EAST YORKSHIRE FIELD STUDIES
Number 3 pages 30 - 43
first published 1970
A Survey of the Jointing and other Structural Aspects of the Yorkshire Chalk
By W. G. Walbank
The Yorkshire Chalk forms the Yorkshire Wolds which stretch from the Humber to Flamborough Head in a wide arcuate line of gently rounded hills (Fig. i.). The area is intensively farmed so that exposures are not numerous, and good exposures are rare. In 1942 Wright and Wright conducted a mainly palaeontological survey of the Yorkshire Wolds and zoned a total of 124 pits and one deep borehole. Each pit was assigned a number and the locality numbers used in this survey are those of Wright and Wright. It is a tribute to the accuracy both of their map and of their locality descriptions that these exposures can be found at the present day. In the course of time a great many of their exposures have become overgrown or been used as rubbish dumps, and this survey had to rely on a mere twenty of the 124 pits available in 1942. Not that only twenty exposures remain, but only this twenty showed the type of jointing considered useful to this survey. One well known example will serve to illustrate the point. Wright and Wright gave the pit by the railway at Kiplingcotes the number 37 (see fig. 1), and this is quite a large exposure being perhaps 40' high and 100' across and is well known for its fossil content. This pit however proved useless from the point of view of a jointing survey. The pit face was covered with scree with only a few hard bands protruding and seen in situ. Nevertheless, where good exposures were to be found inland, they were usually excellent. The Melton, Hessle and particularly the Ruston Parva lime- works were excellent exposures which amply repaid a visit. On the coast the superb exposures on the northern, and more particularly on the southern, flanks of Flamborough Head proved invaluable.
Fig. i. Outcrop of the chalk in Yorkshire and localities mentioned in the text.
General Geological Setting of the Area
The Yorkshire Wolds form one limb of a gentle trough fold or syncline in which it is unusual to find beds dipping at more than 2°-3° from the horizontal. The fold axis though difficult to place accurately, trends approximately NW-SE through Great Driffield and plunges to the south east so that while the base of the Chalk reaches 600' O.D. at Leavening, it is seen at sea level at the Humber. The base of the chalk is not seen in the cliffs of Flamborough Head. To deal fully with the faulting in the Wolds would take more space than is available here. There is one major fault, the Hunmanby fault, which displaces the chalk escarpment by about one mile near the village of Hunmanby. There are, however, a great many smaller faults in the Chalk such as those seen in Selwicks Bay on Flamborough Head, where the Chalk is quite violently disturbed between two fault planes. Other smaller faults with a throw of a foot or so abound. One author has even called some structures faults whilst I have called the very same structures joints. The distinction is so fine as to make the difference between faulting and jointing very hard to determine in many cases. The criteria for joints are examined later.
The Hunmanby Fault is not the only major line of disturbance seen within the Chalk. Stretching across the northern part of the Wolds are very narrow belts of highly contorted chalk which may be called shatter belts. These are zones of highly contorted, faulted, and thrust chalk at the most 150-200' wide. Unfortunately they are poorly exposed and the only good exposure is on Scale Nab on the northern flank of Flamborough Head. This locality is inaccessible except by sea but can be seen from the headland to the east. The cliff at this point is almost 300' high and the huge contortions and overfolds in the vertical face are very impressive. The origin and implications of the shatter belts are discussed later.
To the WNW of the Yorkshire Wolds are the Howardian Hills, which according to Fox-Strangways are 'one of the most intricate pieces of geology in North East Yorkshire'. Here large scale faulting affects Jurassic rocks in three separate phases of movement.
3. Post Cretaceous
This has produced an intricate network of block faulting which reaches the Wolds in the Leavening—Burythorpe— Acklam area. Here the chalk is affected but apparently not to any great degree. Differing from the faults and shatter belts, the Market Weighton structure deserves mention. This exercised the dominant control on sedimentation in East Yorkshire in Jurassic and Lower Cretaceous times and its influence can even be detected in the thinning in the Lower Chalk. Later, the area was glaciated, the coastline in those times being much as it is today. The ice swept round Flamborough Head in the north and left other evidence of its presence in the extreme south at Melton. Its effects are important, and in my opinion vital, to the understanding of the jointing of the chalk, even though these effects are difficult to assess.
