(You can download this educational site to SurfOffline, WebCopier or similar software to keep a safe permanent offline copy, but note that at present there is periodic updating of the live version.)

Safety

There are significant risks on this coast section to the west of Staithes harbour. Please read the Safety Webpage , as the dangers with regard to rock falls etc. discussed there apply to these cliffs. A major problem at this particular location is the tide. The exposures and views of the cliff are on a wave-cut platform, as will be clear from the photographs in this webpage. The foot of the cliff is reached by the mid-tide in places so study can only be made in safety on a falling tide. Tide tables need to be consulted. Most of the cliffs are vertical or nearly vertical and they contain much unstable shaley material. Hard hats are essential and the foot of the cliff should not be approached in most places. The rock platform is extremely slippery with algae in places, especially immediately to the west of Staithes harbour wall. Great care is needed to prevent a backward fall. Adequate clothing, a rucksack and a hard hat can reduce chance of injury in the case of such a fall. At Old Nab there is an unstable upper cliff of the Whitby Mudstone Formation (shales) from which much debris has already fallen. Rock platforms at the foot of the cliff have very good trace fossils but there is danger here and great care must be taken.

INTRODUCTION

Location Map and Geological Maps

The left hand map shows the location of Staithes on the Yorkshire Coast. The central map, modified after Rawson and Wright (2000) is a slightly simplified guide to the exposures of the main units of the Staithes Sandstone Formation and Cleveland Ironstone Formation on the shore in the Staithes area as ledges or scars. The right-hand map, based on the work of Howarth (1955) contains more detail around Penny Nab and Old Nab, but note that it is a map of subzones rather than lithostratigraphic units. It is useful, however, not only for palaeontological studies but also for observing structural features such as faulting and the axes of a small syncline and an anticline. Notice that there is an orientation difference between this and the central map and corrections need to be applied to any directional readings made from it.

Exercise for students: Using the more detailed, subzonal, geological map, briefly discuss the relationship of the headlands of Old Nab and Penny Nab to the structural features.

Cleveland Ironstone - Introduction

There were initial attempts to work parts of the Cleveland Ironstone Formation from the coastal exposures in the 19th century but they were unsuccessful apparently because of poor selection of ore and contamination(Hemingway, 1974b). Systematic working of the Domerian ironstone begun t Grosmont in 1839 and continued there until about 1892. The Main Seam (discussed below) was worked systematically at Skinningrove in 1848 and the Eston Hills in 1851. From this time onwards Jurassic ore production exceeded that from the Carboniferous. Transport to the blast furnaces of the growing Teesside towns was easy and short and iron ore production soared. By 1870 more than 4 million tons of ore were raised annually in Cleveland, which increased to 6 million tons by 1875, and this massive production dominated British bedded ore production for almost fifty years.

Exploitation of the Main Seam moved progressively southward but there was a steady southward deterioration in the iron content. There were also increased haulage distances to the iron works. Production diminished to less than half after 1920 and ceased in 1964, leaving untouched reserves of about 232 million tons in the Cleveland area (Hemingway, 1974b).

Of the iron-rich minerals in the ore pure siderite contains 48.2 per cent metallic iron while chamosite contains about 32 per cent. Variations in the proportions of the various minerals across the ore field are clearly important. The proportion of clastic material such as quartz sand is another variable (Hemingway, 1974b) .

The cliff section at Staithes which is discussed here consists mainly of shale but with ironstone beds or "seams" at intervals. There has been mining in these cliffs back in the past and examples of this will be seen at Old Nab.

STRATIGRAPHY:

General Succession and Simplified Cliff Section

This simplified table is intended to show the general Lower Jurassic (Hettangian to Toarcian) succession with the Cleveland Ironstone forming the middle of it in Yorkshire. The following parts of this webpage will mostly be concerned with the Cleveland Ironstone and to some extent with the Staithes Sandstone. The Whitby Mudstone Formation with Alum Shales occurs in the cliff above but is not accessible here from the beach and is only considered briefly. The table is modified after Rawson and Wright (2000) , which should be consulted for further details.

