It is difficult to verify the numbers of all such waves of Pontic Greeks from the Pontic Alps region to Georgia and the South Caucasus between circa 1520 and 1800, which according to Anthony Bryer is the most obscure period in the history of Pontus and the Pontic Greeks, owing to the scarcity of contemporary Greek and Ottoman Turkish sources on the subject. Modern historians suggest that following the Ottoman conquest of 1461, many, if not most Pontic Greeks retreated up into the highlands, where it was easier to maintain their culture and freedom from the encroachments of the Ottoman authorities. This movement was reinforced in the early 1600s by the growing power along the coastal valleys districts of the derebeys ('valley lords'), which further encouraged Pontic Greeks to retreat away from the coast deeper into the highlands and up onto the eastern Anatolian plateau, before some moved further east into the neighbouring Lesser Caucasus around Kars and southern Georgia. Modern historians also suggests that a major migratory movement of Pontic Greeks onto the eastern Anatolian plateau and the Lesser Caucasus occurred in the reign of Sultan Mehmed IV (1648-87), during which a common pattern was initiated in Ottoman history: the Ottomans and their clients the Crimean Tatars suffered a string of severe defeats at the hands of the expansionist Russian Empire and so followed this up with a wave of repression against the Greeks of both the southern Balkans and the Pontic Alps region, on the pretext that Greek statesmen and traders had colluded with the Tsar. As a result, many Pontic Greeks felt pressured into following their cousins who had left Pontus as refugees in previous generations, and so they too decided to migrate to southern Russia or neighbouring Georgia and the South Caucasus.