Thus the chalk has quite a complex tectonic environment and any one of the above mentioned geological features could have played a contributary part in the formation of the jointing. Conversely, none of them need have played any part whatsoever.
Terminology and criteria of joints
Price (1966) classified joints as:—
1. Master Joints which cut through a number of beds or rock units and can be traced for many tens or hundreds of feet depending on the scale of the exposure.
2. Major Joints, an order of magnitude smaller, but still well defined structures.
3. Minor Joints which are smaller, relatively unimportant breaks.
It is obvious that the choice of names is largely subjective and that a master joint to some, may be a major joint to others and vice versa. In the Yorkshire Chalk the joints fall quite clearly into two categories—minor joints and other larger structures. In my opinion the latter do not fit the dominant role demanded of master joints in both size and structural influence and hence are here called Major Joints. It is these that were measured in this survey.
Minor Joints are small in lateral and vertical extent and are rarely opened more than a fraction of an inch. They cut at most one bed and often die out within one bed. Even in small exposures their total number must reach hundreds. They are probably useless to any structural study and were ignored in this survey. Major joints, as defined for the purpose of this survey, in most cases showed no displacement and three criteria could be used—1. the fracture must measure at least ten feet along the joint plane; 2. the joint must cut a number of beds and must not, regardless of thickness, be restricted to one bed; 3. the degree of openness. The latter criterion was difficult to apply, some joints being much more open than others, but widely opened joints certainly added to the impression of being major structures. With experience a certain amount of flexibility was permissible in selecting joints for measurement, the criteria being stringently applied in large exposures with well developed jointing whilst where joints were sparse they were relaxed somewhat.
In most bedded sedimentary rocks the jointing is developed perpendicular to the bedding planes. Wager (1931) in his study of the jointing in the Great Scar Limestone excluded all joints dipping at more than 50 to the vertical but to do this in the chalk would cut down the number of joints to a mere handful. In the Yorkshire Chalk the joints vary markedly from the vertical, most of them having dips of between 40 and 80°, only a very small number being outside these limits. This preponderance of dipping joint planes is unusual.
The main criterion differentiating a fault from a joint is movement along its plane. On occasion, however, it is possible to detect slight movement on joint planes especially in areas of localized faulting like Selwicks Bay. The joints in Selwicks Bay showing this movement are very smooth indicating shear movement along them which has smoothed out any irregularities. Similarly Lamplugh (1895) made a traverse round the southern part of Flamborough Head and in the cliff between Danes Dyke and Old Falls measured 70 small 'faults' with typical throws of 7", if, 1" 2", 4" etc. These are the same sort of movements as those found along the 'joint planes' in Selwicks Bay. It is difficult to decide if these structures are faults or joints but the small scale of the movement suggests that they may be regarded as either.
Inside the shatter belts jointing was straight, sometimes with calcite developed on the joint planes indicating movement. However the shatter belts were poorly exposed and only pit 87 at Weaverthorpe was entirely within a shatter belt. Because of lack of exposure it is impossible to say if shatter belts do affect the jointing. Elsewhere in the Chalk, where folding and faulting were minimal, jointing was less straight with little or no calcite developed on the joint planes. The joint planes were also seen to be interlocking; if shear movement had taken place, as at Selwicks Bay as shown by faulting, surely then the sides would have been smoothed? Thus two types of joints have been discerned; type (a) major joints with shear movement along their faces and type (b) major joints caused by tension fracture. As Firman found, tension and shear joints differ so little in the field that they are impossible to tell apart.
Joints show varying degrees of openness and only those fractures which have dilated sufficiently to be detected from a reasonable distance can be recorded. Tightly closed and incipient joints are passed over unknowingly. Joint data thus represent only those fractures easily visible in the field. The bearings of the major joints were measured at the various localities and plotted as rose diagrams and as a histogram (fig. 2). All bearings are magnetic and should be corrected for 9I0 of westerly variation to give true bearings.