STAGE/SUBSTAGE

AMMONITE ZONE

FORMATION

TOARCIAN (Upper Lias) (Top - 182 Ma)

Dumortieria levesquei   Grammoceras thouarsense Haugia variabilis Hildoceras bifrons Harpoceras falciferum Dactylioceras tenuicostatum

Blea Wyke Sandstone Formation   Whitby Mudstone Formation (with Alum Shale Member in the middle)

UPPER PLIENSBACHIAN - DOMERIAN (Middle Lias)

Pleuroceras spinatum Amaltheus margaritatus

Cleveland Ironstone Formation

LOWER PLIENSCHACHIAN - CARIXIAN (Lower Lias - continues down)

Prodactylioceras davoei   Tragophylloceras ibex Uptonia jamesoni

Staithes Sandstone Formation   Redcar Mudstone Formation

UPPER SINEMURIAN

Echioceras raricostatum Oxynoticeras oxynotum Asteroceras obtusum

ditto

LOWER SINEMURIAN

Caenesites turneri Arnioceras semicostatum Arietites bucklandi

ditto

HETTANGIAN (base - 204 Ma)

Schlotheimia angulata Alsatites liasicus Psiloceras planorbis

ditto

Notes: The zonal correlations shown here in simplified and approximate form. In fact, the Staithes Sandstone Formation does not equate exactly with the Prodactylioceras davoei Zone. The base is a little above the base of that zone and the uppermost part is in the Amaltheus margaritatus zone above the top of the davoei Zone. The Cleveland Ironstone Formation includes most of the Amaltheus margaritatus Zone, but not the basal part, and almost all of the Pleuroceras spinatum Zone, finishing a very short distance below the top of the zone. The Cleveland Ironstone Formation is about 29m thick with the Penny Nab Member beneath at 19m and the Kettleness Member above at 10m. See Rawson and Wright (2000) for more information.

This very generalised cliff section shows the geology of the coast around Staithes. The field trip described here is concerned with the Middle Lias of the Lower Jurassic. It is specifically on the upper part of the Staithes Sandstone Formation and on the Cleveland Ironstone Formation to the east of Staithes Harbour on to Old Nab. Here the Main Seam at the top of the formation reaches sea-level. The Upper Lias, including the Jet Rock and the Alum Shales is well-developed in the higher parts of the cliffs but is not discussed in detail here (but see Rawson and Wright (2000) ). Fallen debris from this beds may be found at the foot of the cliffs in places such as Old Nab.

STRATIGRAPHY:

Succession within the Cleveland Ironstone Formation at Staithes

Two stratigraphic logs are given here of the Cleveland Ironstone Formation between Staithes Harbour and Old Nab. The left-hand one provides a slightly simplified sequence that can be easily recognised in the field. The more-detailed right-hand one is primarily for reference purposes. It is after Howarth (1955) and has the horizons of ammonite occurrences (these ammonites are not very abundant in the field). Some shell beds are indicated.

In general, the Cleveland Ironstone sequence here is varied sedimentologically and is very fossiliferous. Although it is thus of great interest this is not the richest area in terms of iron content and not all the ferruginous beds are highly conspicuous as ironstones. The sequence is in general one of shales with silty shales and with hard beds of sideritic and berthierine-rich ("chamositic") ironstone. The dip is low so that shales form extensive areas of the intertidal erosional platform with harder ironstone beds projecting as reefs. The shales are not of the dark-grey bituminous type and are generally not "alum shales"; they are lighter in colour and rather silty. The general direction of dip is towards the east so that higher beds are seen near Old Nab (east of Staithes). Here the Main Seam of ironstone is near sea-level. Much of it has been mined away in the cliffs and under the headland but it is well-seen near the end of the promontory.

INTRODUCTION:

Village of Staithes

The car park is at the top of the hill above Staithes Harbour. There are toilets here and also down at the harbour where there are some shops, a cafe and a pub. The road down is in a steep-sided gorge cut by Staithes Beck into the Staithes Sandstone Formation. Notice that the dip is very low so that the sandstone appears almost horizontal in the road-side and beck-side exposures. The old town at the bottom of the hill is attractive and interesting; Captain Cook once lived here. You may be able to purchase guide books such as Eccleston and Eccleston (1998) on the history and geology of Staithes from the Post Office which you would pass on the way down.

INTRODUCTION:

Staithes Harbour

Top three images: Staithes Harbour is protected by sturdy breakwaters. The location is, of course, controlled by the mouth of Staithes Beck. It is not very clear why the stream does not enter the sea at the eastern end of the Staithes Sandstone outcrop on the shore. It could cut through the less resistant shales and not the sandstone if slightly further east. The top left image is of an old monochrome photograph from Steers (1960). It is of interest in showing that significant erosion has taken place at the end of Cowbar (Colburn) Nab. An older and a modern view have been placed together in the top right image. The cliff shows some differencies with regard to fallen material and there is less vegetation in the upper part towards the end. The harbour is a little different now in that large stone blocks have subsequently been added to the harbour breakwaters to increase the defence against storm waves. Incidently, notice the high Boulby Cliff of Liassic strata in the distance.

