West, Ian, M. 2016. The Needles: Geology of the Isle of Wight. Internet site: www.southampton.ac.uk/~imw/Needles.htm . By Dr. Ian West, Romsey, Hampshire. 16th August 2016.
The Needles: Geology of the Isle of Wight

Ian West,
Romsey, Hampshire

and Visiting Scientist at:
Faculty of Natural and Environmental Sciences
Southampton University,
Webpage hosted by courtesy of iSolutions, Southampton University
Aerial photographs by courtesy of The Channel Coastal Observatory , National Oceanography Centre, Southampton.
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A view from the cockpit of a light aircraft showing the western part of the Isle of Wight, June 2011

Aerial view of the Needles, Isle of Wight, by Alan Holiday, June 2011

A pre-1950 postcard, aerial photograph of the Needles, Isle of Wight

The Needles, Isle of Wight, an aerial view

The Needles and Scratchell's Bay seen from the sea and looking east, Isle of Wight, southern England, 10th June 2009

Upper Chalk with rhythmic flint bands, southern part of Scratchell's Bay and Grand Arch, Isle of Wight, 10th June 2009

A small island formed by part of the Shingles Bank, near the Needles, Isle of Wight, seen from Barton-on-Sea, January 2011

The Needles and the Needles Lighthouse, Isle of Wight, viewed from the  the north and looking southward, in calm weather conditions, 10th June 2009

The top of a sea stack, the most shoreward of the Needles, Isle of Wight, with Shags resting on the top of the steeply dipping slabs of Upper Chalk, 10th June 2009

The Needles Lighthouse and adjacent sea stack seen from a boat, Isle of Wight, southern England, 10th June 2009

Needles from Alum Bay

Needles from Alum Bay beach

Compton Bay

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INTRODUCTION:

General

National Trust notice at the Needles, Isle of Wight

The Needles are well-known sea stacks of Chalk at the western end of the Isle of Wight. This webpage describes the stacks and the adjacent Chalk cliffs.

The Needles can be seen very clearly from the cliff top promontory. To go there either drive to the car at Alum Bay, or take a bus to Alum Bay and the Needles. The end of the promontory is an old battery. The National Trust protects and conserves this important and historic area. There is a fee to enter it. Quite close views of the Needles from the sea can be obtained by taking a boat trip from Alum Bay, or from Keyhaven or Lymington on the mainland.

A tunnel in the Chalk at the Old Battery above the Needles, Isle of Wight

One of the several interesting features of the Old Battery is the presence of a tunnel in the Chalk for safe access to a lookout point.

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INTRODUCTION:

Safety

The main hazard here is so obvious it hardly needs expanation. If you stay on the main paths there is little risk. If, however, you were to divert from the main footpaths it would be possible to approach the cliff too closely. The risk of falling would then be great and a fall from these high Chalk cliffs would undoubtedly be fatal. Very strong winds may occur at times in this exposed site above the cliffs.

With regard to the base of the cliffs, it is not usually possible to land at the Needles or in Scratchell's Bay. If this could be done, though, then there is risk of falling fragments or blocks of Chalk. Climbing is not encouraged here, but if it were to take place there is, of course, a major risk of falling from unstable Chalk. Any activity at the base of the cliffs would have to take into account conditions of tide, wind, wave action and currents. As elsewhere flints should not be hammered because of the danger of high velocity splinters.

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THE NEEDLES PARK - INTRODUCTION:

The Needles Park

For more information on the history and geology of Alum Bay and the Needles, go to:

The History and Geology of the Needles.

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INTRODUCTION:

Geological Maps

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A redrawn version of the geology of the Needles promontory, western Isle of Wight, with some notes by Ian West, December 2014, based on published maps

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Old geological map, I.O.W.

