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In page Celts:

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Archaeological evidence suggests that the pre-Roman Celtic societies were linked to the network of overland trade routes that spanned Eurasia. Archaeologists have discovered large prehistoric trackways crossing bogs in Ireland and Germany. Due to their substantial nature, these are believed to have been created for wheeled transport as part of an extensive roadway system that facilitated trade.[1] The territory held by the Celts contained tin, lead, iron, silver and gold.[2] Celtic smiths and metalworkers created weapons and jewellery for international trade, particularly with the Romans.[citation needed]