Lower two images: Having arrived at Staithes Harbour it may be necessary to wait for the tide to fall. On the far (western) side of the harbour is Cowbar Nab, originally know as Colburn Nab. This has a vertical cliff of Staithes Sandstone with some boulder clay at the top, debris from which slides down the cliff. At one time it was thought that there was a fault through the harbour and that this was the reason for the mouth of the beck here. In fact mapping at very low tide by Howarth (1955) has shown that this is not the case and the Staithes Sandstone Formation dips eastward under the harbour so that the top is present at Penny Nab. In the harbour, though, part of the outcrop is obscured by beach material and boulders. Near the breakwater staff from the Open University introduce the students to the general geology of the coast here.

Staithes Harbour to Penny Nab - The Staithes Sandstone Formation

Proceeding eastward from the harbour along the beach requires scrambling down over some large rocks placed for sea defence purposes beyond the eastern harbour wall. This is not a difficult scramble but requires a little care. Then walk over the shore ledges near the foot of the cliff to Penny Nab. Here, and shown in the photograph, there is some very slippery, algae-covered rock to cross. It may be necessary to walk very carefully on this green slime. Once these obstacles have been passed the route is much easier .

Around Penny Nab the topmost part of the Staithes Sandstone Formation is seen. As a sandy unit the Staithes Sandstone appears to thin to the south and southeast from a maximum of 30m at their northern outcrop in the Eston Hills and decreasing only slowly to about 20m at Staithes according to Hemingway (1974). As noted above, and in spite of the simplified scheme shown as a table, the highest part of the sandstone actually belongs to the Amaltheus margaritatus zone and a part belonging to the Prodactylioceras davoei zone is underneath (12.5m). This illustrates the general principle that lithostratigraphic units such as the Staithes Sandstone Formation do not necessarily match chronostratigraphic units such as zones.

The Staithes Sandstone Formation consists of shallow marine sandstones and siltstones. The sandstone is fine to medium-grained and usually blue-hearted (blue-grey under a yellowish outer part) and weathers to rusty, flaggy slabs. That means that the rock is probably blue-grey and reduced at depth, but becomes oxidised and yellowish-brown where oxidising water has affected it at the surface. On the Dorset coast the Upper Lias, Bridport Sands are similar in being blue-grey beneath a yellow weathered surface. In boreholes, the blue-grey colour of such sequences is in great contrast to the yellow cliff exposures. The Staithes Sandstone Formation is both micaceous and carbonate-cemented, again like the Bridport Sands and other Liassic sands of Dorset . There are scattered lenses of sideritic mudstone, and this shows some trend towards the sedimentology of the Cleveland Ironstone Formation above.

The Staithes Sandstone Formation shows various sedimentary structures of types that occur in marine sandstones. The thicker-bedded units are often cross-bedded with hummocky cross-stratification, a structure probably produced by storm waves which have had an oscillating effect on the sands on the shallow sea-floor (Rawson and Wright, 2000). Because only the top of the Staithes Sandstone is present at Penny Nab, though, not all the features may be clearly seen on this eastern side of the harbour. Parallel lamination, low-angle cross lamination and wave ripple lamination also occur. There are both current and interference ripples in the formation in places (Hemingway, 1974). Some beds are highly bioturbated, as can be clearly seen at Penny Nab, and this bioturbation has destroyed sedimentary structures produced by currents.

LOCATION:

Penny Nab - Penny Nab Member of the Cleveland Ironstone

On the eastern side of Penny Nab, on the shore ledges, find the top of the Staithes Sandstone and the base of the Cleveland Ironstone Formation. Look for a change to more shaley sediment and the occurrence of brown siderite (ironstone) nodules. There are also some marine fossils to be found here.

Looking up at the Penny Nab cliff from the beach on the eastern side a large part of the Cleveland Ironstone succession can be seen, as shown in the photograph. Above a vertical cliff of shale is the Avicula Seam; this descends to the beach further east and we will look at it in more detail there. Higher is the Raisdale Seam with the curiously banded shales of the upper striped beds beneath it. Further up is the Two Foot Seam (although less than two foot in thickness here) and then the five thin beds of the Pecten Seam. The Main Seam of Ironstone is in two blocks and has not been mined just here and so is clearly visible in the cliff. The upper and more sloping part of the cliff is composed of the Whitby Mudstone Formation of the Toarcian or Upper Lias. These are shales, some of them alum shales with much pyrite. They are crumbly and will not stand so easily in a vertical face. Debris from them can pour down the cliffs.

