The Western Carpathians are part of the Alpide belt. To the west they longitudinally join the Alps, but the exact boundary is hidden under the Neogene sedimentary fill of the Vienna Basin. To the east, their boundary with the Eastern Carpathians is the valley of the Hornád or Uzh River. The northern boundary with the East European craton and Bohemian Massif is well marked by the thrust of nappes of the Carpathian flysch belt. The southern boundary is less clear, because later postorogenetic evolution caused formation of basins, penetrating the mountain chain non-uniformly.