Chapter 4 Jurassic: Oxford Clay to Kimmeridge Clay

The Belemnite Sands in North Skye are succeeded by a series of dark blue-grey shales with thin grey cementstones and lines of calcareous nodules. At the base the shales become bituminous but they are never sufficiently rich to be classed as oil-shales. The fauna consists predominantly of ammonites and belemnites, but at certain horizons are bands locally rich in lamellibranchs. Plant fragments, foraminifera and ostracoda are found throughout and occasionally a brachiopod.

A large collection of ammonites was made during the survey of the area and submitted for identification to the late L. F. Spath who gave valuable assistance in the delimitation of the zones into which the shales have been divided. Except for occasional specimens extracted from a cementstone the material is very badly preserved; nevertheless it has been found possible to apply with certain modifications demanded by the field evidence the zonal scheme which is in use in England. There can be no doubt that the blue shales above the Great Estuarine Series of North Trotternish are equivalent to the Oxford Clay, Corallian and the lower part of the Kimmeridge Clay of England, but they are unique in that the lithology of these beds in Skye is the same throughout, in marked contrast to the varied lithology which is the rule in England.

In the field these shales fall naturally into five divisions according to the ammonite faunas they contain. The zonal scheme is a subdivision of these major groups. With the possible exception of the Cardioceras cordatum Zone it is not easy to distinguish the individual zones except by detailed collecting, whereas the major divisions are recognizable without difficulty.

The following are the major divisions based on the ammonite succession:

e. Pictonia Beds

d. Amoeboceras Beds

c. Cardioceras Beds

b. Quenstedtoceras Beds

a. Kosmoceras Beds.

The zonal scheme adopted in this memoir is: (Table Unumbered 1)

Oxford Clay

The Oxford Clay in North Skye is very like its equivalent in Dorset, a dark blue-grey clay with grey cementstones and lines of concretionary nodules. Its presence in Skye was first recognized by Forbes (1851, p. 106) who described a section in Staffin Bay obviously not far from the locality here referred to as Point 3 [NG 473 698], (Figure 9). He stated that overlying the Loch Staffin Beds he found 'numerous specimens of Ammonites cordatus and Belemnites Owenii and Beaumontianus indicating the age of these shales beyond a question to be that of the Oxford clay'.

These beds contain ammonites and belemnites in great abundance and in addition: Astarte cf. cordata Trautschold, Nuculana sp. nov., Terebratula cf. subsella Leymerie and Pholadomya sp. Arkell (1945) recorded Cardioceras ashtonese Arkell, C. costicardia S. S. Buckman, C. (Scarburgiceras) excavatoides Maire and Peltoceras gerberi (Prieser) from the Oxford Clay of Staffin Bay but only the second species in this list was found in recent collecting.

On the basis of the ammonite fauna the Oxford Clay has been divided into the following zones:

i. Zone of Kosmoceras jason. The K. jason Zone has been recognized at Points 3 [NG 473 698), 4 (NG 473 701) and 5 (NG 472 708) in Staffin Bay<ref>Reference points in Staffin Bay are indicated in Figs. 8–12.</ref>. It is represented by about 20 ft of shale with ammonites determined as Kosmoceras (Gulielmites) spp . some of which are allied to K. conlaxatum S. S. Buckman (K. conlaxatum S. S. Buckman = K. jason Reinecke sp.).

ii. Zone of Kosmoceras castor and K. pollux. The 6–7 ft of shale succeeding the K. jason Zone at Point 5 contain Kosmoceras (Gulielmites) sp. with Kosmoceras (Spinikosmoceras?) sp. and according to Spath represents the Zone of K. castor and K. pollux = the reginaldi Zone of A. Morley Davies (1916) = the coronatum Zone of Buckman = the elizabethae Zone of Neaverson (1925). (The name coronatum Zone proposed by d'Orbigny in 1825 has priority.)

iii. Zone of Peltoceras athleta. The succeeding 3 ft of shale at Point 5 contains Kosmocerates of the proniae type and may therefore be tentatively correlated with the P. athleta Zone which is here taken to be the equivalent of the duncani and athleta zones of Morley Davies and Neaverson, the duncani Zone of Spath 1926, the spinosum = ornatum Zone of Buckman (1913), the proniaeathleta zone of Pringle (1930), and the athleta–duncani zone of Arkell (1933).

