The Junction Bed or Beacon Limestone (Middle Lias) Lead authors: Jo Thomas and Alan Holiday
The Middle Lias clays and sands produced very few stones strong enough for building, though the clays were used for bricks. The Middle Lias only has the Junction Bed (now re-named the Beacon Limestone) as an acknowledged building stone.
The bed between the Middle and Upper Lias, previously called the Junction Bed, but renamed the Beacon Limestone (from its first appearance in the Dorset cliffs under Thorncombe Beacon) was originally a lime mud containing the remains of ammonites, belemnites and bivalves. Formed in shallow water, some of the mud was repeatedly washed away as conditions changed.
The resulting limestone now appears mottled in colour because of the mix in sediments and contains seven different ammonite zones. In the south, around Symondsbury, the stone is mottled pinkish orange, but in Trent near Yeovil it is white.