, few fossils. Concretionary Limestone Band4. Calcareous Shales with phosphate nodules3. Dark Shales, few fossilsz. Black Shales with ammonites ..1. Igneous Sill

Shales below low-water. mark.The lower part of these shales contain Prionodoceras and some

obscure Perisphinctids, whilst Pictonia is common in the upperpart, with occasional primitive Rasenia. A form attributed toRingsteadia was also obtained, and occasional lamellibranchs,chiefly Astarte and Avicula, are present.

The shales in division 7 present a curious appearance, as theycontain abundant large belemnites which, owing to the beddingbeing vertical, stand upright and project for several inches abovethe surface of the shale. The ammonite fauna of this exposureindicates the presence of the extreme top of the Corallian, andespecially the base of the Kimeridge Clay.

There is evidently a gap in the record, representing most ofthe Corallian, between the top of the Staffin Bay section andthe bottom of the Kildorais section. Shales filling the gap maybe present, though unexposed.

An ammonite fauna similar to that at Kildorais is found inshales on the south side of a dyke which traverses the shore in anE.-W. direction at Point C. The dyke is probably intruded intoa fault which throws the beds down on the south side.

SEDIMENTARY ROCKS OF NORTH TROTTERNISH. 40r

In the quarry in the grounds of Flodigarry House, beds repre­senting the Marine Shales are seen beneath a sill, by which theyare much altered. They contain Prionodoceras of the lower bedsof the Kildorais section.

Marine Shales probably underlie part of the flat ground atthe head of Kilmaluag Bay and the passage beds are representedby the calcareous sandstone with Cyrena, just south of the road(see p. 395). Bryce [2, p. 332J states that the Oxford Clay" ispresent at Duntulm. Though a complete search of this neigh­bourhood was made, no exposures of Marine Shales could befound, unless the road-side pits on the hill down to the Bay southof the Castle are in this material greatly altered. Screes of BlueShales were observed just below the lava scarp at Totescore. AtMugstok (or Monkstadt) Murchison collected ammonitesand belemnites as mentioned on p. 397, which indicate the pre­sence of the Marine Shale Series. No exposures occur in theneighbourhood at the present time, but the topography indicatesthe outcrop of some soft material. At Dig, "Oxford Clay" wasfound by Bryce [2, p. 33rJ 80 years ago at the" west angleof the Bay near the headland" which yielded to him a plentifulsupply of fossils. A very poor exposure of shales at the topof the shore about i mile east of Rhu Idrigil yielded to the authorsome fragments of ammonites which appear to be Cardioceratids.The configuration of the shores of the Bay indicates the presenceof some soft material such as shale.

The most interesting fact that has come to light in the in­vestigation of the Marine Shale Series is the presence of Jurassicstrata of Kimeridge Clay age in Skye. Both in Strathaird andthe Island of Eigg the Cretaceous rests on Marine Shales withCardioceras, of not later age than Corallian [6, p. 28 and 7, p.129J. Beds representing Kimeridge (jay occur in Mull, [13,P.I49J and are well developed in East Sutherland and Ross [9,p. 103 et seq.J.

(c) Sandstones and Pebble Beds (possibly Cretaceous).On the north side of Dun Skudiburgh at the boat slip there

is an exposure of 6 ft. of sandstones and pebble beds (Plate36B) dipping to the west at 12°, which are of uncertain age.

The Sandstones are composed of angular quartz grains andfresh felspar with a calcareous cement, and the Pebble Bedscontain abundant pebbles of a very fine-grained limestone,dark in colour when fresh, but standing out as white lumps on aweathered surface. These do not usually exceed t" in length,and are well rounded. One large rounded pebble of iron­stained quartzite about 2" across was obtained.

These rocks are quite distinct in type from any others seenin the area, and their position just below the lavas leads to theconclusion that they are younger than the Marine Shale Series.

40 2 M. MACGREGOR,

They conform to the description of the Upper Cretaceous rocksin the neighbouring districts, and in particular in Eigg [6, p. 331and it is suggested that here may be a representative of that for­mation in North Trotternish.

It is unfortunate that, except for one fragmental shark'stooth, no fossils can be found even in thin section. The abun­dant fresh felspar would be accounted for if the land in Cretaceoustimes was being denuded under desert conditions, as has beensuggested by E. B. Bailey [1J.

IV. FAULTING AND RAISED BEACHES.The area has been subjected to regional faulting, and, as

many of the faults are filled with dolerite, this took place, atany rate in part, before the end of the igneous activity of Ter­tiary times.

