Complex sentence: St. Cuthbert's beads (or Cuddy's beads) are circular columnals of Carboniferous crinoids which were strung together as a necklace or rosary in medieval Northumberland, and became associated with St Cuthbert.
Split into simpler sentences: St. Cuthbert's beads (or Cuddy's beads) are fossils. They are circular columnals of Carboniferous crinoids which were strung together as a necklace or rosary in medieval Northumberland, and became associated with St Cuthbert.