 The mapmakers continued to differ on the boundary between the lower Don and Samara well into the 19th century.The published by the Russian Academy of Sciences has the boundary follow the Don beyond Kalach as far as Serafimovich before cutting north towards Arkhangelsk, while other 18th- to 19th-century mapmakers such as John Cary followed Strahlenberg's prescription.To the south, the Kuma–Manych Depression was identified circa 1773 by a German naturalist, Peter Simon Pallas, as a valley that, once upon a time, connected the Black Sea and the Caspian Sea, and subsequently was proposed as a natural boundary between continents.