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Geology of the western I.O.W

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STRATIGRAPHY;

Chalk - Introduction

The Chalk cliffs of the Needles, Isle of Wight, as seen from Hurst Spit, Hampshire, but enlarged, April 2007

A landslide or rock-fall in the Chalk on the south side of Alum Bay, near the Needles, Isle of Wight, as seen from Hurst Spit in July 2012

There are from time rock falls or landslides from the steep or over-steepened Chalk cliffs, the "White Cliffs" on the south side of Alum Bay and extending to the Needles. The one shown above in the 2012 photograph, taken from Hurst Spit, is quite substantial. From a distance it looks quite fresh and new. However, it is already present in the higher photograph above taken in 2007. In fact there has been obvious increased erosion of the basal rock fall slope between 2007 and 2012.

Now look at the old photographs below from other years. Is this the major chalk rock-fall which seems quite fresh in the photographs taken in 1995?

Alum Bay, chair-lift above Barton Clay with Chalk cliffs in distance

Seen in the distance here from the steps are the Chalk cliffs. This chalk is of the Belemnitella mucronata Zone of the Campanian Upper Chalk (Upper Cretaceous). It consists of chalk with flints steeply dipping to the north. The Needles peninula extends east-west so the cliff line of Chalk here is along the strike and almost represents the Chalk/Tertiary boundary. With this steep dip towards the sea, large chunks of chalk separated by curved slip-planes fall from the cliff from time to time. The scars can be seen in these photographs (taken in different light and weather conditions).

Needles from Alum Bay, 1995

Needles from Alum Bay beach, 1995

The two photographs of the Chalk cliffs immediately above were taken 1995. They show the details of some cliffs falls at the time. They can be compared to later photographs of the same cliffs.

Chalk cliffs at south side of Alum Bay, 2002

Notice the results of erosion of an old cliff-fall in the photograph above, from 2002. The pile of debris orginally sloped fairly evenly as a fan towards the sea. Recent wave erosion has cut a small cliff in the debris cone, leaving only a part with the original top surface slope. There are other small seaward slopes with grass, here and there on the cliffs. It is not clear as what it is the origin of these.

The Chalk cliffs from Alum Bay to the Needles, seen from above at the eastern end, Isle of Wight, 25th June 2009

The photograph above, from 2009, shows the remarkable uniformity of the steep northerly dip in the Chalk here. There seems to be no suggestion of curvature, which would be expected in the monocline. No beach is developed for most of this stretch, but occasional cliff falls add small areas of chalk debris to the base. Future "Needles" will be formed from this stretch, as Scratchell's Bay erodes and moves eastward.

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STRATIGRAPHY:

Chalk Stratigraphy and Fossils

Stratigraphical sequence of the Chalk

Portsdown Member of the White Chalk Formation dips northward, near the Needles

Chart for the Chalk of southern England relating older Chalk Zones to the modern lithostratigraphic schemes of Mortimore and the British Geological Survey

Chalk belemnites of zonal importance, in relation to the Chalk of Dorset and the Isle of Wight, England

The common Chalk echinoids - Micraster cortestudinarium and Micraster coranguinum, both used for zoning the English, Upper Cretaceous Chalk

The Chalk in the southern part of Alum Bay is Campanian, of the zone of Belemnitella mucronata. At the end of the beach is the soft white chalk of the Studland Member. By walking about 50m along the base of the cliff from the Tertiary erosion surface there is a change to the harder chalk with thin marl bands which belongs to the Portsdown Member (Insole, Daley and Gale, 1998). There is no easy access to older parts of the Chalk succession here. To study older parts in the western part of the Isle of Wight go to Freshwater Bay and Compton Bay. (to be continued)

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CHALK - THE NEEDLES

The Chalk Cliffs and Sea Stacks - Aerial Views

A view from the cockpit of a light aircraft showing the western part of the Isle of Wight, June 2011

The Needles, Isle of Wight, an aerial view

Aerial view of the Needles, Isle of Wight, by Alan Holiday, June 2011

An old oblique aerial photograph of the Needles area with Chalk Cliffs, Isle of Wight, southern England

