 The western third of the state can be considered a separate large geophysical unit, distinctive enough that it may best be described on its own.Several important, complex factors set Western Pennsylvania apart in many respects from the east, such as the initial difficulty of access across the mountains, rivers oriented to the Mississippi River drainage system, and above all, the complex economics involved in the rise and decline of the American steel industry centered on Pittsburgh.Other factors, such as a markedly different style of agriculture, the rise of the oil industry, timber exploitation and the old wood chemical industry, and even, in linguistics, the local dialect, all make this large area sometimes seem a virtual "state within a state".