iv. Zone of Quenstedtoceras lamberti. At Points 3, 4 and 5 in Staffin Bay this zone is represented by 4 ft of shale with Kosmocerates of the 'Bikosmoceras' deficiens group of S. S. Buckman, Quenstedtocerates like Q. gregarium S. S. Buckman, Eboracicerates like E. cadiforme S. S. Buckman, and at the base forms resembling Kosmoceras spinosum (J. de C. Sowerby) ( = ornatum Douvillé). The fauna is suggestive of the lamberti Zone as developed in Dorset.

v. Zone of Quenstedtoceras mariae. The next 24 ft of shale at Points 3, 4 and 5 have been referred to the Q. mariae Zone. In the lowest 6 ft the fauna belongs to the Quenstedtoceras group (see p. 62) whereas the uppermost 18 ft contain a quite distinct Cardioceras fauna, so that the zone is readily divided into two subzones.

a. Subzone of Quenstedtoceras vertumnus.

Six feet of shale containing Quenstedtoceras (Vertumniceras) vertumnum (Leckenby), Quenstedtocerates allied to Q. lamberti (J. Sowerby) and Poculisphinctes sp.

b. Subzone of Cardioceras praecordatum.

Eighteen feet of shales containing Cardioceras (Scarburgiceras) praecordatum (R. Douville), Cardioceras sp. like a large C. costellatum—anacanthum and Cylindroteuthis oweni (Pratt). This fauna may be compared with that found at Warboy's Pit, Huntingdonshire.

vi. Zone of Cardioceras cordatum. The uppermost beds of the Oxford Clay in Staffin Bay consist of 60 ft of shale and as exposed at Points 3, 4 and 5 contain a rich fauna of Cardiocerates indicative of the C. cordatum Zone. It is here divisible into two subzones :

a. Subzone of Cardioceras cardia (= lower cordatum).

Thirty feet of shale with Cardioceras cf. costicardia S. S. Buckman, C. cf. costellatum S. S. Buckman, C. cf. cardia S. S. Buckman, C. cf. cordatum auctorum, C. cf. rouilleri (Nikitin) Lahusen, and Peltoceratoides aff. williamsoni (Phillips). This subzone probably represents the equivalent of the uppermost Oxford Clay at Studley Brickyard and of Dorset, and of the Lower Calcareous Grit of Yorkshire.

b. Subzone of Cardioceras excavatum (= upper cordatum).

Thirty feet of shales with Cardioceras excavatum (J. Sowerby), C. cf. costellatum S. S. Buckman, C. cf. nikitianum (Lahusen), C. cf. papilioniense Boden, C. cf. tenuicostatum (Nikitin) and C. cf. coelatum Pavlov. The fauna is very like that of the C. excavatum limestone of Ardassie Point. This subzone is probably the equivalent of the Lower Calcareous Grit of Wiltshire and approximates to the Zone of Ammonites catena Arkell 1936.

Details

The most complete and least disturbed section of the Oxford Clay is that to be found at Point 5, Staffin Bay, for though several small faults cross the beach they are parallel to the dip of the beds and do not cause any great dislocation of the strata (see section p. 52). At Points 3 and 4 the Oxford Clay is disturbed by small faults and dykes, especially at Point 4 where the Kildorais Fault ends in numerous virgations which split up the strata into wedge-shaped fragments.

Section in the beach of Staffin Bay, midway between Points 3 [NG 473 698] and 4 [NG 473 701]

The section recorded above is seen in the beach between high-and low-water marks but higher beds are exposed in a small cliff just above high-water mark at Point 3. They are as follows:

At the southern end of this section the beds are disturbed by a small fault and a few yards to the south of this a larger fault occupied by a dyke brings the Upper Oxford Clay on its northern side against Middle Corallian shales on the south. There must be 5–6 ft of strata unexposed between the top of the Staffin Bay beach section and the beds seen in the small cliff.

Owing to the disturbed nature of the section at Point 3 systematic collecting is difficult and it was not found possible to establish all the zones found to be present to the north at Point 5. The general sequence however is identical at the two localities.

