Fault Reactivation – Inversion structures of the Bristol Channel Basin

Coastal sections observed between Watchet and Kilve contain some of the best exposed normal fault systems in the UK. The normal faults which formed as a result of extension, during the formation of the Bristol Channel Basin show evidence of reactivation. The reactivation of the faults in the Bristol Channel Basin is thought to have occurred in the late Cretaceous and Tertiary due to contraction attributed to the Alpine collision which formed the Alps.

The rocks exposed in the cliffs of the coastal section belong to the Mercian Mudstone Group and the Penarth Group of the Late Triassic, and the Lower Lias of the Lower Jurassic. The faults offset the beds and along the fault plains gypsum has been spread, which record slickensides indicating the movement of the faults.

The photo shows one of the faults which show evidence of mild inversion in the Bristol Channel Basin. The normal fault has clearly offset of the red Triassic rocks of the Mercian Mudstone Group in the footwall and the pale grey Penarth Group in the hanging wall. Along the fault plain is a thin layer of gypsum. This fault has been reactivated and has been interpreted as an inversion structure due to the evidence of contraction, which is highlighted by the tight folding in the hanging wall against the fault. The contraction cannot be accommodated on the original fault due to the steep angle of the fault, therefore folding occurs in the hanging wall. Occasionally later reverse faults called shortcut faults form to accommodate the stress caused by the contraction but this was not observed at this location.

An afternoon fossil hunting on the south coast

The following are a few fossils that I found while walking along the beach.

Hay Bluff

Hay Bluff overlooks the beautiful town of Hay on Wye, which is world famous for its wealth of bookshops. Just a short drive from the town down a winding country road takes you to the northern most tip of the black mountains where you are greeted by the peak of hay bluff. Hay Bluff is 677m in height and boasts excellent views of the surrounding area on a clear day. The mountain is a very popular place for families, hikers of all levels and Hang gliders. Once at the top of the mountain you can follow the Offa’s dyke trail which follows the England/Wales border for 177 miles.

While walking along it is hard to ignore the large outcrops of rock that stick out of the landscape. While walking along I noticed and looked at 3 large outcrops, from this I identified 2 different sedimentary units, the first was a coarse grained sandstone unit. This is typical of the old red sandstone which dates back to the lower Devonian around 400 million years ago. At this time the first terrestrial rocks were being deposited over Herefordshire and the surrounding area. This was due to the world’s continents colliding with each other which went on to form the supercontinent Pangea. Looking on geological maps the sandstone looks to belong to the Senni Formation which is characterised by green coarse grained sandstone and is believed to have been deposited in fluvial channels.

The second sedimentary unit was a matrix supported conglomerate which underlay the sandstone. The clasts were on average 3-5cm wide and were well rounded. The matrix looked to compose of fine sand which ‘glued’ the clasts together. This unit is also dated at just over 400 million years old, at the start of the Devonian. This unit outcrops over a larger area than the previous unit and I think relates to the St Maughans formation which is characterised by many rock types not just conglomerate. The formation is believed to have been deposited in a braided river system on flat open land.

References:

http://79.170.44.138/earthheritagetrust.org/pub/learning-discovery/aggregates/lithology-profiles/senni-formation/

http://www.blackmountainsite.co.uk/other-places-to-visit/peaks/hay-bluff/

http://maps.bgs.ac.uk/geologyviewer_google/googleviewer.html

http://79.170.44.138/earthheritagetrust.org/pub/learning-

discovery/aggregates/lithology-profiles/st-maughans-formation-ditton-group/    

 http://www.offasdyke.demon.co.uk/   

http://www.goherefordshire.co.uk/

The coolest place in Stockholm

Stockholm the capital of Sweden and acclaimed capital of Scandinavia is a beautiful city which has something for everyone. On my trip I visited many places but the coolest (literally) has to be the Stockholm ice bar, which is situated in the lobby of the Nordic Sea Hotel. The room is completely made of ice from the Torne River that is situated in northern Sweden.  The constant -5°C temperature in the bar is needed to stop the ice from melting and to prevent the artistic ice sculptures that decorate the bar from being destroyed. If being in a room made of ice was not enough, the hotel also provides a specially designed cape which keeps you warm and in my opinion adds to the uniqueness. Included in the entrance fee is a free cocktail (both alcoholic and non alcoholic are served) which is vodka based. There is a wide range to of cocktails to choose from and all come served in a glass made completely of ice!

I highly recommend the Ice bar Stockholm to anyone who wants to do something unique while in Stockholm.

Mapping Aliaga, Spain

The town

Aliaga is a small settlement in Teruel province, Spain. The village provided the opportunity to experience life in a real Spanish village far away from the usual tourist trails and hotspots which many people associate with a trip to Spain.

The village had all the basic amenities needed, there were several bars which served a brilliant beer just what was needed after a hard day’s work in the field. The only restaurant in the village served a good range of food and had some very interesting outcrops just meters away from the building. During my 2 week trip the village held one of its yearly fiestas, during this time the village’s population doubles if not triples with people coming from all over Spain, I met some students that had come from Barcelona for the few days that the fiesta was on. The people were very friendly and welcoming, especially the younger citizens who were keen to try the English they had learnt out on us.

The geology

The geology of the Aliaga area is outstanding, for this reason it is protected by geopark status and hammering is not allowed. All the rocks are sedimentary and are packed full of fossils such as ostracods, and in some areas dinosaur footprints can be observed. These dinosaur footprints are thought to belong to a Theropod. The geology becomes complicated in places where there is folding which is the result of plate tectonics, and faults which displace the rock units by up to a few metres. These features are key to exploring and understanding the Mesozoic and Tertiary evolution of the Iberian Plate. In the town of Aliaga there is a small museum which is geology based and is worth a visit. It shows finds from around the area, such as fossils and explains the evolution of the area through time. A simple web search can provide extended information on the geology of the region.

The beginning

To begin with we walked a road section with the lectures. This section cut through all the units that we would be mapping in the area, so it was important to split the section up into mappable units. The units were defined by the sudden changes, for example a sudden lithological change from sand to mud. Two days were spent walking the section getting detailed descriptions of each unit, these descriptions would be useful to refer back to when writing up a report and to help identify the units while out mapping.

Mapping

The first day of mapping was slow and not much ground was covered but as the week progressed more ground was covered because my field techniques such as taking measurements and triangulating my position on the map were getting quicker. The type of mapping that I used was boundary mapping this was easy to do because the exposure was good in the area. Because the exposure was so good, Google map images of the area could be used to help identify large scale features such as faults in the field.

The final product

The final product was a geological map covering approximately 2km². I was really pleased, as this trip had been the first time I had done geological mapping and doing it independently added to the satisfaction. However I knew that this was just the beginning, because when I got back to the UK I would have to analyse and write up a report using the data collected in my field notebook and from the map.