A large portion of glacially-produced sediment entering Bow and Hector Lakes, Banff National Park, southwest Alberta, is composed of dolomite and calcite derived from surrounding carbonate bedrock. Analyses of sediment samples from the two lakes show that both total carbonate and the ratio dolomite/(dolomite + calcite) decrease with decreasing median grain size and, by implication, with distance from source. In Hector Lake, total dolomite decreases and total calcite increases with decreasing grain size. Results of observed mineral distributions, saturometry experiments, and percent saturation calculations suggest that neither precipitation nor dissolution of carbonate are important, and biological contributions are negligible. The relationships are believed to be the result of normal sorting from suspension. Initially coarser carbonate is replaced in the finer sizes by clays, and dolomite is enriched relative to calcite in the coarser sediments because (1) it is 8% more dense, and (2) its glacially-produced size distribution is initially coarser than calcite.