From the base of the blue shales (Bed 7, Staffin Bay section) Kosmoceras (Gulielmites) aff. conlaxatum S. S. Buckman was collected but is not at all common and 16 ft higher Kosmoceras (Gulielmites) sp., Pseudocadoceras (Longaeviceras ?) sp. and Hibolites hastatus (Blainville) were found. Thus approximately the lowest 20 ft of these shales may be regarded as belonging to the K. Jason Zone.

A few feet above the two cementstones (i.e. at the base of Bed 11, Staffin Bay section) Quenstedtoceras (Prorsiceras), cf. gregarium (Leckenby), Eboraciceras cf. cadiforme Buckman and the outer whorls of large Kosmocerates such as are found at Tidmoor Point Weymouth, indicate the presence of the Q. lamberti Zone which cannot however occupy more than a few feet of shale, for 6 ft above the upper of the two cementstones the Q. maria Zone (Q. vertumnus Subzone) is indicated by the presence of Quenstedtoceras (Vertumniceras) vertumnus (Leckenby), Quenstedtoceras cf. lamberti (J. Sowerby) Lahusen pars., and a doubtful Poculisphinctes sp. Cardioceras cf. praecordatum (R. Douville) occurs a few feet above the red weathering cementstone (Bed 12, Staffin Bay section) so that the lower part of Bed 13 is in the C. praecordatum Subzone of the Q. mariae Zone. The upper half of Bed 13 near the top of the beach section is crowded with ammonites mainly Cardiocerates. The following forms have been collected here:

Cardioceras cf. cardia S. S. Buckman, C. spp. (cardia group), C. cf. costicardia S. S. Buckman, C. cf. cordatum auctorum, C. cf costellatum S. S. Buckman, C. spp. (costellatum group), C. cf. excavatum Lahusen non Sowerby?, C. spp. (nikitinianum group), C. spp. (praecordatum group), C. cf. rouillieri (Nikitin) Lahusen, Peltoceratoides aff. williamsoni (Phillips), Peltoceratoides sp., Perisphinctes (Pachyplanulites?) sp., (Properisphinctes?) cf. indogermanus Boden non Waagen, Cylindroteuthis sp., and Pachyteuthis aff. panderiana (d'Orbigny) auctorum.

This fauna is taken to indicate the C. cardia Subzone of the C. cordatum Zone. The higher beds in the cliff at Point 3 (Beds 14–19) yielded a fauna indicating the upper C. cordatum Zone (C. excavatum Subzone): Cardioceras (Scoticardioceras?) cf. excavatum auctorum, C. sp. (cordatum group), C. cf. cordatum (J. Sowerby), C. cf. schellwieni ? Boden, C. sp. (schellwieni group), C. cf. papiloniense Boden.

This section of the Oxford Clay can be traced northwards along the beach to Point 4 where the beds are even more badly shattered by faulting and indurated by dykes. At the two most northerly dykes at Point 4 higher beds are exposed than any at Point 3. Seawards and slightly north of these dykes about 60 ft of Oxford Clay is seen in the beach at low tide. Almost the whole of the C. cordatum Zone is exposed and though the ammonites are not particularly well preserved the two subzones C. cardia and C. excavatum can be recognized. The C. cardia Subzone, extending from a cementstone at low-water mark upwards through 30 ft of strata contains a finely ribbed form of Cardioceras (Scarburgiceras) praecordatum (R. Douville), C. cf. cardia S. S. Buckman and Cardiocerates probably of the costellatum—anacanthum group. Above, the C. excavatum sub-zone, 20–30 ft thick, contains an evolute variety of Cardioceras (Scoticardioceras?) cf. excavatum (J. Sowerby) and several Cardiocerates which appear to be the inner whorls of forms belonging to the excavatum group.

Northwards of Point 4 the Oxford Clay extends along the beach to Point 5 where very good sections are to be seen (see section p. 52). As at Point 3 the lowest 12 in of blue shale contains a Kosmocerate fauna indicative of the K. Jason Zone. The ammonites are badly preserved but may be referred to Kosmoceras (Gulielmites) sp. Immediately below the two cementstones (i.e. at the top of Bed 18, section p. 52) the shales contain Kosmoceras (Gulielmites) sp. and Kosmoceras (Spinikosmoceras?) sp. which suggest the presence of the Zone of K. castor and K. pollux. The shales immediately above the two cementstones probably belong to the Peltoceras athleta Zone for they contain poorly preserved Kosmoceras sp. (cf. proniae var. crassa R. Douville). Three to four feet higher the Q. lamberti Zone is indicated by Quenstedtoceras sp., Kosmoceras cf. spinosum (J. de C. Sowerby = ornatum R. Douville) and Eboraciceras sp. The zones of K. castor and K. pollux, P. athleta and Q. lamberti are very poorly developed and together occupy less than 15 ft of shale.