LOCATION:

Jet Wyke - General

This is a broad embayment cut into the shaley Cleveland Ironstone Formation which dips at a low angle towards the east. The bay has no beach but vertical shale cliffs which are reached by the high tide. At low tide there extensive scars or ledges of shale with some thin ironstone beds and brown nodules of siderite (ferrous carbonate ironstone) at certain levels. Numerous modern Littorina gastropods ("winkles") together with barnacles live on and around the ironstone ledges. In the shales and ironstone marine bivalves and belemnites are common.

It is the lower part, or Penny Nab Member of the Cleveland Ironstone Formation which is well-exposed on the shore of Jet Wyke (the higher part, the Kettleness Member, is seen further on at Old Nab and is inaccessible high in the cliff). The Penny Nab Member mostly consists of light blue and grey, rather mottled shales with thin beds of hard ironstone here and there. In terms of fauna the the Penny Nab Member is in the Amaltheus margaritatus Zone which roughly corresponds here to the lower part of the Cleveland Ironstone Formation. The member has within it the Avicula Seam and the Two Foot Seam of ironstone. According to Hemingway (1974) bivalves in this part of the sequence include Protocardia truncata, often in extensive shell beds, Entolium sp., Gryphaea sp., Cardinia sp. and Goniomya sp. There is good preservation in places.

The sedimentology is interesting. There are a series of cycles up to 7m in thickness in the Penny Nab Member. Each usually commences above an ironstone bed. A shale unit occurs followed in some cases by a striped unit with some silt or sand content. Sand linsen produced by ripples may be present. The stripes are thin fining-upward sheets ((Rawson and Wright, 2000). The "upper striped beds" (informal term) which occur under the Raisdale Seam contain gutter marks which have probably been produced by sea-floor currents under storm conditions. These interesting striped strata will be considered in more detail below. Above such silty shale comes an ironstone. These are quite conspicuous because these hard beds form reefs extending out on the shore. They are relatively thin for what might be expected within a well-known ironstone formation, but the ironstone is more thickly devevoped in the next member above. You will notice as a matter of interest that the Two Foot Seam near Old Nab is not two feet thick, but significantly less. This is explained by a general pattern of thinning of ironstones towards the coast. They are more thickly developed inland in the Cleveland Hills. Individual ironstone beds are sideritic and "chamositic" (actually with the greenish mineral - berthierine, not chamosite with which it is sometimes confused). In places the Cleveland Ironstone deposits are oolitic with berthierine ooids. These are not conspicuous in the field in the ironstones in the Penny Nab Member, but thin-sections are really needed for proper study. Ooids can be seen, though, in the field within parts of the Main Seam of the overlying Kettleness Member at Old Nab but the strata are not obviously oolitic just here.

Jet Wyke - Faults

Two faults in Jet Wyke are shown in the photographs here. Notice that they are not photographs of the same fault. Look at the position of the Pecten Seam and the Main Seam. In the case of one of the faults these are closer to the beach. In one of the photographs there is a debris-filled mine in the Main Seam.

Siderite Nodules in Jet Wyke

The siderite nodules in the Penny Nab member are of ferrous carbonate. When fresh material is broken this is usually grey in colour. When slightly weathered on the exterior such nodules turn brown and, thus, are easily recognised. Intense weathering, as seen in other formations, can leave a crumbly rusty ovoid, but this does not usually occur near sea-level and is a feature seen high in cliffs under soils. The nodules can in some cases show some internal cracking of septarian type, although this is more frequently a feature of calcitic nodules.

For comparison similar ironstone nodules from the Eocene strata of the south of England are shown. These are from the Hengistbury Beds at Hengistbury Head near Bournemouth. These are about 120 million years younger. In both cases there has been very warm, equable conditions in temperate latitudes, with much vegetation, and significant weathering on the land. In both cases there has been input by rivers carrying the ferruginous results of chemical weathering into the sea probably as organic colloids. Plant debris as lignite is present in both, particularly as compressed lignitic logs at Hengistbury Head. Indeed, logs are notable at Staithes too, but particularly a little higher in the Lower Jurassic sucession where they form the jet gives its name to Jet Wyke. Differences are that the Eocene strata do not contain berthierine ("chamosite) and that the strata are poorly lithified. Flourishing vegetation in a warm climate, chemical weathering from soil acids, and river transport to a shallow sea are factors in the formation of ironstone. Within the sea, some mechanism for separation of the iron from the clastics is needed, and within the sediments a low sulphate activity favours siderite precipitation (as against precipitation of the ferrous sulphide - pyrite) .

