Six to twenty feet of loess, with no rock described in the profile
The Menfro series underlying much of this metro is close to an ideal grinding medium. The USDA-NRCS Official Series Description records it as thick loess six to twenty feet or more deep, silt loam grading to silty clay loam, well drained, with no rock fragments described anywhere in the profile to 80 inches. Nothing abrades teeth the way chert or Piedmont saprolite does, and a 6 to 12 inch grind stays in the friable upper horizons.