Reprinted from Newsletter 98, dated 2001 January–February

Glastonbury’s geological history

Adrian Pearse

A talk by Susanna van Rose

Susannah van Rose’s explanation of the geological context of Glastonbury Tor provided a fascinating excursion into periods far more remote than usually covered in the lecture programme. This meeting was organized by the Antiquarian Society in the new library on November 17.

It is important to relate the Tor to the nearby formations of Brent Knoll and Pennard Hill, she said. Both share its geological makeup and are part of a formation including the adjoining lowlands and an area extending beneath the Bristol Channel into South Wales.

In 1929 it was recognized that the Bristol Channel comprises two distinct parts: an east-west section and the upper part aligned southwest-northeast. 30,000 years ago the Mendips crossed the Bristol Channel into South Wales, but during the Ice Age the Severn, which had previously flowed north towards Cheshire, was blocked by advancing glaciers to form a massive lake, which eventually spilled south via Ironbridge Gorge and eroded a channel through the Mendip range, thus dividing and obscuring the visible geology of this area.

The rocks of the area southwest of the Mendips were formed in the Jurassic epoch, 200 million years ago, when tectonic movements split apart the huge supercontinent of Pangaea, separating North and South America from Eurasia and Africa. Volcanic activity formed and expanded the Atlantic ocean floor, resulting in rising sea-levels flooding the central Somerset area and the formation of sedimentary deposits with their characteristic fossils such as ammonites. Areas of hot rock intruding beneath the crust caused uplift of these deposits, exposing them to erosion. Glastonbury Tor, Brent Knoll and Pennard Hill were in situations where a downfold occurred, resulting in the compression and preservation of the rocks above.

Subsequent subsidence of the region during the last Ice Age and inundation by the sea resulted in the deposits of marine clay across the central Somerset area. Peat formed in the shallower water; the Tor and its relatives remained as islands.

Susannah concluded by noting that the region is still sinking. In 1607 floods came up as far as St Benedict’s church, and will surely do so again!

Questions from the audience resulted in a lively debate and much interest in the nature of the springs emanating from the Tor.

(Susannah van Rose, née Jones, grew up at the Copper Beech in Magdalene Street.)