Pruned has discovered Harold Fisk’s 1944 Geological Investigation of the Alluvial Valley of the Lower Mississippi River, the text and plates of which are available for download from the U.S. Army Corps of Engineers web site (though at hundreds of megabytes per zip file). Has this to say: “The maps, scanned at high resolution and full scale, are some of the most beautiful I’ve seen.” Thanks to Gerard for the link.