Prehistoric burial grounds have been discovered at The Bridestones near Congleton (Neolithic) and Robin Hood's Tump near Alpraham (Bronze Age). The remains of Iron Age hill forts are found on sandstone ridges at several locations in Cheshire. Examples include Maiden Castle on Bickerton Hill, Helsby Hillfort and Woodhouse Hillfort at Frodsham. The Roman fortress and walls of Chester, perhaps the earliest building works in Cheshire remaining above ground, are constructed from purple-grey sandstone.