South West Geotechnical were the selected specialist drilling contractor to assist the British Geological Survey to log the entire sequence of Lias strata to help further understand the geology in the Glastonbury area.
Drilling requirements for the two 100metre deep cored holes were understandably onerous; nothing less than 100% recovery could be acceptable if an entire sequence was to be logged.
A commentary on this pioneering research has been kindly supplied by Steve Booth of the BGS:
‘As part of its national remit, the British Geological Survey provides geological information for South West England, a region extending from Cornwall to Gloucestershire. Geological surveying at 1:10 000 scale underpins this objective. Currently, the mapping team are working in and around Glastonbury; recent work has focussed on improving our understanding of the Lower Lias part of the Jurassic sequence. This sequence is well known along the Dorset and East Devon World Heritage Site coastline and has been the subject of much interest by professional geologists, enthusiastic amateurs and the general public since Victorian times. Key to this interest is the lure of finding a hitherto unknown dinosaur or ammonite fossil.
Some way inland, north of the Heritage coast, particularly around Glastonbury, recent mapping has indicated that the early geological mapping (conducted between 1850 -1973) significantly under estimated the frequency of faulting in the Jurassic rocks. This faulting has made calculating (at times guesstimating) the thickness of the various Lower Lias units extremely difficult.
To address this problem, two boreholes were strategically located on a hillside very near Glastonbury to provide a stratigraphical marker for the District. The 100 metres or so of sediment drilled were anticipated to prove the sequence in the diagram to the right:
