Flysch is a sequence of sedimentary rocks that is deposited in a deep marine facies in the foreland basin of a developing orogen. Flysch is typically deposited during an early stage of the orogenesis. When the orogen evolves, the foreland basin becomes shallower and molasse is deposited on top of the flysch. It is therefore called a syn-orogenic sediment (deposited contemporaneously with mountain building).
