published 1975
The President, Mr. L.H. Emery, opened the meeting with a short list of apologies, following which those members present (a total of five!) proceeded to an examination of the many specimens which were on display. Firstly, Mr Witham had brought collection of Callovian fossils mostly ammonites, gathered from the Kellaways Rock. These were finely preserved, ranging in size up to about six inches in diameter. Included were two specimens of Nautiloidea; one of these was sectioned to display the perforations of the septa, showing the central position of the siphuncle in this order, one of the characteristics which distinguishes it from the related order, the Ammonoidea, in which the siphuncle is displaced outwards from the centre. Some discussion took place concerning the form and complexity of the suture line, another distinguishing characteristic of these two genera. The specimens had been collected over many years, mostly from the South Cave Station Quarry; it was pointed out that owing to recent operations at that exposure much of the Kellaways Rock formerly available there had been removed, making collection at present more difficult.
Next, Mr. Latham, Keeper of Geology at Hull Museum, assembled a variety of specimens from the museum collection. One of these, a laminated hornfels, led to an animated discussion about the nature and origin of hornfelses in general. Also on view was a specimen of obsidian, leading to comparison with a sample of glassy material brought in by a member for identification. The general opinion was that the latter was an artifact, based mainly on the impurities evident in it and that it felt too heavy. This opinion was confirmed by Mr. Latham, who demonstrated an ingenious device for measuring specific gravity without the use of known weights or any kind of volumetric apparatus. This was the "White Instantaneous-reading Specific Gravity Balance", consisting of a hand-held two-armed balance, with one arm calibrated in s.g. and the other carrying one or more moveable balance weights of convenient but arbitrary size. Also needed was a container of water of any size sufficient to sink the specimen, which could be of any reasonable weight: the actual weight need not be known. The specimen was hung by a cotton thread from a reference mark on the calibrated arm, and brought to balance with one of the moveable weights clamped on the other arm. The specimen was then immersed in water, and its point of suspension on the calibrated arm adjusted by trial until balance was re-established. The s.g. was then read off from the calibrations. Tests made with this instrument showed that the specimen of obsidian was indeed significantly less than the unknown sample, clearly not a lava, but with its exact nature remaining obscure - possibly a slag or a mass of bottle glass.
The instrument described above folded up to pocket size; although apparently not well known, it could have considerable use in the field. It is not precision instrument, but would be sufficiently accurate for many field tests. Only cotton or thin wire is needed in addition; a quiet stream or pool would serve as the water container.
Following this experimental interlude further brief examination and discussion of the Museum specimens took place. These included some from the Llangollen area, which led naturally to the next stage of the evening's proceedings, namely display of slides taken by members. Mr Latham had a mixed set, acquired on various excursions: he had chosen these at least in part for their artistic merit, including some fine views of the 'picture postcard' variety, but he took the opportunity of explaining much of the geology exhibited on the photographs. They were taken in many districts including Llangollen, near which is the Clogau Quarry, specimens from which had already been included and discussed.
The Clogau Quarry lies in a region of strongly-folded Silurian turbidites and shales (Wenlock), showing a very mild cleavage. Numerous features of interest are to be seen at this exposure. The quarry had been used in the past as a source of slabs, because the thick grittier beds divide easily along the thinner shaly partings. The cleavage is not strong enough to cause splitting, though in fact these rocks are known locally s 'slates'. They are currently being worked into ornamental products for sale as souvenirs - they can be cut readily, but will not polish.
The phenomenon of refracted cleavage can be observed in this quarry, as had been demonstrated in hand-specimen: the lines of cleavage are seen to be roughly 30° to the bedding within the grits, and at a lower angle within the shales, resulting in stepped appearance on some fracture surfaces. The bedding at this exposure is now near-vertical; two views of the existing quarry face showed, on the one, a large expanse of ripple marks, and on the other a bedding plane with numerous large concretions, all of similar size, and evidently originally formed at the same depth, when the strata were level. It was noted that this phenomenon of concretions occurring in well-defined planes can be seen in many regions in widely different strata - the Kellaways Sands and the Kimmeridge Clay were quoted as examples.
Next to be projected was a set of slides taken by the writer on recent visit to Stonehaven, Kincardineshire. The photographs were taken of and from Downie Point, on the south-east side of Stonehaven Bay. The first was of the bay itself, with its harbour at the south end, and it also showed the headland on the north-east side of the bay, not visited on this occasion. This headland follows the line of the Highland Boundary Fault, and was commented on by Mr. Latham, who, having at one time lived nearby, is familiar with the area. He explained the fault zone, which is a complex one, is made up of rocks tectonically hardened, the rocks on either side of the zone, although of different series, being in both cases comparatively soft. On the south-east side, the seaward side, erosion is virtually complete, and hence the fault zone stands out as the shore-line of this promontory, for a distance of about half a mile. At one point, the sea has breached the hardened rock and eroded the softer rocks inland into a cove - structure similar to the more familiar Lulworth Cove, in Dorset.
