Figure 18.18A typical large marine delta, many kilometers in extent, in which the fine-grained foreset beds are deposited at a very low angle, typically only 4° to 5° or less. Sandbars form at the mouths of the distributaries, where the current velocity suddenly decreases. The delta builds forward by the advance of these bars and the topset, foreset, and bottomset beds. Between distributary channels, shallow bays fill with fine-grained sediments and become salt marshes. This general structure is found on the Mississippi delta.