The Yazoo Clay

The Yazoo Clay, a late Eocene marine clay, lies beneath the Natchez Trace Parkway north of Jackson in Madison County, and at the Choctaw Agency site, milepost 100.7 in Ridgeland, it surfaced in the record early. In 1811 the Choctaw agent Silas Dinsmoor dug a well there in an unsuccessful search for water and struck fossil oyster shells in the clay. He displayed them in the agency house, where, by one geologist's account, early travelers on the Trace saw them. The parkway crosses the same clay again west of Jackson in Hinds County.

The formation is a fairly uniform blue-green to blue-gray, limy, fossiliferous clay, 400 to 500 feet thick where the whole section is present, cropping out in a belt 6 to 30 miles wide across eleven counties from Yazoo to Wayne. Parts of it are rich in montmorillonite, an expansive clay that takes up water like a dried-out sponge and swells, then shrinks as it dries. The Mississippi Bureau of Geology's 1988 circular on the clay names its everyday results in central Mississippi: broken house foundations, sticking doors and windows, cracked plaster and roller-coaster roads.

Near the surface the clay weathers in three zones: a brown soil half a foot to a foot and a half thick; a tan to yellowish brown zone 10 to 30 feet thick, split by desiccation cracks that may be one to two inches wide at the surface, 20 to 30 feet deep and traceable for 100 feet or more, sometimes filled with selenite, a clear gypsum crystal resembling broken glass; and below it the unweathered blue-green clay. Repeated swelling and shrinking polishes the crack walls into shiny slickensides, which engineers read as a telltale mark of an expansive clay.