Since the ancient Thracians lacked an indigenous writing tradition, the most important sources for the reconstruction of their history are archaeological remains, coins as well as accounts of ancient Greek historians. Said historians considered the Thracians to be a numerous people and their country, Thrace, to be of barely comprehensible size, so large that Andron of Halicarnassus (4th century BC) thought of it as a continent of its own. While the boundaries of Thrace fluctuated throughout history, Thrace can be divived in a northern and a southern half, which were also culturally different. The border between the two halves has been identified as the Haemus Mountains or the Danube sligthly further north. Southern Thrace covered the fertile valley between the Haemus and the Rhodopes, the Strandzha and the shores of the Propontis and the Aegean and Black Seas. The western boundary was marked by the Strymon and the upper Morava. Northern Thrace was defined by the Danube, the Carpathians and the adjacent western tip of the Pontic–Caspian steppe, thus enclosing parts of the territory now comprised of modern Romania, Moldavia, Serbia and Ukraine. Thrace also extended into what is now northwestern Turkey both west and east (Bithynia, Mysia) of the Propontis.