The next indoor meeting is Wednesday 21st May when Roy McGurn will give a talk entitled: “Fracking: A hot topic but what’s it all about?”
This could turn out to be quite a lively talk!
There was unusual interest in the April evening meeting when Duncan Hawley, from Swansea Metropolitan University, spoke on Early Geological Maps. He also brought along a splendid selection from his own collection. ‘The geological map is the dynamic force in geology, and the history of its evolution and development is the history of the birth and growth of geology’, so said FJ North in 1928, when he was the first keeper of geology at the National Museum of Wales.
Packe, in 1743, used hatching to depict the topography of Kent, probably the first geomorphological map. Early maps were monochrome; some, like Wm Maton, in 1793, believing that colour would be a distraction. By early 1800s colour became widely used, as in Gimbernat’s map of the Tyrol. In 1809 Jamieson produced the first guide ‘on colouring geognostical maps’. The earliest maps merely noted spot descriptions of rock and correlation over long distances began with Guettard’s 1746 map of France and southern Britain, an early mineral map which picked out the chalk on both sides of the Channel, but depiction of continuous boundaries was common by mid 19th century. Guides on mapping geology were introduced in the 19th cent, for example Henry de la Beche’s Geological Manual (1831) and Ami Boué’s Guide du Geologue Voyageur.
Early geological cartographers frequently worked alone, a serious disadvantage, resulting in considerable inaccuracy. Wm Smith first attempted to delineate geological strata in 1801. His 1815 map is a magnificent one-man effort, with about 100 of the 400 printed copies still surviving. But the collaborative efforts of men like George Bellas Greenough achieved more, by compiling the work of local experts like Fitton, Buckland and Aikin. That effort, through the young Geological Society of London, was in fact already mostly completed by 1812, though not published until 1819. The earliest mappers even had to work without ordnance survey maps. Even with basic maps the geological contours were often inaccurate, changing markedly within a few years, as shown by comparing Murchison’s map of Wales and the borders in 1839 with that of the Geological Survey only six years later.
Colour posed a problem for early geological map publishers, first being added to individual copies by hand, often by young women, thought to be more skilled in that work; though hand colouring also sometimes led to inaccuracy, and darker colours could obscure the underlying information. Smith’s map used gradational colouring to show strata becoming younger, and Farey in the 1810s showed how structural features like faults can be interpreted using colour to show the outcrop patterns they produce. However, by the mid-1800s printed colour maps began to emerge and was cheaper but less accurate and more technically difficult to produce until the 1880s when printing became inevitable as more detail was added to maps. Today’s mapping is done with the geological equivalent of an iPad and the modern maps are things of beauty. What would the early workers have thought. The audience was much taken with Duncan’s evident enthusiasm for his subject, and a vote of thanks was proposed by Tony Thorp.
Two ½ hour talks:
Professor Mike Rosenbaum gave a very interesting talk as he took us back through time looking at some of the events that have led to the development of the Shropshire Landscape that we see today.
The talk commenced by looking at the effects of glaciation. The Anglian glaciation ( commenced 450,000 years ago) was the most extensive but in Shropshire and in Powys it’s effects were obliterated by the later Devensian glaciation (commenced 100,000 years ago). The Devensian ice moved Eastwards from Wales meeting the Irish sea ice from the North. During this period features eg. the Berwyns stuck up from the ice as Nunataks. This ice retreated about 20,000 years ago.
One noticeable effect of this ice is the Iron Gorge which was cut by a sub-glacial river and this has produced a gorge that has very steep sides. The steepness of the sides leads to instability and landslides have occurred and the land is still slipping.
One other very interesting idea that was discussed is one that arose from observations made by OT Jones in 1951. He observed that the river systems in Wales are radially centred on a high point in Anglesey. Later Brown in 1960 observed that that there is a wide elevated plateau covering central and southern Wales and the Marches. It has been suggested that the cause for these observations is due to the fact that the region was once over an ancient mantle plume. Luckily it is now inactive!
Next field trip will be Sunday 28 April. Gather at 10.30 am, car park at Visitor Centre, Elan Valley, Rhayader LD6 5HP (SN 928 647). Driving south on the A470 to Rhayader, turn right at the crossroads and stay on the B4518 for 3½ miles. Bring a packed lunch. Hard hat not essential but advisable if you wish to approach close to rock faces. We aim to return to the visitor centre by around 3 pm (toilets at visitor centre and lunchtime at Claerwen dam). The localities examined lie on or near a single five mile stretch of quiet reservoir road between Caban Coch and Claerwen dams, so we will move mostly by car; the walking is easy.
The roads which wind around the exceptionally beautiful and peaceful reservoirs of the Elan and Claerwen valleys reveal the Cerigwynion Grits which ended the Ordovician here, and the mudstones which began the Silurian. Rising and falling sea-level 440 million years ago produced a range of different types of deep-water sediment deposition. This sandstone and mudstone succession was deposited as a series of gently sloping lobes with slumps, debris flows and a wide range of turbidites. An unusual feature is the record of a very different depositional environment, the Caban Conglomerates, an erosive high energy submarine channel flow which cut through the sandstone/mudstone slope-apron and left its own story in the rock. Local folding (Rhiwnant Anticline) followed by natural erosion and man’s rock cutting and has left different rock facies conveniently juxtaposed.
The next indoor meeting will be on Wednesday 16th April 2014 when Professor Mike Rosenbaum will give talk entitled “Shropshire’s Evolution: highlights of a journey through time.”
