Environments in which sediments are deposited on land include alluvial fans, river beds, lake beds, sand dunes, marshes, deltas, barrier beaches, lagoons, and tidal flats. Sediments also accumulate on reefs, continental shelves, continental rises, continental slopes, and abyssal plains. The type of sedimentary deposit varies according to the type of rock. Knowledge of the sedimentary processes active today, the environments in which they occur, and the types of rock they produce facilitate the interpretation of ancient deposits.