The Upper Cambrian (St. Croixan Series) in the northern portion of the upper Mississippi Valley consists primarily of sandstone with minor amounts of shale and carbonate. This paper presents data on the relation of the mineralogy of the sandstones, particularly feldspar content, to grain size and depositional environments. Feldspar is far more abundant in some formations than previously recognized and consists of detrital K-feldspar grains with low-temperature K-feldspar overgrowths. The abundance of feldspar in these sandstones is nearly linearly related to the volume of very fine sand and, to a lesser extent, to the coarse silt size grains present because most of the feldspar grains are < 0.125 mm except where greatly enlarged by overgrowths. As a result of the small size of the feldspar, quartz arenites (< 10% feldspar) usually have mean sizes > 0.177 mm. Arenites having mean sizes between 0.177 mm and 0.105 mm are usually feldspathic (10-25% feldspar), while arenites and siltstones with mean sizes between 0.105 mm and 0.053 mm are usually highly feldspathic (25-70% feldspar). Feldspar and glauconite abundance and distribution are strongly related to the energy regime of the depositional environments. The quartz arenites are characteristic of high energy regimes (littoral and nearshore environments). The feldspathic and highly feldspathic arenites are characteristic of low energy regimes (shelfal environments). Feldspathic and highly feldspathic arenites in which glauconite is abundant were formed primarily on the shoreward side of the shelf environment. A hydrodynamic model is proposed to explain the small grain size of the feldspar, the repetitive stratigraphic occurrence of feldspathic units, and the regional distribution of feldspathic and nonfeldspathic facies. Because of its excellent cleavage, the feldspar was differentially abraded during passage through extensive high energy environments then sorted and deposited in low energy environments.