|St. Peter Sandstone, Illinois Basin
The St. Peter Sandstone is a mature quartz arenite that contains little or no silt and clay, except near its perimeter. Lamar (1928; his fig. 10) provided results of grain-size analysis for a number of St. Peter Sandstone samples in which the percent clay ranged from 0 to 5.8, with a mean of about 2.5 percent. Because clay and silt are minor constituents of the St. Peter Sandstone, we calculate net sand thickness (c4stpeterg) as Total St. Peter sandstone thickness (total St. Peter sandstone thickness • 0.025).
Lamar, J. E., 1928, Geology and economic resources of the St. Peter Sandstone of Illinois: Illinois Geological Survey Bulletin No. 53, 175 p.