A vertical, non-rectified, aerial photograph, of the Needles, Isle of Wight, 8th June 2007, courtesy of the Channel Coastal Observatory, NOCS, Southampton University

A wider, composite, vertical aerial photograph of the Needles, Isle of Wight, courtesy of the Channel Coastal Observatory, National Oceanography Centre, Southampton University

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CHALK - THE NEEDLES

The Chalk Cliffs and Sea Stacks - Views from Sea and Cliffs

The Needles and the Needles Lighthouse, Isle of Wight, viewed from the  the north and looking southward, in calm weather conditions, 10th June 2009

An old colour illustration of the Needles, Isle of Wight, probably from the 1960s, and showing the gap left by the fall of Cleopatra' Needle or Lot's Wife in 1764

The Needles, Isle of Wight, the outer two of the three Chalk stacks and the lighthouse, as seen from the cliff top at the Old Battery, with Ian West, June 2009

View eastward from near the Lighthouse, Needles, Isle of Wight, to show relationship of the structures of the stacks to that of the cliffs of Scratchell's Bay, 2009

View eastward into the cliff of Scratchell's Bay, in line with the Needles, Isle of Wight, 2009

The photograph above is part of attempt to determine the lithology of the Chalk in the Needles by comparing their location with the Scratchell's Bay cliff section. Looking along from the Lighthouse it would seem that the Needles relate to the Chalk with very closely-spaced cyclicity. However, this may be incorrect, because examination of the photographs of the Needles seem to show rather wide cycles. The matter is not proven from the above photograph, but it can be used within a continuing argument. Other views of northern Scratchell's Bay are probably needed.

The western extremity of the mainland Chalk cliffs of the Isle of Wight, just before the Needles sea stacks, 10th June 2009

The Needles and the Needles Lighthouse, Isle of Wight, viewed from the  the north and looking southward, in calm weather conditions, 10th June 2009

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THE NEEDLES STACKS

The First Stack

The First Stack of the Needles, Isle of Wight, shown obliquely in relation to the narrow arrete and promontory in front of the Old Battery, 10th June 2009

The First Stack, the most shoreward of the Needles, Isle of Wight, with Shags resting on the top of the steeply dipping slabs of Upper Chalk, 10th June 2009

Details, close-up, including shear planes in the first Needle or stack at the Needles, Isle of Wight, 2009

The first or most landward Needle is easy to examine from the cliff top at the Needles Fort. The most obvious feature is that there are many vertical or near-vertical shear planes. The displacement on these suggests that Sigma One, the maximum principal compressive stress was vertical here. This might not surprising because much of the north limb of the fold was above. However, there is a problem. The northward dip is such that the upper surface of the Chalk was not a great distance above the stack. There is the possibility, of course that there was a tectonic structure above, like the Ballard Down Fault, that resulted in additional loading but there is no specific evidence for this.

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THE NEEDLES STACKS

The Second Stack (after the gap)
Western End

The Second Stack or Needle, western end,  showing some shear planes and a small double snakes head, Isle of Wight, 2009

The Second Stack or Needle, western end, showing north-dipping shear planes, Isle of Wight, 2009

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THE NEEDLES STACKS

The Second Stack (after the gap)
Eastern End

The Second Stack or Middle Needle, eastern end, with a conspicuous, north-dipping shear plane in the Mucronata Chalk, Isle of Wight, 10th June 2009

A view from the sea of the eastern end of Second Needle seems to show north-dipping shear planes. This is unusual and contrasts with the sheared Chalk of the Lulworth Cove - Durdle Door area, Dorset. There the shear planes are dominantly south-dipping.