Avicula Seam in Jet Wyke

The Avicula Seam forms a flat ledge projecting seaward, about 150m east of Penny Nab. The upper surface shows many specimens of Oxytoma cygnipes (formerly Avicula cygnipes) from which the bed is named. The base is conglomeratic, indicating shallow water conditions and erosion (Rawson and Wright, 2000) . According to Hemingway (1974) the Two Foot Seam at some places also has a basal conglomerate and, thus, these may be significant indicators of the conditions of initiation of such ironstone beds. The "Avicula " and other Pecten-like bivalves seem to indicate sandy-shoal conditions and certainly some ironstones are oolitic, indicating shallow, high-energy conditions. If this really is the general pattern, the implication seems to be a trend from quiet-water clay deposition at moderate depth to a very shallow, wave-influenced environment, soon followed by shallow shoal-water conditions. The marine fossil fauna and lack of freshwater molluscs show that deposition was in marine conditions throughout.

LOCATION:

Jet Wyke - Jointing in the Cleveland Ironstone Formation

Jointing is present throughout the Cleveland Ironstone Formation in Jet Wyke and elsewhere, but is not very conspicuous. In the photograph an unusual case is shown of marine algae, and other marine life preferentially attached to joints. There are numerous Littorina littorea (winkle) shells associated with the joints and scattered (seen as black dots) between them. Presumably the shale does not provide a good attachment surface for algae but the cracks are more suitable. Notice how the joints are also visible in the shale away from the pool. Nothing much is attached to them here though. Three joint directions are visible. They seem to be nearly vertical but the trends of these could be measured in the field with a compass-clinometer.

The "Upper Striped Beds" with Gutter Marks

In about the middle of the Cleveland Ironstone Formation is a distinctive unit of thinly striped shale, two metres in thickness. The stripes are of fining-upward siltstone within shale. These upper striped beds (informal name), occurs just beneath the Raisdale Seam of ironstone. Do not confuse this thin and subtle striping with the conspicuous five bed cyclicity, and thus five thick stripes, of the Pecten Beds. Many of the cliff photographs show the Pecten Beds. Go down from these, below the Two Foot Seam and further down just below the Raisdale Seam. In the field you will need to as close as a few metres from a scar or cliff exposure to appreciate the character of the upper striped beds.

The striping and associated sedimentary sinuous, narrow, channel-like features known as gutter marks have been described by Greensmith, Rawson and Shalaby (1980).

(For more on gutter marks, or gutter casts or scour and fill see:
Gutter cast in the Bencliff Grit, Osmington Mills, Dorset. and also Stow (2005) )

The upper striped beds form the uppermost two metres of bed 34 in the Staithes Cleveland Ironstone succession. They are within the Amaltheus gibbosus Subzone of the Amaltheus margaritatus Zone of the Upper Pliensbachian Stage. Amaltheus margaritatus has actually been found within the upper striped beds. The striped strata, which are also known at Hawksker Bottoms, form a distinctive marker bed indicating a synchronous event according to Greensmith, Rawson and Shalaby (1980).

The same authors commented that the most informative section through the upper striped bed is at the clean-washed cliff foot in the middle of Jet Wyke, immediately above a distinctive wave-cut knotch. This location is shown in the left photograph above and in others higher in this web page.

The base of each layer of pale-coloured laminated siltstone in the Upper Striped Beds is erosive. The gutters are at the base of some and cut down into and even undercut up to six underlying layers (Rawson and Wright, 2000). On the scars near Old Nab the hard bases of the anastomosing gutters can be up to 0.5m wide and 5m long. Examples are shown in the photograph, but please note that in the photographic image the colour of the gutter marks has been darkened by computer adjustment to increase their visibility and contrast. In reality they are lighter and less conspicuous than shown, although quite easily found. Notice the transverse jointing, an indication that the gutter marks or gutter casts have cemented by carbonate so as to be more brittle than the associated shales. They have thus been subjected to brittle fracture during compaction under burial, while the shales have been distorted in a more ductile manner. At Hawsker, 20km to the southeast similar gutters in an equivalent bed are less deep and have a finer grained infill as if they are more distal from the shoreline indicating currents from the west. Rawson and Wright, (2000) considered that deposition had occurred under storm surge conditions.