From 7 ft above the upper of the two cementstones, for a thickness of 22 ft, the shales contain ammonites of the Q. mariae Zone. Here the Q. vertumnus Subzone is obscure, containing only indeterminate Quenstedtoceras but the upper 18 ft have yielded Cardioceras (Scarburgiceras) spp. (praecordatum group), Quenstedtoceras. sp. and Cylindroteuthis oweni (Pratt) and therefore belong to the C. praecordatum Subzone.

The top of the Q. mariae Zone may be conveniently taken at a 4 to 5-in cementstone (Bed 25, section p. 52), above which for the next 30 ft the shales contain a fauna indicative of the C. cardia Subzone (lower C. cordatum Zone) i.e. Cardioceras cf. costellatum S. S. Buckman, Cardioceras sp. (costellatum group), Cardioceras sp. (praecordatum group) and Peltoceratoides aff. williamsoni (Phillips).

At the northern end of the Point 5 section the uppermost beds of the Oxford Clay are exposed, i.e. the C. excavatum Subzone (upper C. cordatum Zone). The ammonites are badly preserved but the following forms can be recognized:

Cardioceras aff. costellatum S. S. Buckman, C. cf. cordatum (J. Sowerby), C. cf. excavatum (Sowerby) Boden pars., C. sp. (excavatum group), C. cf. nikitinianum (Lahusen) Boden, and C. cf. papiloniense ?Boden.

A small exposure a few yards inland from High Water Mark at Point 8 [NG 467 723], has yielded ammonites indicative of the uppermost Oxford Clay. They are badly preserved Cardiocerates probably of the C. excavatum group.

At the seaward end of a small burn entering the south-west corner of Staff n Bay between Glashvin and Digg, blue-grey shales are exposed which contain obscure Cardiocerates with Pholadomya sp. and Terebratula sp. Apart from these there are no inland exposures of undoubted Oxford Clay but a small patch of blue clay 300 yd N.E. of the cross-roads at Brogaig may belong to this division. The lowest 5 ft of drift in Brogaig Burn north of the main road contain pebbles of shale with Cardioceratid and Perisphinctid fragments and belemnites so that this stream probably flows over Oxford Clay before it descends to the raised beach.

Where Lon a' Mhuilinn joins Lon Glas and turns to flow northwards, is a small exposure of grey shale in the east bank of the stream in which a fragment of belemnite was found. This is probably an inclusion of Oxford Clay in the sill which is here transgressing upwards.

Bryce (1873, p. 332) thought that Oxford Clay was present at Duntulm but a careful search of this neighbourhood has so far failed to find any trace of it.

Oxford Clay is almost certain to be present on the western side of the valley running from Lincro to Knockhoe according to the fossils recorded by Murchison from the canal cut to drain Loch Chaluim Chille (see p. 54), but it is not now exposed.

Blue shale in the screes below the lava scarp south of Totscore probably indicate the presence of Oxford Clay under the superficial deposits.

Corallian

The Corallian beds in Skye are exceptional in that they consist entirely of blue shales with exactly the same lithology as the Oxford Clay below and the Kimmeridge Clay above. As exposed in Staffin Bay the Corallian shales are 140 ft thick and have been divided into the following ammonite zones:

Upper Corallian. Ringsteadia anglica Zone

Middle Corallian. Perisphinctes variocostatus Zone Lower Corallian. Cardioceras cawtonense Zone

(i) Zone of Cardioceras cawtonense.

The lowest 40 ft of the Corallian contains a Cardiocerate fauna and is exposed for some distance south of Point 1 [NG 474 694], and north of Point 4 [NG 473 701], Staffin Bay. The Zone is poorly exposed and the ammonites are badly preserved but the fauna certainly includes a large number of Cardiocerates which can be referred to C. cawtonense (Blake), and C. maltonense (Young and Bird), together with Cardioceras (Cawtoniceras) aff. blakei Spath, and C. (Cawtoniceras) cf. zietini (Rouilier) auctorum. Correlation is suggested with the Headington Shell Bed and the lowest 18 ft of the Ampthill Clay at Gamlingay.

