 The boundary between the Western Carpathians and the Eastern Alps is formed by the Vienna Basin, the Hainburg Hills of the Little Carpathians at Devín Gate, and a gap carved by the Danube.To the east and northeast the mountains are bounded by the East Slovak and Sandomierz Basins, but it is less striking and passes through highland terrain that continues to the Eastern Carpathians.The Western Carpathians are part of the northern branch of alpine orogeny, which was formed by the closure of the Tethys Ocean millions of years ago.