Changes in shape of sand and silt in streams may be a reflection of changes in source rock rather than the progressive effects of transport. If so, shape characteristics should change as the stream crosses geologic contacts. To clarify the problem, stream sediment samples were collected in two drainage basins on the west flank of the Bighorn Mountains, Wyoming. Three size fractions--fine sand, very fine sand, and coarse silt--were studied, using Fourier analysis and the scanning electron microscope. Provenance discrimination improves with increasing grain size. In all three size ranges sediment derived from a glaciated crystalline rock source can be differentiated from that derived from sedimentary rock sources. Sand from sedimentary rock sources can be subdivided into shape families according to the extent of diagenesis. In very fine sand and fine sand, sediment from the Flathead Sandstone is distinguishable from that derived from other sedimentary rocks. In fine sand the Tensleep and Darwin sandstones are also distinguishable as separate sources. Along Tensleep Creek, changes in the shape composition of sediment occur rather continuously due to transport of sediment by past glacial processes. Along Paintrock Creek, however, shape proportions do not change in a continuous manner but instead change by abrupt, discrete jumps which occur where additional sources provide sediment. The shape changes are due to the dilution of upstream sediment by sediment from adjacent sources. This suggests that downstream sources provide sediment at a greater rate than the upstream sources even though the gradient decreases with distance. This can be explained by the fact that the younger, downstream sedimentary rocks are more friable than the upstream rocks.