(ii) Zone of Perisphinctes variocostatus.

The middle Corallian consists of 40 ft of shales exposed at Points 1 [NG 474 694], 2 [NG 473 696], 3 [NG 473 698] and 7 [NG 468 715] (Figure 11) in Staffin Bay and at Flodigarry. The Cardiocerate fauna of the zone below has been succeeded by abundant Amoebocerates such as A. alternans, and forms like A. serratum (J. Sowerby), A. marchense Spath, A. vagum Ilovaisky, and Perisphinctids comparable with P. variocostatus auctorum, and P. dichotomum (S. S. Buckman). These beds are probably the equivalent of the Middle Ampthill Clay and the Trigonia Beds of Dorset.

(iii) Zone of Ringsteadia anglica.

The uppermost 50 ft of the Corallian exposed in Staffin Bay at Points 1, 2 and 7 is referred to the R. anglica Zone. Ammonites are very abundant and consist predominantly of Amoebocerates.

The shales contain numerous individuals of Amoeboceras (Prionodoceras) serratum (J. Sowerby), A. (Prionodoceras) aff. serratum (J. Sowerby), A. (P.) alternans of Russian authors, A. (P.) cf. alternans (Nikitin) and A. (P.) cf. excentricum (S. S. Buckman), with an occasional Ringsteadea sp., Pictonia sp. and Prorasenia sp. (witteana group).

Details

Corallian shales are extensively if not well exposed on the shores of Staffin Bay. At the boat-slip south of Point 1 shales of the C. cawtonense Zone are faulted against Mottled Clays. The strike is here north-east, parallel to the direction of the fault but as the beds approach Point 1 they swing round parallel to the coast-line, bend round the irregular intrusive mass of dolerite at Point 1 and continue northwards to within a few yards of Point 3. A very large boulder on the south side of the boat-slip stands on C. cawtonense shales. From here the following fossils have been obtained: Cardioceras (Cawtoniceras?) sp., Perisphinctes (Dichotomosphinctes?) sp., C. (Cawtoniceras?) cf. zietini (Rouillier) auctorum, Gryphaea dilatata auctorum, Parallelodon cf. aemulum (Phillips), Pleuromya alduini (Brongniart), Pholadomya sp.

Nearer Point 1 higher beds are exposed and though somewhat disturbed and altered by the intrusion, the P. variocostatus and R. anglica zones can be recognized. Shales belonging to the former zone are seen near low-water mark and contain: Amoeboceras (Prionodoceras) alternans (Nikitin) and A. (P.) marchense Spath. Near the seaward margin of the storm-beach shales with Amoeboceras (Prionodoceras) spp. of the ogivale—marchense group and forms suggestive of A. (P.) semicoelatum Spath indicate the lower part of the R. anglica Zone.

Burnt shales exposed on the footpath over the cliff spur at Point 1 are probably Kimmeridge in age.

In the beach at Point 2 the upper half of the Corallian is well exposed.

Section in the beach immediately north of Point 2 [NG 473 696], Staffin Bay

The lowest beds, which are only exposed at very low tides, contain a rich ammonite fauna indicative of the P. variocostatus Zone. Below a 2 ft 6 in leaf of sill the following were collected: Amoeboceras (Prionodoceras) alternans auctorum (Nikitin 1916, pars) A. (P.) alternans of Russian authors (= aff. freboldi Spath) A. (P.) alternans Sokolov 1912, A. (P.) aff. marchense Spath, A. (P.) cf. marchense Spath, A. (P.) sp. (serratum group).

From the 5 ft of shales above the sill an abundant fauna was obtained: Amoebaceras (Prionodoceras) alternans Ilovaisky 1904 (= aff. freboldi Spath), A. (P.) aff. alternans of Russian authors, (trans. to serratum group), A. (P.) aff. marchense Spath, A. (P.) sp. (prionodus—excenticus group), A. (P.) cf. serratum (J. Sowerby), A. (P.) aff. serratum (J. Sowerby), Perisphinctes sp. (biplex group, cf. variocostatus auctorum), P. cf. dichotomus (S. S. Buckman), P. sp.