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THE NEEDLES STACKS

The Outer Stack (with the Lighthouse)

The Needles Lighthouse and adjacent outermost sea stack seen from a boat, Isle of Wight, southern England, 10th June 2009

Outermost Stack on Needle, with lighthouse, with shear planes, joints and a possible curved fault plane,  Isle of Wight, 2009

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NEEDLES - COMPARISON

Similar Chalk and Shear Planes at Arish Mell, Dorset

Shear planes in the Portsdown Chalk at Arish Mell, near Mupe Bay, Dorset

Chalk of the mucronata Zone or Portsdown Chalk is exposed in the cliffs of Arish Mell, between Worbarrow Bay and Mupe Bay, East Dorset. A photograph above shows similar north-dipping shear planes with small displacements toward the north. This is a particular section in a particular cliff and is shown merely as an interesting example of similar structures; the present writer has not undertaken any systematic study. It is of interest with regard to the hardness of the Chalk at the Needles that the Arish Mell section is quite close to a location of very hard Upper Chalk at Cocknowle Arkell's (1947).

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CHALK - NEEDLES:

The Original Four Needles - Descriptions

Isaac Taylor's 1759 sketch of the Needles, Isle of Wight, showing the narrow needle or stack, known as Lot's Wife

The Needles, Isle of Wight, in old drawings, one of these showing Lot's Wife, a tall needle-like stack

An old etching showing the Needle, Lot's Wife at the Needles, Isle of Wight, the tall stack which fell in 1764

Sir Richard Worsley's etching of a needle at the penultimate stack in 1762, the Needles, Isle of Wight

Until about the middle of the 18th century, there were originally three pinacles of dipping, Upper Chalk with flints at the western end of the Isle of Wight Sir Richard Worsley (1781), p. 274. The tallest of these was Lot's Wife, which rose 120 feet (36.6m) above low water mark. These gave rise to the location name "The Needles". Thay have all been destroyed now and there are no true pinacles present. However, three elongate Chalk stacks remain and is to these which the name "Needles" is still (wrongly) applied.

"Lot's Wife" (or "Cleopatra's Needle") was the name of the tallest spire of chalk. It seems to be the separate pinacle shown above in a drawing by Isaac Taylor in 1759.

A rather lower pinnacle was attached to the penultimate stack. This is shown in two images above. One is from Hurst Castle. In addition this same "needle" is shown is a small engraving of the Needles on page 25 of Sir Richard Worsley's (1781) History of the Isle of Wight. His publication confirms that it was attached to the penultimate stack.

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CHALK - NEEDLES

Failure of the Pinnacles

Sir Richard Worsley (1781) wrote, regarding the Needles:
The Western termination of this parish, and of the island, is an acute point of high land, from which has been disjoined by the wasting of the sea, those lofty white rocks, called the Needles, formerly three in number [see his map], but about seven years ago [i.e. 1774], the tallest of them, called Lot's Wife, which rose a a hundred and twenty feet above low-water mark, and in shape resembled a needle, being undermined by the constant efforts of the waves, overset, and totally disappeared.

Some more details has been provided by Norman (1887). He commented on the spiral shaped rock, Lot's Wife, 120 feet (36.6m) high. He said that it fell in 1764, conflicting with the date give above. He stated that the sudden collapse was with great force and produced a local seismic shock so great that it was felt as far away as Portsmouth.

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CHALK - NEEDLES

The Needles Lighthouse

An old lighthouse was present on the cliff top above Scratchell's Bay. The lighthouse out at the end of the Needles was erected by Trinity House in Victorian times. This lighthouse was first in operation on the night of the 1st January, 1859 (Norman (1887).