Old Nab

Here, some way east of Staithes, the easterly dip brings the upper part of the Cleveland Ironstone Formation, particularly the Kettleness Member, to the foot of the cliffs. The Two Foot Seam, the Pecten Seam and the Main Seam are conspicuous features just to the west of the headland. These are attenuated here compared to some inland locations (Hemingway, 1974). The Pecten Seam is easily recognised by its five thin, hard subdivisions. It is very fossiliferous with the large clam shell or bivalve Pseudopecten equivalvis and many belemnites. The Main Seam has been extensively mined resulting in much collapse of the cliff. You can see this in the left-hand photograph above. The collapse has rendered unstable the Whitby Mudstone Formation above, including some very loose Alum Shale. See Eccleston and Eccleston (1998) for the history of the mining here. They show an old map on p. 135 with a waggon way along the beach from the Old Nab mines to Staithes. Incidently, almost the same view as that in the top left image is shown in a monochrome photograph of Howarth (1955), fig. 2, p. 146. The main difference is that there is more loose and dusty debris on the cliff in the recent photograph, but no doubt this will vary according to the extent of storm effects on the cliff.

The Main Seam, a relatively hard, sandy bed. When freshly quarried elsewhere the Cleveland Ironstone can be green in colour because of the presence of the iron-rich silicate mineral "chamosite" or berthierine. The Main Seam does not appear green in the cliff; it is greyish-brown probably because of weathering and some oxidation of the iron. The Main Seam is accessible at the end of the headland, but beware of the danger of rock falling from the cliffs above. It contains bivalves, sometimes paired, and other fossils. A very gentle anticline and syncline are present at Old Nab as shown on the geological map, and the effect of the folding is visible beyond the headland. Trace fossils in the Main Seam at Old Nab are shown below.

Rhizocorallium at Old Nab

The trace-fossil Rhizocorallium is common in relatively-shallow, marine strata of the British Jurassic. It has been excavated by shrimp-like crustaceans. These examples are from the top of the Main Seam of the Cleveland Ironstone Formation at Old Nab (beware of debris falling from the rather unstable shales of the Upper Lias, Whitby Mudstone Formation in the upper cliff). The Rhizocorallium burrows are remarkably well-displayed because of an iron content apparently higher than that of the adjacent strata. This clarity means that even scratch marks made by the claws of the crustaceans are visible.

LOCATION:

Old Nab - Mushroom Rocks

The Main Seam descends to sea-level at the end of Old Nab. This photograph taken at the very end of the promontory on the beach is of a view eastward towards Penny Nab. In the foreground are mushroom rocks or "tables" of ironstone supported on shale. There has been much mining here back in the past which has modified some natural features. According to Rawson and Wright, 2000) small rectangular stacks have been formed by the unworked pillars of Main Seam left from ironstone mining; the debris in the worked areas and the overlying rock has been washed away by the sea. Thus, it is not clear as to whether these particular mushroom rocks are entirely natural features or the result of marine erosion on ironstone that has already been affected by mining.

Brackenberry Wyke

Fossils of the Middle Lias

Of the fossils in the Cleveland Ironstone Formtion, bivalves seem most common. Brachiopods occur, but not conspicuosly. Belemnite guards are easily seen at various horizons, but particularly in the Pecten Seam. In some cases the phragmocone may be preserved. Ammonites can be found in places. In a brief visit these do not seem very abundant and perhaps only poor impressions like the small example in this photograph may be found. However, systematic search would no doubt yield much better material, and, indeed, numerous ammonites have been collected in the past and have aided in the establishment of zonal and subzonal schemes (Howarth, 1955). Some illustrations of well-preserved ammonites are given below. The occurrence of ammonites, belemnites and brachiopods indicates that the depositional environment was fully marine in terms of salinity.

Left and central images: The ammonite Pleuroceras spinatum characterises the upper part of the Middle Lias (Upper Pliensbachian). Here this is the Kettleness Member or upper part of the Cleveland Ironstone Formation. The genus Pleuroceras of Hyatt (1867), the "ribbed horn ammonite", has a subevolute to evolute shell that is quadrilateral in whorl section. It has strong radial ribs often with tubercles or spines. The venter is flat with a prominent keel bordered by flat areas or grooves. The keel ranges from corded with strong chevrons to smooth.

Right: Ammonites of the genus Amaltheus occur in the Amaltheus margaritatus Zone of the lower part of the Middle Lias. This approximately corresponds to the Penny Nab Member of the Cleveland Ironstone Formation at Staithes. Amaltheus, like Pleuroceras, belongs to the SuperFamily Eoderocerataceae. The shells are typically subevolute to involute. Notice that in these specimens the whorls overlap in an involute manner. The ammonite is compressed to very compressed and oxyconic with triangular to circular whorl sections. The compression is obvious here and the whorls are almost triangular in section in these examples. The ribs bifurcate to trifurcate, passing forward to produce a prominent corded keel. Spiral ornament is present in some oxycones, as in the Amaltheus margaritatus shown here.

