Five Pleistocene glaciations lowered seal levels sufficiently to rejuvenate the Mississippi River system, enabling it to cut valley systems in Louisiana to depths of 100 to over 300 feet. Rising seas of waning glaciation permitted alluvial drowning of these valley systems and the deposition of five Quaternary formations, the latest being the Recent.
Regional tilting, caused by subsidence of the deltas and a compensating elevation inland, has preserved the surfaces of the four Pleistocene formations as terraces, which extend continuously both coastwise and inland along major streams. In central Louisiana the oldest surface slopes at 10 feet per mile, younger surfaces at rates of about 6, 2, and one feet. The Recent surface slopes less than one-half foot. Slopes of terraces flatten inland and intervals are more constant in northern Louisiana. Slopes steepen coastward, particularly toward later deltas, and each formation passes below more recent deposits. Sediments grow finer upward in each formation.
Basal gravels of each formation have been identified under the Recent deltaic plain and positions of maximum deposition have been located for four of the five Quaternary formations. Each of these positions is characterized by lenticular thickening downward, so that at least three of the formations attain a maximum thickness of about 3000 feet. Each lens marks the position of the delta of its period. No two center in exactly the same place.
Seven subdeltas of the latest Recent have been identified, heaviest deposition being centered east of a line southward from Baton Rouge. North about 100 miles, in southwestern Mississippi, is the highest land of the lower Mississippi Valley, the result of subsidence of the Recent delta.