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CHALK:

Scratchell's Bay - General

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The Needles and Scratchell's Bay seen from the sea and looking east, Isle of Wight, southern England, 10th June 2009

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Scratchells Bay and Alum Bay seen from the Needles, Isle of Wight in 1908 by Professor Armstrong, in Rowe, 1908, image restored by Ian West

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Southern part of Scratchell's Bay, near the Needles, Isle of Wight, showing cyclicity in the Chalk, 2009

Upper Chalk with rhythmic flint bands, southern part of Scratchell's Bay and the Grand Arch, Isle of Wight, 10th June 2009

Details of cyclical or rhythmic sequence of flint bands, Chalk, Scratchell's Bay, Isle of Wight, 2009

A view from the cliff top of the Upper Chalk with rhythmic flint bands, southern part of Scratchell's Bay and the Grand Arch, Isle of Wight, 25th June 2009

Cyclical Chalk with flints in the Grand Arch, Scratchell's Bay, Isle of Wight, 2009, with some small thrusts

Scratchell's Bay does not seem to show any major faults. There are, however, some minor shear-planes or thrust planes. Two of these in the lower part of the Grand Arch appear as "Yellow Bands" like those of the Upper Chalk in Ballard Cliff to Harry Rocks, between Swanage and Studland. The Yellow Bands there seem to be minor bed-over-bed shear planes. The small thrusts shown in a photograph above have some slickensides and appear to have thrust in either a generally east or generally west direction. They have not been accessed for detailed study and more investigation is needed.

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CHALK:

Scratchell's Bay - Hopson, Farrant, et al. - Update of 2011.

See the key paper: Hopson, Farrant, Wilkinson et al. (2011), The Lithostratigraphy and Biostratigraphy of the Chalk Group (Upper Coniacian to Upper Campanian) at Scratchell's Bay and Alum Bay, Isle of Wight, UK, Proceedings of the Geologists Association, vol 121. Special Issue - The Geological History of the Isle of Wight. (See also the key introductory paper on the Isle of Wight - The Geological History of the Isle of Wight: an overview of the 'diamond in Britain's geological crown'. Proceedings of the Geologists' Association, Special Issue, The Geological History of the Isle of Wight, vol. 2011, pp. 745-763.)

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Scratchell's Bay, although inaccessible without a boat, provides a key cliff-section within the Seaford Chalk, Newhaven Chalk, Culver Chalk, and Portsdown Chalk Formations. In old terminology this is a substantial part of the "Upper Chalk" and represents strata from Coniacian to Campanian. There is no "Lower Chalk" or "Middle Chalk" exposed in this bay. The top of the Chalk, the Maastrichtian strata are not present and have been eroded away in early Tertiary times just here. However they were once present in the general region to the south, and the pebbles of Maastrichtian flint in the Lutetian at Bournemouth may have come from the southern Isle of Wight or adjacent region. So the Scratchell's Bay section is a good one, but is a partial one, and rarely reached by the general geological public. Therefore, the Hopson, Farrant, Wilkinson et al. (2011) paper is particular valuable in describing this rather inaccessible place.

There has been detailed work on Scratchell's Bay in the past in several papers by Gale and by Mortimore These are not discussed here, but can be found from useful reference list of the Hopson, Farrant et al. paper. Some are listed in the Isle of Wight Geological Bibliography, associated with this webpage.

Stratigraphical details from the Hopson, Farrant et al. (2011) paper are not repeated here. Their fig. 3a give an extensive graphic log, showing the positions of flint nodules and some marl bands in the Chalk of Scratchell's Bay. These flint bands show up well in photographs. Their fig. 4. shows the distribution of key foraminifera in the Scratchell's Bay section and in relation to the BGS zonal scheme, which is used in the Hopson, Farrant et al. paper. Fig. 5 gives correlation with Culver Down and Whitecliff Bay Chalk sections. There is also thickness comparison between Whitecliff Bay and Scratchell's (and Alum) Bay.

The following extract is from the "Discussion and Conclusions" of the paper. The main point is further evidence for syn-depositional tectonic control (faulting) on sedimentation, an aspect which has already been established in the works of Mortimore.

"The determination of the lithostratigraphical succession at Scratchell's Bay (BGS), its correlation with other logged successions on the Needles promontory, and with the Whitecliff GCR [Geological Conservation Review] succession, demonstrates various differences that provide insight into differential development and sedimentation along the Purbeck-Wight Structure.