Bed 5, 22 ft higher, contains a fauna clearly indicating the R. anglica Zone: Amoeboceras (Prionodoceras) aff. alternans of Russian authors (= regulare Spath), A. (P.) cf. alternans of Russian authors (= regulare Spath), A. (P.) aff. semicoelatum Spath, A. (P.) sp. (serratum group).

Between an 8-in sill and the cementstone 15 ft above (Bed 6) the shales yielded: Amoeboceras (Prionodoceras) sp. juv. (inner whorls of A. prionodes group) and Pictonia sp. The following species were obtained from above the cementstone (Bed 8): Amoeboceras (Prionodoceras) aff. alternans of Russian authors (= aff. freboldi Spath), A. (P). cf. alternans of Russian authors, cymodoce cf. freboldi Spath, A. (P.) alternans of Russian authors (= aff. regulare Spath), A. (P.) spp. (inner whorls of regulare, freboldi, excentricum etc.), A. (P.) sp. juv. (inner whorls of forms of regulare—excentricum group).

In dark grey shales immediately below a cementstone (Bed 10) the basal fauna of the Kimmeridge Clay (P. baylei Zone) is represented by Pictonia cf. cymodoce Salfeld and Pictonia sp. (normandiana–latecostata group). The sequence here (see section p. 66) agrees very closely with that at Kildorais (Point 7).

From Point 2 the Corallian shales cross a small bay, on the north side of which they can be seen faulted against Oxford Clay by a N.N.W. fault, the site of which is marked by a large dyke.

North of Point 4 [NG 473 701] two large dykes exposed in the storm-beach penetrate lower Corallian shales. The beds are badly burnt by the dykes; thus the fossils are poorly preserved so that specific determination of individuals is difficult. There is no doubt, however, that the fauna includes Cardioceras (Cawtoniceras) aff. blakei Spath, C. cawtonense (Blake) and C., maltonense (Young and Bird) with Pachyteuthis aff. panderiana (d'Orbigny) auctorum, Lima (Plagiostoma) laeviuscula J. Sowerby, Inoceramus sp. (near nitescens Arkell), Gryphaea sp., Perampliata ampliata (Phillips), and Parallelodon (Beushausenia) cf. aemulum (Phillips). Out of the large number of specimens collected, the two species Cardioceras cawtonense and C. maltonense were so markedly predominant that these lowest beds of the Corallian have been separated off as the zone of C. cawtonense ( = perarmatum Zone of Spath 1933, table ii, p. 872).

A small triangular mass of Corallian of the R. anglica Zone is faulted in just south of Point 6 [NG 470 712]. In the few feet of shale exposed Amoeboceras (Prionodoceras) cf. alternans of Russian authors and A. (P.)sp. (ogivale–marchense group) were obtained.

Between Points 6 and 7 [NG 468 715] the whole of the R. anglica Zone and the basal beds of the Kimmeridge Clay are well exposed in the beach. A typical section is that seen a little way south of Kildorais (Point 7).

Section in beach at Kildorais, Point 7 [NG 468 715], Staffin Bay

Beds 1 and 2 of this section are exposed near low water-mark and represent the upper part of the P. variocostatus Zone. They contain the following ammonites: Amoeboceras (Prionodoceras) aff. alternans (Nikitin) (= aff. regulare Spath), A. (P.) cf. marchense Spath, A. (P.) cf. vagum (Ilovaisky), A. (P.) aff. serratum J. Sowerby sp. (cf. Spath 1935, p. 74).

Above the lower pair of cementstones (Bed 7) the shales contain a fauna indicative of the lower part of the R. anglica Zone: Amoeboceras (Pionodoceras) cf. alternans (Nikitin) of Russian authors, A. (P.) cf. excentricum (S. S. Buckman), A. (P.) sp. (prionodes group), A. (P.) serratum (J. Sowerby), A. (P.) aff. serratum (J. Sowerby), A. (P.) sp. (serratum group), Ringsteadia sp. (cf. Spath, 1935, pl. 9, fig. 2).

In the shales 3–5 ft above the band of calcareous concretions (Bed 10) are ammonites indicating the middle of the R. anglica Zone: Amoeboceras (Prionodoceras) sp. (cf. alternans Nikitin = regulare Spath), A. (P.) aff. serratum (J. Sowerby), A. (P.) sp. (serratum group), Pictonia ?, Prorasenia sp. (witteana group).

