The sands of the Frenchman Formation (Maastrichtian) and the Ravenscrag Formation (Palaeocene) in southwestern Saskatchewan are mineralogically immature lithic arenites and wackes. Volcanic rock fragments, which in the Frenchman Formation make up to 30% of the fraction coarser than 0.0625 mm, are less common in the Ravenscrag Formation, where quartz and carbonate rock fragments are significantly more abundant in the same size fraction. Sand composition is influenced by grain size in both formations. Over the range from 0.59–0.074 mm there is, in general, a decrease in the percentage of rock fragments and an increase in the percentage of quartz, with decreasing grain size. Although the Ravenscrag sands mostly are finer grained than those of the Frenchman, compositions of the fraction coarser than 0.0625 mm fall into separate but overlapping fields in a conventional quartz – feldspar – rock fragment (QFR) diagram. Differences, in terms of QFR ratios, between the formations, are clearer if compositions of limited size ranges (six size ranges between 0.59 and 0.074 mm) are considered, but there is still overlap. Significant compositional differences are obvious only when the relative amounts of different types of rock fragment are taken into account.Grain-size distributions of different components in the sands are, for the most part, the same in both formations, and reveal trends that are anticipated by studies of sand transport in modern streams. The proportions of quartz and detrital carbonate grains increase with decreasing grain size, whereas relative amounts of nonvolcanic rock fragments and chert decrease with decrease in grain size. Volcanic rock fragments and feldspar show an irregular increase with decreasing grain size in the Frenchman sands, and an increase followed by a decrease in the Ravenscrag sands. The differences in the latter two components between the sands of the two formations are attributed to a reduction in both the abundance and the variety of volcanic detritus supplied to the area in Palaeocene times.The bulk of the volcanic material in both formations likely was derived from western Montana. In particular, the Elkhorn Mountain Volcanics (Campanian) and the Adel Mountain Volcanics (Maastrichtian to ?Palaeocene) probably were important sources. The uplift of Palaeozoic carbonate rocks and Mesozoic clastic deposits in the Front Ranges of the Rocky Mountains may account for the relative reduction in supply of volcanic rock fragments and feldspar, and the increased contribution of detrital carbonate and quartz, to the sands of the Ravenscrag Formation.