Some Middle Liassic bivalves which might occur in the Cleveland Ironstone succession at Staithes are shown here. Oxytoma cygnipes is the "Avicula" of the Avicula Seam. Pseudopecten equivalvis occurs in the Pecten Seam. Both are shown in photographs on this webpage. Protocardia is a bivalve that is easily recognised if well-preserved because it is rather like a cockle (Cardium) but with ribs on only one side. Modiolus is a mussel-like bivalve. Plicatula is small and oyster-like in appearance. Many other bivalves which might be found in the Middle Lias are not shown.

Nests of brachiopods, Tetrarhynchia tetrahedra can occur in the Kettleness Member, the higher part of the Cleveland Ironstone Formation which belongs to the Pleuroceras spinatum Zone. In this part of the sequence, apart from large Pseudopecten equivalvis, referred to above, these beds yield several species of Pleuroceras, the bivalve Pholadomya and many belemnites as shown in a photograph above (Hemingway, 1974).

Palaeoenvironment and Origins of the Jurassic Ironstones

There are several Jurassic ironstones in the British Isles. They are a common facies and similar ones occur in Cretaceous strata. Apart from the Cleveland Ironstone, Jurassic examples include the Northampton, Frodingham, Banbury, Raasay and Abbotsbury Ironstones. They are most common in the Lower and Middle Jurassic. The minerals present may include "chamosite", siderite and goethite. The greenish iron silicate mineral listed as "chamosite" is usually berthierine, in fact. Siderite is ferrous carbonate, bluish-grey when unweathered but usually oxidised and rusted brown on the outside. Goethite is hydrated iron oxide which may be of penecontemporaneous origin, but is often of secondary origin. A greenish and reduced ironstone weathers at the surface to a brown rock rich in goethite (or "limonite"). The Cleveland Ironstone is brownish-grey in the coast sections.

Ironstones, and also shelly limestones, often cap coarsening-up mud to sand sequences in the British Jurassic (Anderton et al., 1979). Chamositic ironstones are often cross-stratified and contain abundant and diverse shallow-marine faunas dominated by suspension feeders. In comparison with shaly and sandy sequences the ironstones are stratigraphically condensed. There may be conglomeratic structures, as mentioned above in the Cleveland Ironstone Formation, and stromatolitic structures can also occur. These features and the reduced amount of clastic terrigenous input have led Sellwood and Jenkyns (1975) to argue that the ironstones have originated on a shoal or swell type of setting.

Chamosite (or berthierine) and siderite require reducing conditions for their formation, but the oolitic fabric, the cross-bedding and the evidence of a flourishing benthic fauna suggests that mobile aerobic substrates actually existed (Anderton et al., 1979). Probably some early diagenesis in the initial sediment was involved; oxidising conditions can occur at the surface of the sea-floor sediment with reducing conditions just a few centimetres down. A high organic content would facilitate this situation and, indeed, in modern analogues of chamosite occurrences in the Niger and Orinoco deltas there is a close association with faecal pellets. Thus an organic-rich, high energy, shallow-water shoal environment with a rich benthic fauna seems to have been the type of place where these ironstones were formed. Note that the British Jurassic environments were not equatorial, though, but temperate (approaching 40 degrees north). It was, however, a much warmer geological period than at present. Tree remains are common in the Jurassic strata (jet and lignite) and clearly there was a very well-vegetated land. The temperate rain forests of conifers and cycadophytes would have developed good soils facilitating chemical weathering and thus a supply of iron. Jurassic deltaic deposits (in the Middle Jurassic of Yorkshire) show that rivers were actively transporting much material to the shallow seas and, in addition to clastics, this would have included iron as organic colloids.

Comparison with the Dorset Coast

The Domerian or Middle Lias on the Dorset Coast is seen at Eype, west of West Bay or Bridport Harbour. The two zones of Amaltheus margaritatus and Pleuroceras spinatum include there the lithostratigraphic units f the Three Tiers (at the bottom), the Eype Clay, the Down Cliff Sands, Margaritatus Beds, the Thorncombe Sands and the Marlstone Rock Bed. Of these the Eype Clay is the largest unit at 68m (House, 1993). The proportional thicknesses are quite different from Yorkshire. The Amaltheus stokesi Subzone of the Amaltheus margaritatus Zone is 96m thick according to Howarth (1955) out of a total Domerian of 125m. With such thickness variations in some parts of the succession it is not surprising that cyclicity is not easy to interpret in the Dorset Jurassic (but in any case the notoriously thin Junction Bed overlies the Domerian at Eype!).

