The kingdoms of southern Arabia continue to thrive as their cities take a central part in the expanding incense trade. Indeed, this period probably sees the height of southern Arabian prosperity in ancient times, with the irrigation systems at its most productive and the overland trade in myrrh and frankincense at its most flourishing. Greek and Roman traders are in direct contact with the producers of south Arabia, as they are with the trading cities further north, where the Nabatean kingdom rules. The material culture of the region is increasingly influenced by classical Mediterranean models, and it is at this period that the beautiful city of Petra, in modern Jordan, is adorned with its marvellous rock-carved temples, which are some of the most outstanding examples of Hellenistic architecture.