At the last indoor meeting Tony Thorp and Michele Becker held a light-hearted evening of rocks. Tony's volcano had everyone holding their breath whilst awaiting the eruption. Michele then went on to explain fractional crystallisation in magma. This was followed by practical demonstrations of turbidity currents, normal and reverse grading. The latter actually being edible!
This evening was followed by a field trip in the steps of Darwin as detailed below.
Our September field trip, on the 20th, followed "In Darwin's footsteps" under the leadership of member Roy McGurn. Our meeting point was at the first location, Pen y Foel Lane, Llanymynech, where Roy explained the background to the dozen assembled members.
In August 1831, England's foremost geologist, Prof. Adam Sedgwick, set out on an expedition to map the "transition beds" of Wales, that had hitherto defied meaningful interpretation. The period was one of dramatic change in the understanding and concepts of geology. Although a biblical interpretation (according to Genesis) was not seriously regarded by many researchers by this time, the concept of some kind of creation was still strong.
For the previous 50 or so years, the theories of Abraham Gottleb Werner had held sway, that the rocks we see are the result of some sort of fluvial action, perhaps floods or deposits left by a retreating sea. The school of thought was broadly known as "Neptunism". Sedgwick himself had used his presidential address to the Geological Society to recant his belief in this theory that very spring.
The succeeding counter argument, perhaps broadly termed Plutonism, was the development of ideas presented by James Hutton in his two lectures to the Royal Society of Edinburgh as far back as 1788. Hutton had postulated the rock cycle, with rocks constantly being renewed from igneous rocks breaking through to the surface from great depth. Hutton could see no sign of a beginning , nor any sign of an end to this process, which finally challenged any concepts of a divine creation, which also postulated a day of judgement.
It could be said that Copernicus challenged the concept of the earth being at the centre of the Universe. (He was wise to publish his works when more or less on his death bed, as in his day such ideas would have amounted to Heresy.). Hutton challenged the idea of a biblical creation, again another heresy in an earlier time. In the summer of 1831, the third prophet who was to eventually challenge man's place at the centre of creation, began his journey here.
It is perhaps how fate works that the grandson of Hutton's friend, Erasmus Darwin, who invoked some ire himself with observations on the immutability of species was asked to accompany Prof. Sedgwick on his quest.
A few yards up the lane, they stopped and no doubt could see in the road cutting, steeply dipping shaley rocks on which Darwin could try out his newly acquired clinometer (a small plaque marks the spot). From this spot they could see the nearby limestone escarpment and quarries of Llanymynech Hill. These are almost flat and, if projected, would have been overhead at this point. These radically different dips would indicate an unconformity, as taken by Hutton to show a break in succession and maybe a period of erosion.
The second location was on Trevor Rocks, above Llangollen, with a view of Castell Dinas Bran and up the Dee valley. Sedgwick had business in Llangollen, with Robert Dawson, a surveyor, who was to furnish him with notes on the limestone of the Vale of Clwyd.Sedgwick's mission was very vividly set out for him here. The prominent Mountain Limestone, as it was known is easily identified by its distinctive lithology. It was known elsewhere for the "Herefordshire Beds" to occur beneath the Mountain Limestone, again with a fairly distinctive lithology. Below the Herefordshire Beds, into deepest and darkest Wales, the lithology was a lot less distinctive and geologists at the time were having great difficulty making any sense of it. It was generally referred to as the transition beds..
Sedgwick needed to find a sequence of strata that took him down into the transition beds and from which he could derive fossil sequences to identify them. Ideally following the maps of the time, he needed to follow a sequence down from the Herefordshire Beds. (= Old Red Sandstone). Sedgwick was using a later version of Greenough's map that had partially corrected the Herefordshire Beds error. The red sandstone in the Vale of Clwyd had identified as New Red Sandstone, a rock common in England and known to be some way above the Mountain Limestone. However a small sliver of exposed rock was still indicated below the Mountain Limestone and was known as the Old Red Sandstone.
