Sunday, 14 November 2010

Somerset - Geological Sculpture

The photo shows views of the recently installed wooden sculpture at Staple Hill on the northern edge of the Blackdown Hills in Somerset. Happily the stratigraphy is simple with mainly horizontal bedding and none of the Variscan complexity that we find in west Somerset. One problem was to decide how far the fine detail of the stratigraphy should be shown: do we divide up the Upper Greensand Formation into Foxmould and Chert Beds, or more modern equivalents; was it necessary to show various divisions of the Lower Lias clays as we see them in the cliffs at Charmouth?
A second problem was nomenclature: the British Geological Survey (BGS) Lexicon of Named Rock Units provides BGS definitions of terms that appear on their maps and publications; it revised wholesale the names of formations and members. It was an attempt to bring order into the haphazard terminology that had accumulated over the last 200 years. All very commendable except that well-used names were dumped (e.g. The Yeovil Sands Formation for shame). The Somerset Geology Group is finding problems in advising on which old or new name to use for building stones for the revised Pevsner architectural guide to Somerset. Fortunately, the term ‘Upper Greensand Formation’ and ‘Blue Lias’ remain and are widely understood by the public. Ought we to have used the term ´Charmouth Mudstone Formation´ instead of ‘Lower Lias clays’? Do we include the term ’formation’?
We decided that traditional semi-descriptive terms were most suitable especially as they include the words ‘sand’ ‘clay´, and ‘red marl’. After all we are trying to communicate with the interested public and not frighten them.
The upper photo shows the classic topography: plateau, convex upper slope, linear middle slope, concave lower slope and the vale. For technical reasons it was not possible to show the slip plane of the landslide. The topography is clearly related to the underlying geology: the Greensand makes a resistant capping to the escarpment thus preserving the eye-catching Blackdown Plateau and the Tertiary Clay-with flints and pebble beds that rest on that surface. The pebble beds are shown on the side panel; they are not distinguished on the recently revised BGS Wellington Sheet (2009) and this is a sore point! (Prudden, 2002, 2011 in press, Waters, 1960). They overlie the Clay-with-flints and represent an early Tertiary marine event. They are exposed in ditch sections at Staple Hill. The thicknesses of the pebble beds and the soil have had to be exaggerated on the sculpture. The Lower Lias clays crop out below the Greensand and underlie the clay vale seen in the distance.
The lower photo shows the face of the block. A symbolic landslip is shown. Water percolating through the Greensand emerges where the Greensand rests on impermeable clays; a spring line is shown by a metal strip. These landslides are some of the most extensive in Britain and ring the Blackdown Hills. They are not active today unless loaded or undercut as when the M5 was built; they must date to an environment in the past when melting permafrost resulted in instability. Freborough et al., (2005) point out that many of these landslides tend to have a linear rather than crescent-shaped backscar. Marshy grassland and forest are shown below the spring line.
The sculpture was assembled on the spot from components made in a workshop. The timber was sourced from a fallen oak tree from Neroche Forest. It will be allowed to weather naturally. It is hollow underneath - hedgehogs please note. The sculpture was formed from modules created in a workshop and the components bolted together on site. The sculpture was designed and made by Robert Jakes, sculptor and woodcarver.
Neroche Forest is a Landscape Partnership Scheme supported by the Heritage Lottery Fund and a range of local organisations. Led by the Forestry Commission and working closely with the local community, the Scheme is working to protect and celebrate the heritage of the area, and to provide opportunities for quiet enjoyment, education and training in countryside skills. Neroche Forest is part of the Blackdown Area of Outstanding Beauty. The scheme involving a wooden sculpture was developed by the local stakeholders group.
Visitors may also care to explore via public footpaths the area 2.5km to the east below the site of the Norman Castle Neroche [ST 272 159]. There is a splendid backscar and pressure ridges at the toe of the landslide.
The coast between Charmouth and Lyme Regis shows what Staple Hill would have looked like during periglacial conditions when the landslides were active. Visitors to the coast can clearly distinguish the same rock formations as at Staple Hill and admire the seepages at the base of the Greensand and the active landslides and mudflows. One difference is that the landslides at Charmouth are the result of waves undercutting the cliff.
The Somerset Geology Group was pleased to cooperate in the project as one of its aims is to promote an awareness and understanding of the Somerset’s outstanding geodiversity.

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