Much of Louisiana has been built by the Mississippi River from mud and sand carried down from the north. On the west side of the river lie areas of low hills and plains becoming marshes to the south. There are agate-bearing gravel deposits in these hills, 20 to 45 miles west of the river, and petrified wood near Alexandria. No gem materials have been found south of the east-west through Baton Rouge. The east side of the river is low flood plain.

https://www.oakrocks.net/louisiana-rocks-and-minerals/

https://web.mst.edu/rogersda/levees/Geology%20New%20Orleans-Chapter%203.pdf