Faults south-east of Staffin Bay have already been mentioned,(see p. 394) and there is a fault giving rise to the similar featuresnorth of Kilmaluag Bay.

There must be an E.N.E.-W.S.W. fault at Flodigarry with adownthrow of about 300 ft. to the south, which brings up thesills to form sea-cliffs to the north. Its extension inland explainsthe abrupt ending of the lava platform in Sron Vourlinn andSgurr Mor.

Two main directions of faulting are developed :-N.N.W.­SSE. and E.N.E.-W.S.W. and it is probable that the coastlineis in many places determined by faulting in these directions.Examples are, the line of the coast south east of Staffin con­tinued in Staffin and Flodigarry Islands; the coast north-westfrom Port Kilbride and the coast north of Kilmaluag Bay.

Raised beaches occur in several places round the coast,for example south-east of Staffin (see p. 394), and the peninsularof Rhu Hunish.

V. GEOLOGICAL STRUCTURE AND PHYSIOGRAPHY.The physiography of the northern end of the Trotternish

Peninsula depends to a marked degree on the geological struc­ture. This structure, as can be seen from the map and dia­grammatic section, Fig. 5I, is that of a gentle syncline pitchingslightly to the SSW., with an axis roughly along the courseof the River Rha. The dip of the sediments conforms to thisgeneral structure, though often modified locally by the intrusionof thick sills and by slipping.

The Great Estuarine Series crops out almost continuouslyround the coast line, being interrupted only near Uig and Staffin.These beds, consisting of sandstones and shales with numerousintercalated sills, give rise to scenery which is diversified owing tothe presence of the hard igneous layers. Terraced elevations arecharacteristic. Where sills predominate, the sea has cut back the

SEDIMENTARY ROCKS OF NORTH TROTTERNISH. 403

series into vertical cliffs, as to the south-east of Staffin. The cliffsthere attain a height of 400 feet, and are due to the protectivecover of the great columnar sill. As the sediments are cut backby the sea the columns collapse and their debris forms an effec­tive breakwater. Impressive cliffs also occur round the northcoast and from Flodigarry to Kilmaluag Bay. Wherethe sills are less abundant, particularly near the topof the series, the sea erodes the beds powerfully, as is well seenin the formation of the bays of Staffin, Kilmaluag, and Mor.

The Marine Shale Series crops out in a semi-circle within theGreat Estuarine Series, and comes down to sea-level at Staffinand Dig Bays. The absence of sills and the nature of the depositrender it susceptible to denuding agents, and its outcrop usuallyforms boggy low ground, or broken ground covered by talus andslips from the lavas above.

ALONG LINE

§§Marine Shale Serieso Great Estuarine Series11 LavasIIIIITI SIllsScale in Miles

Vert. Exag. 4x.

5r.-DIAGRAMMATIC SECTION ACROSS ~. TROTTER:\,ISH,

MARKED ON MAP (Fig. 49).FIG.

N.W. by W ..•.......... 1 S.E. by qw by N _. E by S.

1 j~MOladhMOrg ~ ]:I: ~ ~ ~ ~

'~~~Lo

The most noteworthy topographic feature of the area is themarked scarp formed by the lavas and their sills above the lowshale outcrop. The feature is most marked -along the easternedge of the lava scarp.where a magnificent line of precipices hasbeen developed (Plate 37A). The face is subject to rapid disinte­gration and landslides are frequent. Large masses of rockdetached from the face slide down over the shales, and the rockformations, for which the Quairang is famous, are due to stack­like masses moving away from the scarp and leaving depressionsin their rear.

Between the scarp and Staffin Bay great numbers of thesemasses are to be seen in all stages of decay. Close to the scarpface they are large with vertical faces and flat tops, and haveall the features of the rock-mass from which they have beendetached; but in the course of their slow movement towards thesea they disintegrate rapidly, till they are little more thanconical, grass-covered mounds of brecciated igneous material(Plate 37B). The disturbed nature of the shale beds on the shore

M. MACGREGOR,

of Staffin Bay, described above, is attributed to this slipping ofthe overlying lavas.

Numerous small tarns are formed among the slipping masses,specially between Staffin Bay and the Quairang, and, as theynave no visible outlet, must drain away below the lava massesover the surface of the shale. Loch Sneosdal is another exampleof a lake resting on the impervious shales below the scarp.

