Formation of the Moreau-Caminada chenier plain on the southeast Louisiana coast started about 3000 years ago. Autochthonous and detrital peat layers and laminae were found interlayered in the beach ridges. The very fine-grained, well-sorted beach sand contained 1.4 to 7.6 percent detrital lignite grains, eroded from pre-existing peaty marsh deposits. Some time after chenier progradation had ceased, sediments of a more recent Mississippi delta lobe surrounded the chenier plain. Later, a substantial portion of the plain was eroded and a reversal of littoral drift direction took place, possibly as the result of the growth of the Mississippi Plaquemines-Balize delta lobe. Original shore outline and drift and sediment source directions are the most important determining factors in the formation of chenier beach ridge configurations. Periods of mudflat and beach ridge formation were not related to major shifts in the positions of active Mississippi delta lobes but to fluctuations in local wave conditions and sediment supply.