Some of the differences are significant and point to a syn-depositional (syn-rift) tectonic control on the sedimentation and abrupt along-strike lithological change as discussed for example by Mortimore in the Late Cretaceous Chalk Group. These tectonic controls are well illustrated and discussed in Mortimore (2011 and references therein) and further justification for the existence of these controls is provided here. Whilst time-constrained phases of basin-wide tectonic events have been described across southern England (e.g. Mortimore and Pomerol, 1991a,b, 1997)and more widely across the Anglo-Paris Basin into Germany Mortimore et al., 1998 there is growing evidence that more localised events associated with individual elements of larger fault and fold structures have profound effect on the detailed bed by bed deposition of the Chalk (e.g. Gale, 1980 in addition to these basin-wide events. The lithological contrasts determined between the Whitecliff and Scratchell's/Alum Bay sites adds some additional data to that view, and points to differential movement on the along-strike elements of the Purbeck-White Structure as it is seen on the Isle of Wight itself, i.e. the Sandown and Brighstone (Needles) folds and associated reverse faults.

Recent BGS geological mapping has identified a number of faults (with orientations ranging from NW-SE through to NNE-SSW) that cross-cut the Purbeck Wight Structure. ..... [continues].....

It is further envisaged, although not the main tenet of this paper, that these cross-cutting faults have been re-activated following the end of Chalk deposition, on at least two, probably many more occasions. One such re-activation was pre-Palaeogene, influencing the outcrop distribution of the highest Chalk formations (as first discussed by (Rowe, 1908, p. 285). The cross-cutting relationships of the northerly-orientated faults and the Purbeck-Wight structure demonstrated by recent mapping and seismic interpretation (Rowe, 1908, p. 285) points to reactivation during the deposition of the Palaeogene. That this reactivation took place on at least three occasions, is further justified by the evidence of Palaeogene uplift and erosion afforded by Gale et al. (1999) during the Lutetian and Bartonian, and by Newell and Evans (2011) timed at the Bartonian-Priabonian boundary.

[The conclusions end with a summary of the variations in the thickness and lithology of the Upper Coniacian to Upper Campanian Chalk. Further details are not given here; please go to the paper by Hopson, Ferrant et al. (2011), which is easily accessible in the Proceedings of the Geologists Association - Special Isle of Wight Issue, vol. 122, issue 5. ] .

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CHALK:

Scratchell's Bay - Old Account - Rowe, 1908..

Rowe (1908) provided an account of Scratchell's Bay, together with quite good photographic illusrations. One aspect from this historic account is the origin of some rock falls, one of which was produced by the guns of the Needles Battery above. Rowe explains that the surface of the Chalk near sea-level is planed off smooth and that therefore the chance of finding good fossil remains is poor. The brief section by Rowe on p. 228 follows:

"Scratchell's Bay

We now reach this fine bay with the Needles and the Grand Arch. It may be mentioned that owing to strong currents at the Needles it is much easier to approach it from Freshwater [Bay] than from Alum Bay. There are [or were] two natural falls in this bay, and one talus of chalk tipped from above. The tip was made when the fort was being excavated, and together with the fall in the centre of the bay is in the zone of Actinocamax quadratus, while the old fall by the Needles is wholly in the zone of Belemnitella mucronata. This fall was caused by the concussion from the heavy guns of the Needles fort.
As we round Sun Corner [the south corner] in the boat the eye is at once arrested by the wonderful sweep of the beds in the Grand Arch, and crowded flint lines above the grass slope on the projecting point of the bay. Obviously the chalk below the grass slope is in the zone of Micraster cor-testudinarium, and we now seek a junction with the zone above it. Fifteen feet above the grass slope is single strong tabular flint band with a narrow belt of flintless chalk below it; but no marl band can be seen 6 and a half feet above the tabular band. We can find no better junction than this in an accessible surface, and we can only once more point out what feeble reeds to lean on are these long-trusted lithological features. The zone of Micraster cor-testudinarium seems to be about 50 ft. [15m.] thick, and is inaccessible in its entire extent. Mr. Strahan (op. cit. p. 75) says that the "brow of the cliff is known as the Main Bench, which is vertical and descends sheer into the water, was determined by the Ordnance Survey to be 416 ft [127m] above the datum-level while the Grand Arch, which forms the east side of Scratchell's Bay, and overhangs considerably, is 300 ft [91m] in height.
Attractive, as these cliffs are from the sea, they afford but poor sport for the collector. Indeed, but for fall in mucronata chalk, few fossils would ever leave this bay, for the surface is planed smooth by the action of wave and shingle, and unless one can climb up some feet, the chance of finding fossils is remote indeed.

