The eastern terminus of the early medieval linear earthwork Nico Ditch is in Ashton Moss (); it was probably used as an administrative boundary and dates from the 8th or 9th century. Legend claims it was built in a single night in 869 or 870 as a defence against Viking invaders. Further evidence of Dark Age activity in the area comes from the town's name. The "Ashton" part probably derives from the Anglo-Saxon meaning "settlement by ash trees"; the origin of the "under-Lyne" element is less clear: it could derive from the British "lemo" meaning elm, or refer to Ashton being "under the line" of the Pennines. This means that Ashton probably became a settlement some time after the Romans left Britain in the 5th century. An early form of the town's name, which included a "burh" element, indicates that in the 11th century Ashton and Bury were two of the most important towns in Lancashire. The "under Lyne" suffix was not widely used until the mid-19th century when it became useful for distinguishing the town from other places called Ashton.