At this location the scree covers the base of the limestone. Tantalisingly traces of red soil may be found in places along here, a possible indicator of red sandstone (Old or New) beneath.
As Dinas Bran is significantly higher than the base of the limestone, even if the latter is "extrapolated" towards it, there seems to be something else happening between the two. A small fault is marked on the modern maps which must take the southern side up a little, so any base to the limestone will have been above the hilltop and hence long eroded away.
Sedgwick was not going to find much to help him here.
The third location was at Velvet Hill, next to the road up to Horseshoe Pass, at the entrance to Valle Crucis Abbey. Darwin's enthusiasm for his new clinometer is evident, he records:
"Saturday 6th August Vale of Crucis.
The bank above the abbey consists of Clay slate, which breaks at regular intervals, striking nw by n, d,25 to the ne by n. at different parts road observed beds of diluvium very Shrops only no sand:also boulders of trap."
The entrance to the abbey contains a small well. This is likely to have been the original road and the well may well have been a stop Sedgwick made to water his horse ahead of the long climb up Horseshoe Pass. Darwin busied himself with his notebook and clinometer.
Darwin went on to note.
"Beyond Vale of Crucis on the road to Rurhven the Limestone is seen having a grand escarpment to the west: The contrast between this and the more regular slope of the Clay Slatew gives more grandeur to the views. The Greywacke generally covered by gorse, Heath and Fern: the limestone either bare or the verdure very green." However Darwin's observation shows how they were using geomorphology to identify the outcrop of limestone. Beyond the pass the escarpment changes, with the rounded, heath covered greywackes he noted, seeming to break into the line of limestone hills, and the limestone outcropping into the greywacke area.
Next Step. Darwin notes "About 1/2 mile beyond Daforn, a black bituminous Limestone organic remains veined quartz in parts reddish in one part strata exactly arched. The line N by E."
This note refers to a disused quarry at Plas Newydd where, indeed we found limestone crags, reddish in parts. No arch, but much limestone may have been removed.
Darwin goes on "1/2 mile further a tortuous valley through Clay Slate generally dipping to the E. About a mile from the Ruthven beds of sandstone"
This is Nant y Garth, the faulting brings up the Silurian beds again and forms the end of the Vale of Clwyd. At its end, the road opens into a plain with hills either side, especially a clear line of them to the north. We are into Triassic New Red Sandstone, but outcrops are rare although the soil is red.
Appropriately, we ended our trip at the Castle Hotel, now a Wetherspoons, where Sedgwick and Darwin spent the night. Darwin notes:
"Ruthin: takes its name from the new Sandstone on which it is built. The soil is for some miles about the town and the plain may be considered of that formation. In most places covered up by diluvium. Mile to the west of the town a quarry of worked. The rock is spotted with brown the stone at Cardeston. Overlying Magn conglomerate. It is very irregular strat but the rock on which the castle is built nearly horizontal seams. petrifications this there are some beds of Old red Sand striking the same ways as the overlying stone but dipping at a greater angle. A further is more clearly visible in a water"
The elusive ORS has finally reappeared and Sedgwick may have thought he had an opening into the lower succession, the notes stress this rock is UNDER the limestone and does have all the appearance of the beds found in Herefordshire. However the boundaries into the lower beds are generally faulted here, in its simplest form the Vale of Clwyd is a rift valley, down faulted during the Upper Carboniferous-Triassic. Sedgwick was not to find a succession into the transition beds anywhere in North Wales, as the unconformity Darwin didn't observe way back in Llanymynech represented a break in deposition and a period of erosion that covered the entire Devonian period and the beginning of the Carboniferous.
Members had enjoyed a fascinating trip and Roy's research and leadership were really appreciated. Most members started to return home at this point, but noted that Ruthin contained a remarkable assortment of building stones, particularly evident in St. Peter's Church, where Sedgwick and Darwin attended Matins on Sunday 7th August 1831. They perhaps viewed the pot-pourri of stone used in its construction.
Ruthin seems worth a full day trip in its own right. Perhaps we could follow more footsteps at a future date.