Immediately below the two upper cementstones (upper two feet of Bed 11) the shales contain a fauna indicating the P. baylei Zone of the Kimmeridge Clay.

In the lower part of this section the dip is westwards at 30–40° but the beds turn over rapidly and at the top of the section are practically vertical.

At Point 7 an E.N.E. fault, the position of which is marked by a dyke, brings Corallian on the southern side against Kimmeridge Clay to the north.

Between Points 7 and 8 [NG 467 723] several small leaves of sill cross the beach usually with a little burnt shale on either side. Some of this shale must be of Corallian age but the fossils are altered beyond recognition.

Inland the exposures of Corallian shale are few and poor. In the grounds of Flodigarry Hotel is a small quarry showing a lenticle of indurated shale in dolerite sill. The beds contain fragmentary and much altered ammonites referable to Amoeboceras (Prionodoceras) aff. serratum (J. Sowerby) and indeterminate Perisphinctids and therefore probably belong to the R. anglica Zone.

In a small roadside quarry west of Flodigarry Hotel and across the main road are several small exposures of much weathered and altered shale. Some of these beds have yielded a fauna which suggests a horizon not met with elsewhere in North Skye, i.e. the A. alternans Zone of Spath (1935, p. 74), which in the zonal scheme here adopted would be at the base of the P. variocostatus Zone or at the top of the C. cawtonense Zone. This fauna consists of: Amoeboceras sp. (alternans group), A. cf. tuberculato-alternans (Nikitin), Perisphinctid, Pictonia ?, Prorasenia ? sp. (witteana group), Ringsteadia?, Astarte sp., Lima (Plagiostoma) sp., Nucula ?, Oxytoma sp. and Pholadomya sp.

The dark shales exposed in the north-west side of a cart road 250 yd S.W. of Kilmaruy burial ground north of Brogaig Burn are probably Corallian in age but are too badly weathered and fragmentary to yield fossils.

Corallian shales do not appear to be exposed north of Flodigarry or on the west side of the Trotternish peninsula, north of Uig Bay.

Near Uig uppermost Corallian Beds are exposed near a small waterfall in Lon an t-Sratha at Balnaknock at the eastern end of the River Conon inlier. About 15 ft of shales contain abundant examples of Amoeboceras (Prionodoceras) sp. juv. These appear to be the inner whorls of A. excentricum, A. serratus and allies with perhaps also A. (P.) alternoides Spath or A. freboldi Spath with the lamellibranch Pleuromya alduini (Brongniart). An upper Corallian (R. anglica Zone) horizon is indicated. Some 8 ft of shales above are basal Kimmeridge in age (P. baylei Zone; p. 71).

On the south side of Ru Idrigal are shales partly incorporated in sill and extensively altered but yielding an ammonite fauna indicative of the C. excavatum sub-zone, i.e. Cardioceras (Scoticardioceras) excavatum (J. Sowerby) C. cordatum auctorum, C. (Plasmatoceras) cf. tenuiostatum (Nikitin), C. praecordatum?, C. cf. anacanthum costellatum S. S. Buckman, Perisphinctid (cf. P. sayni de Riaz, Borissjak), C. (Vertebriceras?) sp., with the belemnite Cylindroteuthis oweni (Pratt) and the lamellibranchs Pholadomya sp., Camptonectes sp. and Parallelodon aemulum (Phillips).

On the south side of Uig Bay, about mile south of the mouth of the River Conon, beds of this same zone are again seen on the beach. The horizon may be a little different, for although Cardioceras (Scoticardioceras) excavatum is still the dominant form, there are examples of species not seen at Idrigil Point, i.e. Cardioceras cf. persecans S. S. Buckman and C. cf. caelatum Pavlow. C. cf. tenuicostatum (Nikitin) and C. cordatum auctorum also occur.

Kimmeridge Clay

Only the lowest beds of the Kimmeridge Clay now remain in North Trotternish. Most of the outcrop is hidden beneath the Quirang landslip or is covered by Tertiary lavas. Much of the shale is indurated and distorted by an upper leaf of the dolerite sill which at the northern end of the syncline transgresses upwards to occupy a position 30–40 ft above the base of the Kimmeridge Clay. Such fragments as still remain have special interest for it is here that the P. baylei Zone has what is probably its finest development in Great Britain, occupying a minimum of 20 ft of shale, a thickness which by further collecting might be found to be much greater.

