Somerset Geology Group - Autumn Newsletter

Please click here to download the Somerset Geology Group Autumn Newsletter from Hugh Prudden.

Somerset Geology Group Newsletter Autumn 2012

Hugh Prudden of the Somerset Geology Group has sent the following;-
BUILDING STONES   
The centre of Yeovil is noteworthy for excellent displays of the condensed, algal facies of the Barrington Beds (Beacon Limestone, Upper Lias Limestone, Junction Bed, or what you fancy) in walls and buildings. There has been a long-standing need for a supply of this building stone but it is no longer worked. There were once shallow quarries all over the outcrop. Recycled stone is not always satisfactory as it tends to break up.
This is just one example of the need to find small quantities of new sources; the idea is to exploit temporary exposures and take the stone for preparation in established work shops. Ben Miller, Minerals Planning Officer, Environment Directorate of Somerset County Council, together with Geckoello Consultants, are interested in seeking new sources of building stones, especially for renovation work.
a. They need information on the which building stones might be needed but are no longer available e.g. North Curry Sandstone, Wedmore Stone, Calcareous Grit from the Upper Greensand, White Lias.  We need more suggestions.
b. We also need more potential sources of stones.
At present there are good supplies of Doulting Stone, Ham Hill Stone, Blue Lias, Budleigh Salterton Pebble beds (sandstone facies) at Capton, and Upper Westleigh Limestone. The Morte Slates in the old waste screes at Oakhampton Quarry are being used for walling. I would be happy to coordinate suggestions.

We are in the early stages of setting up a collection of Somerset building stones at the Somerset Heritage Centre in Taunton. We need good examples of North Curry Sandstone, Pennant Sandstone, Downside Stone.
Contact: Hugh Prudden with any information

Somerset Geology Group - other news
We no longer have meetings for various reasons but keep in touch with an occasional Newsletter.  This goes out to some 60 recipients, both local worthies and many in academia. Hugh welcomes news of recent publications, research and meetings. SGG is a loose association with no formal constitution or subscription. We do not seem to have a formal website yet but perhaps one day....this is Somerset! However, if you Google appropriate names and topics you will find much of interest including a useful Somerset Good Rock Guide to Somerset. It is a quick reference guide to the best places of general geological interest in Somerset. It is a starting point for exploring ...  Members are very active in various fields.  Please let me know of anyone who would like to be on the mailing list.

Websites of interest:-
a. this one! It contains news of events at home and abroad and useful links to associated groups in the region, one of which is the Bath Geological Society which has a superb programme of speakers and are recommended if you live in the area. 
b. Outcrop - blog of the Avon RIGS Group-promoting geology in the West Country’.  It contains news of forthcoming events, RIGS of the month spot, local minerals, fossils etc. It is clearly written and usefully highlights the various aspects of Avon and nearby areas.  Worth a look at.
c. Doreen Smith of the Dorset GA  Group issues a fine Newsletter with accounts of Dorset’s  geological highlights and events.  Details on their Website.

Somerset Good Rock Guide

PLEASE NOTE - the Somerset Good Rock Guide has changed location. It can now be viewed from the home page of the Bath Geological Society's website or can be found directly here.

