The upper Cretaceous Belly River Formation of the southern Foothills of Alberta is a unit 2,000–2,500 ft. thick composed mainly of mudstone interbedded with lenticular, fine- to medium-grained sandstones in units 5–100 ft. thick. Thin intraformational mudstone-pebble conglomerates, nodular limestones, coal seams, bentonites, magnetite-sandstone, and pelecypod beds make up the remaining, minor part of the lithology. The sandstones are composed of about one-half quartz (and quartzite), one-third rock fragments (including chert), and one-fifth feldspars. They are mostly lithic sandstones in the cIassification scheme adopted. The quartz rock fragments and feldspars are separable into compositional and genetic groups. Variatal minerals are carbonates, chlorites, red and brown biotide, and two types of glauconite. The principal accessory minerals are allanite, apatite, collophane, almandine garnet, tourmaline, and zircon. Albitization of intermediate plagioclase suggest the grade of diagenetic aIteration has neared the laumontite-albite stage. Quantitative clay analysis of nine mudstones, shales and bentonites shows illite and montmorillionite to be the dominant minerals (with considerable mixed layering). Illite is detrital, montmorillonite and mixed-layer clays are diagenetic.