Complex sentence: The Saxons arrived in England well after the Iron age peoples had vanished and in their ignorance they ascribed the earthworks to their god of the underworld, hence the name Grim's Ditch.
Split into simpler sentences: The Saxons arrived in England well after the Iron age peoples had vanished and did not understand the origin of the embankments they encountered. In their ignorance, they ascribed these earthworks to their god of the underworld, hence the name Grim's Ditch.