The Mississippi and Ohio rivers occupied separate valleys at least as far south as the southern end of Crowley's Ridge at Helena, Arkansas, throughout the majority of the Quaternary Period. Diversion of the Mississippi River through Thebes Gap into the upper end of the St. Francis Basin took place very late in the Wisconsin Stage possibly when glacial melt water topped the dam of Lake Agassiz about 10,000 years ago. Perched along the eastern bluff line of the present Mississippi River Alluvial Plain are "pre-loess" terrace deposits of graveliferous sands and clays. These gravels, also named the Lafayette Formation and upland complex, contain chert gravel, some large chert cobbles, and erratics of sandstone boulders. The westward migration of the ancient Ohio River complex can be inferred by the westward dip of the gravel's lower contact in Marshall and DeSoto counties of northwestern Mississippi, the base of which descends from 600 feet above sea level at Holly Springs to 480 feet at Red Banks, 350 feet at Byhalia, 320 feet near Olive Branch, 300 feet near Hernando, and 190 feet at the bluff line approximately four miles south of Walls. The base of alluvial gravel in the present-day Mississippi River Alluvial Plain west of Walls varies from 0 to 100 feet above sea level.