The Tertiary Lava Series, with included dolerite sills, formsthe high ground which terminates on all sides in a scarp. Thisplateau has a gentle tilt to the south-west, the base of the seriesbeing at sea-level at Dig, but at an elevation of about 400 feetat the north-east end of the outcrop. The eastern scarp of thelavas forms the water-shed of the peninsula from the Quairangto near Portree, and has an average elevation of about 1,000 feetwith a maximum of nearly 1,800 feet above the Quairang.

Drainage. The River Rha with its tributaries runs down theaxis of the syncline, and descends by a series of water-falls oversills to sea-level at Dig. Some of this river's headwaters appearto have been captured by streams working against the scarp,especially those descending into Glen Sneosdal (see Map, Fig. 49).

The Kilmaluag River results from several streams descendingthe scarp, and meanders over the Marine Shales and the EstuarineBeds to a point just south of the road, where it has cut a gorgethrough an intrusive sill. Thence it meanders again over theshale outcrop, and finally drops over another sill tothe shore. The stream has reached base-level, and is cuttinginto its own alluvium between the sills.

The Kilmartin River is a strike-stream draining a largesedimentary area on the east of the central ridge. It has cutsideways on the sediments towards the east until it has comeup against the resistant sills which form the high ground alongthe coast.

Owing to the great difference in resistance to erosion of thesedimentary rocks and the igneous intrusions, the rivers aUtend to have sluggish courses interrupted by waterfalls overthe sills.

VI. SUMMARY AND CONCLUSIONS.

The Great Estuarine Series is made up of beds of typicallyestuarine or deltaic character, and represents a time intervalfrom the base of the Vesulian to the top of the Bathonian. Thelower beds exhibit varying conditions of sedimentation, mud­stones alternating with sandstones, and current-bedding beinga common feature. Cyrena occurs chiefly in the finer beds,and often in sufficient numbers to form limestones. The wide-

PROC. GEOL. Assoc., Y'OL. XLV. (I9.34). PLATE 37.

A.--EASTERN ~CARP OF THE LAVAs--Sron Vourlinn-i-with Loch Langaig,Flodigarry in foreground.

B.-LOOKING S. FROM FLODIGARRY. Showing grass-covered mounds of lavanear the shore of Staffin Bay which have slipped down the hill over

the Marine Shales. Staffin Island is in the distance.[To face p. 404.

SEDIMENTARY ROCKS OF NORTH TROTTERNISH. 40~

spread Oyster-beds indicate temporary brackish-water conditions,and a marine interlude is indicated by the Rhynchonella-bed atDuntulm. The upper beds, characterised by Paludina, ostra­cods and Estheria, were probably laid down in shallow fresh­water lagoons.

The Great Estuarine Series marks the end of a sedimentarycycle which started with marine beds at the top of the InferiorOolite and ended with the upper fine-grained fresh-water beds.The latter indicate almost complete silting up of the area ofdeposition.

The great extent of closely similar shallow water deposits,e.g., Ostrea-beds, for a distance of about 70 miles from Duntulmto the Island of Muck, may be explained by assuming that thezones of deposition lay parallel to the present coastline, asappears to have been the case with the Jurassic shallow-waterbeds in E. Sutherland and Ross. These facts, and the absenceof any trace of deposits of similar age inland, suggest that thecoastline of west Inverness-shire and E. Sutherland and Rosswas approximately the same as at the present time, and thatthe mainland between was above sea-level and undergoingdenudation to yield the material for the Jurassic Beds.

The Passage Beds which occur beneath the Marine Shalesrecord the change from estuarine to marine conditions of thenext cycle of sedimentation. They appear to represent theCallovian and possibly the lower Divesian. The fossils foundin the Marine Shale Series indicate the presence of the UpperDivesian, the base of the Corallian, the top of the Corallian,and the base of the Kimeridge Clay. The presence of thesehigh shale beds confirms the opinion that there is a markedunconformity at the base of the Cretaceous in N.W. Scotland,originally expressed by Judd [4, p. 728J. Beds of estuarinecharacter which may represent the Cretaceous occur in the area.

The Tertiary lavas rest on an eroded surface of the olderrocks. Both the lavas and the Jurassic rocks have been riddledby innumerable sills of dolerite. That block faulting in twodirections-N.N.W. and E.N.E.-occurred in part before thefinal phase of minor intrusion is shown by many of the smallerfaults being filled with igneous material.