Returning to Downie Point itself, the object of interest here was a remarkable conglomerate, no less than six hundred feet thick, occurring as the central member of a series, with sandstone below and another above. The whole is known as the Dunnottar Series (Dunnottar Castle is situated nearby) and lies stratigraphically immediately above the Downtonian, all being Old Red Sandstone. The dip here is almost exactly vertical, with the succession younging towards the south-east. Downie Point, a roughly parallel-sided promontory, consists almost entirely of the conglomerate, visible on both sides of the headland, and with the lower of the two sandstones, on the Stonehaven side, seen only on the shore, and in the cliffs at the landward end. Thus the base of the conglomerate is exposed as an impressive vertical wall over a considerable distance. The slides showed two reasons for this exposure: seawards, erosion has removed most of the softer sandstone; landwards, former quarrying operations have done the same, and left clean, readily-accessible end very photogenic surface showing some of the special features of this conglomerate.
The most striking feature is its extreme coarseness; numerous blocks up to four or five feet across were visible, and very many were above two feet. Most of the components were very well rounded, nearly spherical in some cases; a few angular pieces were seen among the smaller sizes, but even these had rounded corners. Clearly a deposit of this nature must have been transported a considerable distance, and by very fast-flowing water. Another notable feature seen in an oblique view of the quarry face was the sharpness of the contact between the sandstone and the conglomerate, everywhere parallel, or nearly so, to the bedding of the sandstone. It is evident that the change of conditions leading to the deposition of this conglomerate must have been very sudden indeed.
On the south side of Downie Point lies another bay. A final slide in this series showed cliffs of the upper sandstone on the far side; it was said that there is another, less spectacular conglomerate exposed in the next headland. Dunnottar Castle just visible in the distance.
The meeting then moved on to consider and compare several slides of the Speeton region, taken independently by Mr. Gravett and the writer over a period - they were of the years 1968, 1970 and 1974. A number of views of Red Hole, and of the 'Pink Band' exposures further towards the headland, led to a long and very animated discussion, largely inconclusive, and certainly un-reportable, concerning the stratigraphy and palaeontology of these beds, both in relation to each other and to similar beds elsewhere. This discussion, perhaps better called an argument, seemed to be greatly enjoyed by those members who knew what they were talking about!
All this was by the way, as the intention in showing these slides was to demonstrate the geomorphological changes which have taken place in this region over the last few years. Here at least some definite conclusions could be drawn. One member commented that significant storm damage had been caused during the winter of 1972, and some evidence of this could be seen. In the shorter term, it is clear that much slipping of the middle cliff has taken place recently in the region between the red and Pink chalk exposures; also that a bluff in the Pink region has been largely eliminated by wave action. It seems, from the precarious attitude of many of the boulders lying on the slope of the middle cliff, that active slipping is still taking place.
Further along the cliff, beyond the Pink Chalk, there was in 1970 a large talus slope, reaching probably a hundred feet up the cliff face. This has now vanished almost entirely. And finally, still further along the cliff, where the face is near-vertical for some three-quarters of its height, evidence was seen of a slow-moving major slumping of a single huge column of chalk. This column, originally about three hundred feet high to the drift slope, was already on the move in 1968, and had by then cracked away from the cliff near the top, the capping of drift forming a conical pinnacle. By now the whole structure can be seen to have moved downwards by distance estimated to be about sixty feet.
To conclude the evening, the President assembled a large collection of photographs he has taken recently with the Stereo Scanning Electron Microscope in the Geology Department of Hull University. These are in connection with research work he is currently undertaking, involving a study of the surface configuration of certain small fossil shells, typically of the order of one centimetre across. No adequate description of these photographs can be given here: it must suffice merely to record the astonishing wealth of ultra-fine detail which they exhibited.
This had been a long evening, longer than any of its kind in the writer's experience. That it passed so quickly was due to the great interest shown in the various exhibits, and to the stimulating discussion which they engendered: all present had something to contribute to make the evening a success. The one regret was that so few members had availed themselves of the opportunity.
H.R.W.
This meeting has been reported in great detail in the hope of encouraging more members to attend future informal evenings.
B.L.
copyright Hull Geological Society 2021