I came across this interesting snippet in Hugh Prudden's excellent notes on the geology of Somerset -- suggesting that some of the building stone used in the older buildings of Glastonbury has come from Palaeozoic outcrops -- maybe from the Quantocks, Brendon Hills and Exmoor. These of course lie to the west of Glastonbury, and if the Irish Sea Glacier ever did reach Glastonbury (as I have speculated more than once), it might -- just might -- have picked up Palaeozoic erratics on the way through. I don't know whether any more accurate provenancing has ever been done -- can anybody enlighten us?
18. GLASTONBURY ST 5138
Junction Bed-Yeovil Sands-Holocene formations-landforms
Erosion has separated this outlier of Yeovil Sands from the main escarpment which can be seen some 26 km to the east. The intervening vale is Lower Lias clays. To the north are the Mendip Hills and, to the west, the Rhaetic Beds and Blue Lias of the Polden Hills. On a clear day the Palaeozoic massifs of the Quantocks, Brendon Hills and Exmoor can be seen. Fluvial and marine deposits of sand, clay, gravel and peat underlie the Somerset Moors and Levels which extend to the Bristol Channel. This is, perhaps, the best place for a comprehensive view of Somerset. The Junction Bed limestone, which includes the marlstone Rock Bed, forms a marked bench. The main street in Glastonbury has a rich variety of Palaeozoic and Jurassic building stones. The Abbey ruins have Doulting Stone facings with a core of Blue Lias.and Marlstone.