Zone of Micraster cor-anguinum

We take the strong tabular flint line some 15 ft [4.6m] above the grass slope as the approximate base-line for this zone, and obtain the measurements of the lowest beds by using a boat. The upper limit is fixed by the occurrence of the first Uintacrinus plate, and may be taken to be reasonably accurate, as there is no difficulty in finding the plates of this crinoid in the section. The measurement obtained for this zone is 310 ft [94.5m] but no detailed section is given. The measurement was plotted out in sections according to the dip, and each section worked out separately. To look for any fossil save, Micraster coranguinum, Echinocorys scutatus, Echinoconus conicus, and Porosphaera in a shingle-battered section like this is a shear waste of time. These we obtained, and found them to be fully characteristic of the zone.

Zone of Marsupites testudinarius
Uintacrinus - band

Happily, here the cliff is set back a little, and for that reason is not so pounded by the shingle. The chalk is hard and white and distinct flint courses are common, the flints themselves being solid and black, with thin white cortices. We have long since abandoned the hope of finding any reliable zonal guide in the flints of this district. The same horizon at Freshwater Bay yielded flints with a thick pinkish cortex, and we could multiply the evidence ad nauseum throughout the zones in the Island.
We fix the junction with the zone of Micraster cor-anguinum at the position of the first Uintacrinus plate, and the upper limit of the subzone by the presence of the first Marsupites plate. We believe that both these junctions are substantially accurate and that the thickness given here, 34 ft 6 inches [10.5m] is a much more reliable guide than at Freshwater Bay. There are no salient lithological features to coincide with the zoological junctions."
[end of quoted section from Rowe (1908)

[the text of Rowe continues in the same style to include "Marsupites Band", Zone Actinocamax quadratus, Zone of Belemnitella mucronata etc.]
[On p. 232, Rowe discusses the Chalk exposure in Alum Bay. He mentions that at low tide one can walk out to the Needles. This is not recommended here because it is hazardous. He said that the results hardly justify the labour, as owing to the sheared surfaces of rock, which are all cut along the bedding planes, the fossils are not well-preserved, while even the falls yield but a poor fauna. He noted that it is instructive to examine the section at a really low tide, for then the extent of the chalk reefs gives one some idea of the thickness of the mucronata zone.]

Induration of the Chalk at the Needles

Rowe provides some information on the induration of the Chalk at the Needles (p. 232).
"The famous Needles are wedge-shaped rocks of mucronata chalk [Portsdown Chalk Formation in modern lithological terms] which have resisted marine and aerial erosion to a surprising degree. As we traverse the mucronata-chalk in Scratchell's Bay it becomes evident that an increasing degree of induration is met with as we approach the Needles, and this reaches its maximum at the southern surface of the shoreward Needle. Indeed the chalk is so hard that it is almost impossible to remove a fossil from it, and herein unquestionably lies the explanation of the formation and persistence of these famous pinacles. The Needles run out a little south of west, following the almost straight westerly trend of the beds in Alum Bay. Mr. Strahan [Aubrey Strahan - later Sir Aubrey Strahan, F.R.S., known for mapping Dorset etc and later, in 1914, he became the Director of the Geological Survey] tells us that a lofty spire of chalk which once rose as the most conspicuous of the group, fell down in 1764." ...
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continues

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[To see the Chalk cliffs at Harry Rocks across the bay to the West - Go to:
Harry Rocks and The Foreland, near Swanage, Dorset.]

