The next indoor meeting will be held on 15th January 2014. This will be the AGM to be followed by a talk given by Dr Sara Metcalf entitled: The Grinshill Fossil - Footprints in the Sand.
On 20th November Prof. Bill Fitches gave a talk on gneissic rocks, especially in Greenland, in particular those on the western margin of this huge territory, recalling the 1970s working for the Greenland Geological Survey, landed by helicopter to lead isolated mapping teams under canvas for weeks at a time in this beautiful but harsh environment. Greenland is on a craton (ancient continental crust) which extends east beneath the Outer Hebrides and north-west Scotland.
The oldest rocks are 3.75 Ga; even the youngest are 2.3 Ga, but these old rocks are often overprinted by younger events. The gneisses result from extreme metamorphism of mudstones at 600°C after burial up to 30 km deep. The structures at field scale are stunning. The minerals have migrated to large-scale dark and pale bands visible from long distance. In places the gneisses are sheared into puzzling structures, and sometimes extensively cross-cut by igneous dyke swarms. Rocks derived from shale similar to our Silurian black shales, but ten times older, was so metamorphosed that the carbonaceous content has segregated as graphite. Perhaps the organic material came from extremely primitive life 3 Ga ago. We were introduced to a new range of features including en-bayonet dykes, tennis ball gabbros and football anorthosites!
The talk concluded with a look at the Swiss Alps where Bill Fitches, during his time on the academic staff at Aberystwyth University, took students to see similar TTG gneisses (tonalite-trondhjemite-granodiorite) a hundred times younger than those in Greenland, and derived from Permian granites. The Matterhorn, in the alpine pennine nappes, shows rock of the ancient African continent thrust north over the top of European basement gneiss; between the two a band of rock formed from the oceanic crust and sediments of the floor of the long-gone Tethys ocean.
The next indoor meeting will be held on Wednesday 20th November, 7.15 pm at Plas Dolerw. Pfofessor Bill Fitches will present a talk entitled “Greenland’s Ancient Gneissic Rocks”
The last indoor meeting was a talk given by James Cresswell of “GeoWorld Travel”. This was a highly informative and enjoyable talk.
Greenland has some of the oldest rocks on earth which are found in the Isua Greenstone Belt and have been aged at between 3.7 - 3.8 GA. Greenland forms part of the North American Plate whilst Svalbard forms part of the European plate. It is this relative difference in their positions which have produced the differences that can be seen in their geology. With this in mind, commencing at 4.6 GA James worked chronologically through geological time discussing plate movements, glaciation and rises and fall in sea level, sedimentation and orogenesis, the results of which can be seen in their geology. This detailed explanation along with superb photographs of spectacular rock formations made for a very pleasurable talk.
The next indoor meeting will be held on Wednesday November 20th where Professor Bill Fitches will give a talk entitled: “Greenland’s Ancient Gneissic Rocks”
The next indoor meeting will be held on 16th October. The speaker is James Creswell from GeoWorld Travel who will speak on the Geology of Eastern Greenland and Svalbaard.