Book review - Exe Valley

Hugh Prudden of the Somerset Geology Group has reviewed this new book as follows:-
The Quaternary of the Exe Valley and adjoining areas: Field guide.
Basell, L.S., Brown, A.G. and Toms, P.S. (eds).  2011. 
Quaternary Research Association.  
The Guide was produced to accompany the Field Meeting to the Exe Valley and adjoining areas 4-7 April 2011.The recently published field guide is an important contribution to our knowledge of the Quaternary of Somerset.  The following chapters are of particular interest for Somerset readers: -
The archaeological context of the Exe Valley and adjoining areas. (Basell, L. and Brown, T.) This chapter summarises the context, chronology, typology and raw materials of artefacts, and draws together the present state of knowledge.
Quaternary fluvial sequences and landscape evolution in Devon and Somerset (R. Westaway). This is a complicated chapter based on shoreline flats, river terraces and karstic features plus magmatic underplating.  Figure 3.3 shows there to have been an estimated post-Ipswichian uplift of 11-12m around Burroughbridge rising to 13-14m around Weston-super–Mare and the Mendips.
The Haldon Gravels Although in Devon this is a tightly argued assessment of the gravels on Haldon and is highly relevant to the wide spread of early Tertiary gravels on the Blackdown Plateau.
The Devil’s Punchbowl (S. Harrison).Visitors to Exmoor will be aware of the cirque-like feature above Winsford. It has been suggested that this was indeed the site of a small cirque but some of us wondered if non-glacial, as opposed to periglacial, processes could account for the features.  However, Stephen Harrison has researched the landforms and sediments in more detail and has added more supportive detail to his original suggestion that the evidence points to a cirque glacier.  Read the account and visit the site to see what you think. It may be relevant that recent research has suggested that the surface bedrock of Lundy in the Bristol Channel became exposed following deglaciation between 40-30 ka.  The presence of the nearby Irish Sea Ice Stream at a relatively recent date might explain increased snowfall and what seem to be fresh glacial landforms. Chard Junction Quarry and the Axe Valley gravels (Basell et al.) There are very few workable gravel resources in Somerset compared with Dorset except to the south of Chard in the Axe Valley; the Jurassic limestones are rapidly worn down and the clays are a non-starter.  However, there are considerably quantities of durable Cretaceous chert and flint outcropping on the sides of the Axe Valley together with a smaller contribution of Tertiary gravels from the Blackdown Plateau. These gravels are unlike the terrace staircases in most of our rivers; they from a stacked sequence resulting from erodible bedrock, high rates of sediment availability and periglacial transport.  Palaeoliths have recently been found in situ and optical age estimates give an intrinsic measure of reliability and a weighted mean age of 326+/- 22 ka.  The stacked gravels include periglacial structures at several levels; at the time of the field visit an excavated horizontal surface showed narrow vertical fissures filled with fine material as part of polygonal structures; it was like walking on the tundra!.
The Middle Pleistocene Deposits and Archaeology at Broom. (Horsfield et al.) The Broom sites are some 3km SW of the Chard Junction pits and were worked in the 1930s. C.E. Bean made valuable records and collections of Lower Palaeolithic hand axes at the time. This chapter summarises the stratigraphy, sedimentology and age of the deposits. Of particular interest is the Broom Member (Broom Gravel Member and Broom Sand and Silt Bed); the Bean archive suggests the association of the majority of artefacts with land surfaces contemporary with the Broom Member.  Recent pollen samples indicate a regional vegetation of boreal forest, with tree stands interspersed with wide expanses of open country and ericaceous heath.
The Doniford gravels (Basell, et al.) This is perhaps the most useful exposure for the general geologist as it has open access and is well-exposed.  The cliff is subject to erosion but this maintains a clear face.  It lies just to the east of Watchet harbour.  We owe much to the Wedlakes for their collections and observations together with those of C. Norman.  OSL samples do form a stratigraphically consistent time-series spanning from 65+/- 5ka to 25 =/- 3ka. These gravels are associated with periods of hillslope recession, mass wasting and periglacial solifluction which contributed to the gravels we see today.  It was a time of shallow braided rivers choked with rock debris.  The widespread angular unsorted head on lower valley sides is evidence of mass wasting; come the next glacial phase doubtless the head would be on the move again.
The geological setting and landform evolution of the Exe Valley and adjoining areas. (Brown, et al.)
This is a broad brush overview of the structure, topography, bedrock geology and superficial deposits.  It is thought–provoking.
Get your copy today from:
Quaternary Research Association.
Members £15: non-members £20.
Please note that on November 3rd Professor Tony Brown and/or Dr. Laura Basell will be talking to the Bath Geological Society about this new research.

Mendip Rocks!

The Somerset Wildlife Trust and Wells and Mendip Museum are collaborating on a brilliant project called Mendip Rocks! An Earth caching trail will be developed across the AONB landscape. An EarthCache, unlike most Geocaches has no traditional cache container or a log book. The purpose of an EarthCache is to share information about a particular geological feature of our planet Earth, and the object is to learn something about it. When an EarthCache is visited, participants will learn something interesting about the geology and in this case how this has influenced the biodiversity of the area. Investigative work will be required to answer questions in order to ‘claim’ the cache online.
Experience elsewhere has shown that this innovative adventurous method of interpretation attracts new audiences into the countryside and encourages their appreciation of it. This is a cost effective way of producing interpretive material and is aimed at an audience that might not otherwise access interpretation. The caches will be located on SWT reserves in the first instance to determine the success of the approach. Caches will be targeted at robust environments in open access land or on public rights of way to avoid potential damage to features that are being interpreted.
Help is needed! SWT needs assistance to write the geological interpretive information and upload this on to the Earthcache website. If you are interested contact SWT

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.