Deep Borehole Research for BGS at Glastonbury 2007

The following description of the project at Glastonbury has been kindly supplied by Steve Booth of the British Geological Survey:

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.

Middle Lias

Green Ammonite Beds
Belemnite Marls
Black Ven Marls
'Shales with Beef'
Blue Lias

Lower Lias
Approx. 198 to 208 million years old

Penarth Group

Blue Anchor Fm (Fomerly Tea Green Marls)

Mercia Mudstone Group (formerly Keuper Marl)

Late to middle Triassic
Approx. 208 to 233 million years old

It is still early days in the interpretation of the recovered cores but the above borehole prognosis has proved incorrect. Expert identification of key stratigraphical marker fossils found within the cores is still needed to be sure but it would seem that the drilling just reached the top of the Blue Lias.

In this Glastonbury locality, it is possible there is an unusually thick sequence of Lower Lias or faulting has thickened (by repetition) the sequence here or elsewhere the sequence is atypically thin, again because of faulting.

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