The St. Peter Sandstone in Wisconsin, Minnesota, and Illinois was investigated, and size, shape, and grain surface characteristics were determined by modern analytic techniques.

Texturally the St. Peter Sandstone is a fine to medium-fine (av M_z = 2.09 ^phgr), moderately to moderately well sorted (av ^sgr_I = 0.59 ^phgr), finely skewed, mesokurtic sandstone. Textural parameters exhibit little vertical or horizontal variability. The average grain roundness is a ^rgr value of 4.76 (rounded) with a significant percentage of angular grains in the very fine sand and silt fractions. The average grain Elongation Index is 0.67 (intermediate) with an average 23% of the grains in the very elongate class. High magnification studies show grain surfaces to be devoid of the classic abrasion features of frosting. The "frosting" is chiefly minute, crystallographically oriented, rhombohedral and prism overgrowths of authigenic quartz,

End_Page 329------------------------------

with superimposed overgrowths of authigenic clay minerals and etch pits.

Textural data were utilized in a variety of bivariant techniques and linear discriminant functions attempting to determine depositional environments for the St. Peter Sandstone. The results were inconsistent and nondefinitive. More consistent results were obtained by inspecting cumulative probability curves, following the techniques of Douglas, Sindowski, and Visher, which indicate a shallow-marine origin for the St. Peter Sandstone.

The classic textbook description of the St. Peter Sandstone as a very well-sorted sandstone with very well-rounded nearly spherical, and eolian frosted grains, is in error.

End_of_Article - Last_Page 330------------