Note that strata of the same age discussed here occur in the Western Isles of Scotland, the Aveyron district of south-east France and also in southwest Germany.

Acknowledgements

The author wishes to express his thanks to the staff and students of the Open University who participated in field work illustrated in this account. Their cooperation is much appreciated. In particular I am very grateful to Glynda Easterbrook, Mike Widdowson and Jennie Neve.

REFERENCES AND SELECT BIBIOGRAPHY
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Howarth, M.K. 1957. The Middle Lias of the Dorset Coast. Quarterly Journal of the Geological Society, London, 113, 185-204, pl. 17.

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Abstract:
The processes of change on near-vertical coastal cliffs have been quantified using terrestrial photogrammetry and laser scanning. The approach allows discrete rockfall geometry to be measured, and source located in three dimensions. This paper presents the analysis of an inventory of over 100 000 discrete rockfalls, recorded from the Liassic coast at Staithes, North Yorkshire (UK), where a rock face area of over 16 000 m2 has been monitored over a 20 month period. The data obtained on three-dimensional scar geometry and source position give an insight into rockfall characteristics from a range of rock types, cliff heights and geometries. Multiple failure mechanisms such as overhang collapse, constant spalling, fragmentation and large scale, coherent rockfalls have been observed and related to rock-type controlled processes on the rock face. The spatially referenced rockfall scar data are used to assess the influence of environmental controls on variable rock mass properties, such as rock type, structure and cliff geometry. Analysis of rockfall magnitude–frequency reveals notable similarities between coastal rockfalls and inventories from non-coastal environments. The resolution of the monitoring data allows quantification of rockfalls down to volumes of 1.25 × 10- 4 m3 to be consistently sampled and measured. This complete magnitude–frequency relationship suggests that rather than evolving exclusively through isolated, sporadic losses, coastal cliff geomorphology reflects interconnected processes in which each rockfall is part of a continuum of change to the rock face. Further detailed assessment of the rock face reveals the control of the pre-failure morphology on subsequent failure patterns, for example, the quantity of rock protrusion from the cliff is positively correlated with subsequent failure volume. The continuum of activity and the controls on failure identified within these data suggest that the episodic behaviour of coastal cliffs previously assumed may have been overstated by coarser resolution monitoring data. The findings improve our understanding of the evolution of coastal cliffs and highlight areas for further research into both cliff processes and the character of rock slope failures in general.
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Stow , D.A.V. 2005. Sedimentary Rocks in the Field: A Colour Guide. Manson Publishing, London, 320pp. By Professor Dorrik Stow, School of Ocean and Earth Science, National Oceanography Centre, Southampton University.
Preface:
The world of sediments and sedimentary rocks is exciting and dynamic. It is fundamental to our understanding of the whole Earth System and of the wide range of environments that characterize its surface. It also provides the key to a plethora of natural resources - industrial, chemical, metallic, water, and energy resources - that shape the way we live.
Ideas and concepts in sedimentology are fast changing, but fundamental fieldwork and data collection remain at its heart. In the first instance, it is an observational science, closely followed by laboratory, experimental, and theoretical work. The primary skill lies in knowing how and what to observe and record in the field, and then how best to interpret these data. For me, this has always been a distinctly visual process. The unique aspect of this guide, therefore, is in the wide range of graphic material that draws together the very latest ideas and interpretations (over 50 line drawings), coupled with over 425 photographs (from 30 differentcountries) of the principal types of sedimentary rocks and their characteristic features. It is intended for ease of field use by students, professionals, and amateurs alike.
All the field photographs illustrated have been carefully selected from my own collection, except where otherwise acknowledged. All figures have been redrawn and many specially compiled from the latest research knowledge, always with a view to providing the best aid to recognition, classification, and interpretation in the field. The key emphasis is to help with field observation and recognition of the main features of sedimentary rocks. Some pointers are given towards their preliminary interpretation, but further endeavour in this area must remain the province of the broader sedimentological literature, and will depend on the nature of the work in progress. Many different disciplines and sub-disciplines of geology and oceanography, as well as sedimentology, require a field understanding of sediments and sedimentary rocks. They include: geophysics and geochemistry, paleontology and Quaternary geology, physical geography and soil science, archeology and environmental science. Above all, and for all, this is a book to take into the field and use!
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