The Club’s Annual Summer Weekend, 8-11 July 2016, was spent in the Lake District, based in the delightful lakeside town of Keswick, among the northern lakes, and admirably led by Cumberland Geological Society stalwart, John Rodgers. The Lake District, which is also the Cumbrian Mountains, is some 30 miles square of Ordovician and Lower Silurian rocks surrounded by younger Carboniferous rock. A band of Ordovician Skiddaw Slate lies across the north and a band of Silurian rock across the south of the district. Between them is a band of igneous rock, mostly lavas and ash flows, the Borrowdale Volcanic Group, hosted by Ordovician rock. This formed due to arc volcanism, caused by a subduction front as the Iapetus ocean closed only 40 km to the north, across the Solway Firth. An underlying batholith of granite caused the original uplift of older rock. During the Ice Age this erosion resistant dome was sculpted by glaciers to form the picturesque lakes and lakeland scenery.
On the Friday evening our leader gave an introductory talk on the lakeland geology. A month spent there would not do justice to the district, so our two days had to be very selective. On the Saturday we explored an area north-east of Keswick in the Skiddaw Group, around Mungrisdale and Mosedale Bridge, moving on in the afternoon to the nearby valley between the dramatic Bowscale and Carrock Fells. These localities allowed us to examine structures in the Skiddaw slate, and look at the effect of metamorphism on the slates, particularly rocks spotted with cordierite, and containing acicular andalusite. In the valleys of the Caldew and Grainsgill Beck we could see both granite outcrops and glacial action. The weather was mostly kind but, this was our twelth annual weekend excursion, and we can’t expect perfect weather all the time. It literally blew a gale when we walked up to the Tungsten mine. Saturday evening ended with a discussion back at the B&B before the group dispersed to various Keswick restaurants.
Sunday began at Friar’s Crag looking south down Derwent Water and discussing the working of the glacier which formed this, generally flowing north towards us, round the dolerite intrusion on which we stood, continuing north of us to excavate also Bassenthwaite Lake. This area is on the northern part of the Borrowdale Volcanic Group, so again we were able to combine consideration of the local volcanics - this area was a caldera – with an appreciation of the extraordinary erosive power of the glaciers. We drove down the west bank of Derwent Water to see roche moutonneés, lava flows and columnar cooling among many other features. In the afternoon we walked the Rosthwaite moraines just south of Derwent water, in the glacial valley now occupied by the north-flowing river Derwent and its tributaries from Seathwaite and Stonethwaite to the south, endeavouring to understand the glacial movements which left this debris. Another splendid weekend ended on the Monday morning, for most us with a look at Keswick and the Threlkeld Museum just east of the town.
At the last meeting Dr. Chris Simpson presented a snapshot of the geology to be found on the island of Cyprus. The main focus of the talk was the Troodos Ophiolite. Ophiolite is a fragment of oceanic crust and the underlying upper mantle that has been uplifted and emplaced onto continental crust.
The Troodos ophiolite was created during the complex process of sea-floor spreading and formation of oceanic crust and was emerged and placed in its present position through complicated tectonic processes related to the collision of the Eurasian plate to the north and the African plate to the south.( Geological Survey department cyprus) It is seen as the most complete, intact and studied ophiolite in the world.
With many good photographs Dr Simpson worked his way up through the stratigraphic column of the ophiolite commencing with the ultramafic rocks like harzburgite composed of olivine and orthopyroxene. Harzburgite along with dunite are interpreted as a “mantle sequence”, which represents residue from partial melting of the upper mantle. The next sequence was the layered gabbro with massive gabbro above. The gabbro is a residual of magma. During the slow cooling, the heaviest minerals lie in the depths, thereby creating the darker coloured gabbro, with light-coloured varieties in the higher layers. At higher levels and in small pockets, the products of this different crystallisation are to be found, known as plagiogranites or granophyre.
Above this is a sheeted dyke complex. They are pathways through which molten basaltic magma rose from the mantle to the seafloor where it solidified as a pillow lava. Some of the magma did not reach to the surface and solidified as dykes, consisting of dolerite. Above these are the volcanic rocks consisting of pillow lavas and lava flows. Basalts predominate in the pillow lavas and are characterised by large spherical to ellipsoid pillows. Their crust is glassy due to the rapid cooling but the insides of the pillows is honeycombed due to sudden expansion of gases in the lava.
The geology along with the wonderful weather makes Cyprus a lovely and interesting place to visit.
Our next event will be on Wednesday the 15th of June where guest speaker Dr. Chris Simpson will give his talk “Cyprus Geology: a snapshot”
This month we had a talk by Jack Davies on “Reflections on Geomorphology”. It was much more than an investigation of the development Britain's geomorphology, more a geologist's life story. Starting with childhood freedoms exploring Britain by cycle, through student days, to theses typed in DS with multiple carbons, he had developed a deep understanding of the development of landforms and an appreciation of the uniquely diverse geology of the British Isles.
He described, from a personal perspective, the landforms resulting from the Cretaceous transgression of Wales, followed by Mid Tertiary uplift and the removal of the chalky deposits, with the initiation of drainage and erosion. Stripping back of the Mesozoic cover revealed a wave-trimmed peneplain, with superimposed radial drainage cutting down to the Palaeozoic rocks beneath.
Mid Tertiary uplift elevated the Welsh High Plateau and later negative changes in base level produced the Middle and Low Peneplains with consequent adaptation of drainage.
Later Pleistocene glaciations produced the 400ft, 300ft and 200ft platforms
It was a refreshing approach with no Powerpoint!
Our next meeting will be on Wednesday the 18th of May, where guest speaker Jack Davies will present his talk “Reflections on Geomorphology”