Read the passage:
'The ancient city of Kano has served for over a millennium as the premier commercial hub of the trans-Saharan trade in the West African savanna. Enclosed by massive earthen fortifications known as the Badala, the city welcomed merchant caravans bearing Saharan salt, North African silks, and brassware, which were traded in the sprawling Kurmi Market for locally woven indigo-dyed textiles, tanned Moroccan leather goods, and agricultural grain. Rather than diminishing during colonial times, Kano adapted by connecting to the railway network in 1912, transforming groundnut pyramids into the iconic visual symbol of northern Nigeria's export economy.'

What is the meaning of the word 'iconic' as used in the passage?