Joints are so numerous that they lend themselves to statistical analysis although being such small scale structures it is difficult to pin down their formation to a single cause. Within one area it is possible to give a probable cause to any orientated set of joints but difficult to tie the cause down very closely. Many authors have related jointing to nearby orogenic events and it is generally agreed that jointing is always associated with events early in the life of a rock. All this means, for example, is that Carboniferous rocks would be jointed by Variscan (Late Carboniferous—Permian) stresses rather than Alpine (Miocene) which is after all only to be expected. However, when the Yorkshire Chalk is considered the next orogeny after the Cretaceous is the Alpine in Miocene times some 50 million years later and so it is legitimate to look into other possible causes.
Experimental work on rock deformation has shown that limestones all behave in much the same way. Although chalk was not amongst those tested, it is the purest of limestones and there is little reason to expect it to behave differently. All limestones are weak and brittle at atmospheric pressure, supporting less than 1000 kg/cm strain and seldom deforming more than 1% before shattering. They are ductile but intermediate in strength. In air they break with steep shears or shatter violently at low pressures on surfaces inclined at 50O-70O to the least principal stress axis. The fracture is caused by shear and it would seem that flow and fracture are sometimes indistinguishable, there being no sharp dividing line. This work appears to confirm the field observations and the general dip of the structures. If this correlation is accepted then it would seem that the direction of least principal stress in the chalk was horizontal at the time of formation of the jointing. If this is so then one may ignore the Wolds Syncline as a mechanism of joint production, since the syncline would form under horizontal compression, whereas for joint formation this should be the direction of least stress. It would thus seem then that the 'orogenic' or 'tectonic' theories of joint formation cannot be used in relation to the chalk, and other theories must be found. Joints have been ascribed to torsional stresses in much the same fashion as Versey (1930) imagined for his forces causing the faulting in the Howardian Hills. Recently some doubt has been cast on the experimental work on which the theory is based. However, there is a modification of the torsional mechanism which explains joints as a fatigue phenomenon caused by semi-diurnal tidal attraction in the solid rock caused by the gravitational attraction of the moon. According to this theory joints develop quite early in the history of the sediments from joints in more competent rocks below. How these develop is not clear, and how joints originally form is also not explained. Joints have been seen in such soft sediments as Miocene clay but it is difficult to see how these joints survive burial under later sediments and it seems likely that they are of a transitory nature. True joints must form some time afterwards either by orogenic means as mentioned above or by the compressional uplift mechanism first put forward by Price. The latter is backed up by physical and geological observations and seems to provide the best explanation, at least for the greater part, of the jointing in the chalk. The essence of Price's theory is as follows.
Rocks can retain residual strain energy and this can be transformed during uplift to the surface into stresses which will fracture rocks, producing shear and tension joints. It can be shown that rocks approximate to ideal Bingham Bodies i.e. they react to stresses by both fracture and flow, a combination of solid and liquid and, as such, they can store residual strain energy. Joints are essentially surface and shallow depth structures, a reasonable observation, since a joint plane would be closed by the pressures acting at any other than shallow depth. The uplift of any rock body will thus have the following consequences (a) decrease in gravitational loading (b) decrease in lateral stress which is related to Poissons Number (a physical constant for any particular material related to elastic deformation) and (c) the lateral strain developing in a rock during uplift. It will be seen that because, with uplift, part of a sphere expands to form a larger sphere there will be a horizontal expansion which is brought about by elastic stretching. A tensile stress develops which is expressed as E^ (where E = Young's modulus).
Fig. 3. Extension of Bed Length L to L + 8 L after uplift from AC to BD.