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STRATIGRAPHY:

Chalk-Tertiary Junction

The junction between the Upper Chalk and the Tertiary strata is seen at the southern end of Alum Bay. There is a well-defined corner where the cliff direction changes from roughly north-south to roughly east-west. The Chalk forms a high wall here and deeps steeply towards the north. The lowest Tertiary strata here is the Reading Formation of Palaeocene age and consists of reddish clays with a little sand and gravel. The Upper Cretaceous succession is not complete. The highest Chalk is of the Belemnitella mucronata Zone, of the Campanian, and the Maestrichtian strata has been eroded away. Thus the true K/T boundary, notable for the extinction of the dinosaurs, is not seen here. The Reading Formation lies on different parts of the Chalk succession in different places so that in broad terms there is an unconformity. However, within Alum Bay there is no obvious angular discordance between the Reading Formation and the Chalk.

Solution pipe in the Chalk-Tertiary surface, Alum Bay

Solution pipe in Chalk, closer view, Alum Bay

The top of the Chalk is rather irregular and has oval pipes containing grey, argillaceous sand. The pipes seem to be of early Tertiary age and predate the folding. In other words they have in the past descended from the Tertiary surface downward into the Chalk, but since then folding has steeply inclined the top Chalk surface. You will notice the actual junction as a, steeply dipping, nearly vertical face from which the Reading strata is partly stripped away by erosion to reveal the details. It is rugged and deeply furrowed, and the solution pipes attain greater dimensions than at the equivalent position in Whitecliff Bay. Their sides are often undercut and, in many cases, open laterally into sand-filled fissures, some of which follow bedding planes. White (1921) considered this surface to have the general aspect of a foreshore with potholes worn in an approximately horizontal limestone. However, if the pipes are solution features, as they indeed appear to be, then what is visible is a surface affected by early karst rather than mechanical erosion and potholing. Solution pipes which are inclined like this and not simply vertical are rare and interesting features.

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ACKNOWLEDGEMENTS

Ian West much appreciate the arrangement of a geological field trip on a boat from Lymington to the Needles (Puffin boats) by the Probus organisation of Mudeford, and particularly the help of Peter Hollick. Dr. Yining Chen has assisted with recent field work on the Isle of Wight and elsewhere, and this help is much appreciated. iSolution, the computer services organisation at Southampton University has hosted this webpage, and vertical aerial photographs have been provided courtesy of the Channel Coastal Observatory. Alan Holiday is thanked for kindly sending me copies of the oblique aerial photographs of the Needles which he has taken in 2011.

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References and Bibliography

Please go to Isle of Wight Bibliography .

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Copyright © 2016 Ian West, Catherine West, Tonya Loades and Joanna Bentley. All rights reserved. This is a purely academic website and images and text may not be copied for publication or for use on other websites or online courses without permission, or for any commercial activity. A reasonable number of images and some text may be used for non-commercial academic purposes, including field trip handouts, lectures, student projects, dissertations etc, providing source is acknowledged.

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Dr Ian West, author of these webpages

Webpage - written and produced by:


Ian West, M.Sc. Ph.D. F.G.S.

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at his private address, Romsey, Hampshire, kindly supported by Southampton University,and web-hosted by courtesy of iSolutions of Southampton University. The website does not necessarily represent the views of Southampton University. The website is written privately from home in Romsey, unfunded and with no staff other than the author, but generously and freely published by Southampton University. Field trips shown in photographs do not necessarily have any connection with Southampton University and may have been private or have been run by various organisations.