The physiography of the area can be compared with the Islandof Eigg which it closely resembles both in geology and in theresulting scenery, though the minor importance of the ShaleSeries in that island has prevented the formation of a lavascarp which is the most conspicuous feature in North Trotternish.

In conclusion I have to thank Dr. L. F. Spath for identifyingthe ammonites from Staffin and Kildorais, and Dr. W. J. Arkelland Dr. J. Pringle for kind criticism and advice.

PROC. GEOL. Assoc., VOL. XLV., PART 4, 1934. 27

406 SEDIMENTARY ROCKS OF NORTH TROTTERNISH.

REFERENCES.1. BAJLEY, E. B. J924. The Desert Shores of the Chalk Seas. Geol,

Mag., vol. Ixi., pp. J02-J6.2. BRYCE, J. J873. On the Jurassic Rocks of Skye and Raasay.

Quart. Journ. Geol. Soc., vol. xxix., pp. 3J7-35I.3. FORBES, E. J85I. On the Estuary Beds and the Oxford Clay at

Loch Staffin, in Skye. Quart. Journ. Geol. Soc., vol. vii, pp. J04­JJ3·

4. JUDD, J. W. J878. The Secondary Rocks of Scotland, ThirdPaper. The Strata of the Western Coast and Islands. Quart.Journ. Geol. Soc., vol. xxxiv., pp. 660-743.

5. LEE, G. W. and PRINGLE, J. J93I. A Synopsis of the MesozoicRocks of Scotland. Trans. Geol. Soc., Glasgow, vol. xix.,pp. J5 8- 224·

6. MEMOIRS OF GEOLOGICAL SURVEY OF GREAT BRITAIN.J908. The Geology of the Small Isles of Inverness-shire. Sheet 60.

7. 1910. The Geology of Glen Elg, Loch Alsh and South-East Skye. Sheet 71.

8. J920. The Mesozoic Rocks of Applecross, Raasay andN.E. Skye. Sheet 81.

9. ----. J925· The Geology of the Country around Golspie,Sutherlandshire. Sheet 103.

10. 1930. The Geology of Ardnamurchan.II. MURCHISON, R. I. 1827. On the Coalfield of Brora in Sutherland­

shire, and some other Stratified Deposits in the North of Scotland.Trans. Geol. Soc., end Series, vol. ii., pp. 293-326.

12. J827. Supplementary Remarks on the Strata of theOolitic Series, and the rocks associated with them, in the Countiesof Sutherland and Ross, and in the Hebrides. Trans. Geol. Soc.,znd Series, vol. ii., pp. 353-368.

13. SPATH, L. F. 1932. Med. om Crenland, vol. lxxxvii., NO.7.14. WALKER, F. J932. "Differentiation in the Sills of Northern

Trotternish (Skye)." Trans. Roy. Soc. Edin., vol. lvii., pp. 24J­257·

DISCUSSION.Dr. J. W. ARKELL (communicated): I regret that I shall be unable to

hear Dr. MacGregor read his paper, and I write to congratulate him on thecompletion of a valuable piece of work. Of his several important results,that of outstanding interest is the detection in Skye of rocks of LowerKimeridge Clay age with well-preserved ammonites indicative of thethree lowest zones of the formation. This discovery and the simultaneousannouncement of the Rasenia zone in Mull recently published by Dr. Spath(Med. om Grenland, 1932, p. 149) are of obvious importance in the recon­struction of North British paleeogeography.

Concerning the Oxford Clay, Dr. MacGregor states that his earliestfossils of zonal significance are the Cardiocerates that he has shown me,of Upper Divesian date. The presence of older Oxfordian-Callovian stratain North Skye seems to be indicated, however, by the finding of at leastone Kosmocerate by the Survey at Staffin (figured by Buckman as K.degradatum) and by Murchison's record of A. kamigi at Monkstadt.

The very interesting marine fauna with Rhynchonetla, Modiola, etc.,found by Dr. MacGregor in the Upper part of the Great Estuarine Series,looks like nothing so much as a collection of Rh. concinna and its associatesfrom the Great Oolite of the South of England. The many fine specimensof Ostrea hebridica brought back by him allow it to be said definitely thatthat species is identical with our ubiquitous Ostrea sowerbyi. Moreover,Forbes, who named the Scottish oyster, forestalled Morris and Lycett bytwo years. If the rule of priority is to be obeyed, O. sowerbyi mustdisappear from the literature, as a synonym of O. hebridica,