What caused the end of antiquity, the shift of economic activity away from the Mediterranean towards Northern Europe? We assemble a large database of coin flows between the 4th and 10th century. We build a dynamic model of trade and money where coins gradually diffuse along trade routes. We estimate the parameters of this model to recover time-varying bilateral trade flows and real consumption from data on the spatial and temporal distribution of coins. Our estimates suggest that reduced trade openness arising from the cost of crossing the newly formed border between Christianity and Islam, combined with technical progress and increased minting output in Muslim Spain and in the Frankish lands, explains the increased urbanization of Western and Northern Europe relative to the eastern Mediterranean from the 8th to the 10th century.