In a recent paper1 on the petrographic characteristics of the St. Peter sandstone, the writer pointed out that in the upper Mississippi Valley, the petrographic constituents of the upper portion of the St. Peter are more typical of the Glenwood beds than of the major portion of the St. Peter sandstone. Throughout the area of outcrop of the sandstone in southeastern Minnesota and northeastern Iowa, the upper 3–10 feet of the formation is thin-bedded, more poorly sorted, and consists of coarser sand and more fine silt and clay than the greater part of the formation. Further studies of numerous samples from widely separated outcrops of the Glenwood beds and underlying clayey sandstone layers indicate that the early Mohawkian sea reworked the upper part of the St. Peter sands in the upper Mississippi Valley, and that additional clastic sediments from new source areas, differing both texturally and petrographically from . . .