The Kimmeridge Clay follows the Corallian with no change of lithology except that cementstone bands and nodules are rather more abundant than in the beds below. The change of horizon is indicated only by the change in ammonite fauna.

The most complete section is that at Kildorais, Point 7 [NG 468 715] in Staffin Bay. Beds 11–15 (Section p. 67) are exposed near high tide-mark and have yielded a rich ammonite fauna consisting predominantly of Pictonia spp. with some species of Amoeboceras. Belemnites are very abundant. The almost vertical beds owe their inclination to the easterly thrust of the moving landslip. They contain several bands of calcareous concretionary nodules, usually in the form of septaria, and two impersistant cementstones.

In the two feet of shale immediately below the lower cementstone (Bed 11) the following have been collected: Pictonia aff. bigoti Tornquist, Pictonia sp. (cf. bigoti Tornquist), Prorasenia sp. (hardyi group?), Prorasenia sp. (witteana group), Goniomya literata (J. Sowerby), Oxytonia expansa (Phillips) and Pholadomya cf. aequalis (J. de C. Sowerby).

Between the two cementstones (Bed 13) ammonites are very abundant though badly preserved. From here the following forms were obtained :

Amoeboceras (Prionodoceras) cf. banhini Ilovaisky non Oppel, A. (P.) cf. alternans (Nikitin) auctorum = regulare Spath?, A. (Amoebites?) sp., Pictonia cf. bigoti Tornquist, P. aff. bigoti, P. cf. normandiana (Tornquist), P. cf. parva Tornquist, P. cf. laticostata Tornquist, P. aff. laticostata, P. sp. (laticostata–normandiana group), P. sp. (normandiana group) P. aff. baylei Salfeld, P. aff. costigera S. S. Buckman, Prorasenia sp., Astarte subdepressa Blake and Hudleston.

For 15 ft above the two cementstones (Bed 15) belemnites are abundant and ammonites fairly common but are crushed and difficult to extract. The fauna indicates: Amoeboceras (Amoebites?) sp. (beaugrandi (Sauvage) group), Pictonia aff. baylei Salfeld, P. cf. laticostata, P. cf. parva, Prorasenia sp., Astarte contejeani de Loriol, A. subdepressa Blake and Hudleston, and Oxytoma expansa (Phillips).

So far no fossils have been found above this horizon though the shales continue for another 20 ft before disappearing below landslip.

North of the fault at Point 7 [NG 468 715], Kimmeridge Clay underlies the beach for some distance but is poorly exposed. At Point 6 [NG 470 712], south of a large dyke and beneath a mass of slipped Palagonite Tuff-breccia, a few feet of blue shale are exposed containing obscure ammonites not specifically determinable. They are predominantly Pictonia however, which suggests a horizon near the base of the Kimmeridge Clay. Further south at Point 2 [NG 473 696], about 20 ft of Kimmeridge Clay are exposed beneath a leaf of sill (Section p. 66). The shale is poorly fossiliferous and much disturbed by the intrusion, but Pictonia cf. cymadoce Salfeld and P. sp. (normandiana–laticostata group) are present and indicate a horizon in the P. baylei Zone.

Inland a wedge of Kimmeridge Clay is faulted in amongst the Great Estuarine strata exposed in the Kilmaluag River. Three small exposures in the eastern bank below Kendrom show grey shale with an 18-in cementstone. The numerous ammonites collected from the latter include: Amoeboceras (Prionodoceras) cf. alternans Nikitin, A. (P.) sp. (?cf. bigoti Tornquist) and Pictonia spp. suggesting a horizon in the P. baylei Zone.

North-west of Kendrom in the west bank of the river, a small exposure of clay is probably at about the same horizon but appeared to be unfossiliferous. Shales belonging to the P. baylei Zone are also exposed beside a small waterfall at Balnaknock in the Lon an t-Sratha, a tributary of the River Conon.

About 25 ft of shale are exposed including a 6-in cementstone. The uppermost 8 ft of this section contain Amoeboceras (Prionodoceras) sp. and Pictonia aff. baylei (Salfeld); the lower 15 ft represents the upper part of the R. anglica Zone. (p. 70).

References

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