In theory the horizontal extension of the beds during uplift is equal to about one half the change in gravitational loading. This is very approximate but nevertheless is assumed to hold true in practice. Both folded and unfolded rocks display jointing but folding requires lateral compression which was found to be negligible in the Yorkshire Chalk and thus the simpler part of Price's theory dealing with unfolded rocks can be applied. Price proves mathematically that it is difficult for shear joints to develop under such conditions and that tension joints are much more likely. Also in the unlikely event of the tensile stresses being equal i.e. cancelling each other out, then the joint pattern will be random. However, it has been shown that this is rare and joints form perpendicular to the axis of least principal stress in the majority of cases. Once fractures have developed the tensile stress is temporarily relieved and it is possible for another set of joints to form an orthogonal system of joints at right angles. The process just described could occur almost simultaneously and cross joints may be formed by this mechanism. This theoretical discussion does not explain all the features of jointing seen in the Chalk but it is my contention that the samples of limestone shattering violently in the experiments of Mandin and Hager are equivalent to the tension jointing of Price. In Price's theory the distinction between shear and tension joints seems important and this is a definite drawback since the distinction is not possible in the field, in the vast majority of cases.
Whilst the significance of post-compressional uplift has been emphasised it also seems that glaciation has had a significant effect as will be seen in the conclusions.
The large rose diagrams show no preferred orientation of major joints and although the jointing on Flamborough Head shows some preference in orientation it is not enough on which to base a general theory. Preferred strike direc¬ tions are seen in some pits such as 4 and 8 where the roses show fine orientation maxima. The number of joints, however, was insufficient for rigorous statistical analysis. Attempts to combine separate pits in single diagrams generally met with failure as each pit had a weak orientation which was random compared with the maxima of other pits. To explain the distribution of jointing, joint directions and preferred orientation the following theory was formulated.
All joints in the Yorkshire Chalk were formed by Price's mechanism which may have produced a random pattern but probably did show, at the time of formation, well developed maxima and minima in the pattern of joint direction. This pattern has been overprinted by small scale, localized effects which are not widespread enough to apply to more than one pit. The ice of the last glaciation has caused many of these effects either directly or indirectly, as in the case of the ice sweeping round Flamborough Head and pressing in on the Chalk, and the cryoturbation structures at Melton. These localized stresses and strains have caused joints set up by the original forces either to open or to close. Those joints opened or partly closed by the forces have been mapped, those shut tight by them have been passed over. The joints with movement along their faces as at Selwicks Bay must be due to shear movement and their morphology suggests this interpretation. They are seen in areas of localized faulting and are thus best related to this faulting for their origin.
Implications and other features of interest
The Wolds shatter belts are of considerable interest and of obvious linear belt-like morphology. Their structure and structural implications would repay more detailed study. There are two possibilities regarding the conditions under which they were formed: (1) penecontemporaneous deformation of the sea bed in Chalk times or (2) disturbance in the consolidated state. Both have much to commend them but it is difficult to imagine in the second case how such violent contortions could be formed without affecting the surrounding rock. Faulting must remain a possibility, though, in considering the rocks of Selwicks Bay. The soft rock deformation required in the first case is difficult to envisage at Foxholes where a block of chalk has been rotated. This subject is of interest in understanding the structure of the Chalk, but here the matter must rest.
Another feature of interest is the Great Wood Valley which runs across the northern part of the Wolds and is unique in the chalklands of northern England because of its antiquity. It is a wide flat V-shaped valley looking far too large for the Gypsey Race, the small stream that flows down it at present. If, however, the Scale Nab Shatter Belt is traced over a map of the Wolds, it is seen to fall on the northern flank of the Great Wood Valley, and it requires little stretch of the imagination to visualise the valley being formed by preferential erosion along the shatter belt. The Great Wood Valley does die out, and the Gypsey Race makes a 900 turn at Maiden's Grave Farm (098716) bringing the stream into approximate alignment with the Hunmanby Fault. Though far from conclusive, this evidence seems to imply a structural control for the drainage of the northern Wolds. It would also seem that the shatter belts were formed later than the Hunmanby Fault, since it is possible to align disturbed chalk across the fault; if this correlation holds then it would seem that the shatter belts were formed by deformation in the solid consolidated state, assuming that the 900 turn of the Gypsey Race is due to the effects of the Hunmanby Fault.
I should particularly like to thank Mr. Penny for his helpful advice during the course of this work and Mr. Pickthall of A.P.C.M. Ltd. for his help in Melton Quarry.
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