In order to determine detailed variations in cross-bed orientation, 600 measurements were taken in a nested sampling design from one continuously exposed Wasatch (Eocene) channel sandstone in southwest Wyoming. The sandstone is fine-grained, well-sorted quartz arenite and crops out for a distance of 1,830 m. The outcrop nearly parallels the overall flow direction of N. 70° E. Twenty-four sample sites spaced 60 m apart were established along the outcrop. At every site, 25 cross-bed measurements (true dip direction and angle), arranged in a 5 × 5 sample grid with 50-cm spacing, were recorded.
Results show that vector average dip directions for the 24 sample sites fall in the 275° sector between 319° and 234° (0° = North), with a grand average of 070°. Arithmetic mean dip angles for 24 sites range from 9° to 31°, with a grand mean of 18°. Sand-body thickness (5.0 to 15.5 m) is directly proportional to the amount of variability in cross-bed orientations within each sample site.
Analysis of variance indicates that a minimum of 125 and a maximum of 375 cross-bed measurements are required to estimate adequately the overall flow direction in this sand body. Because local cross-bed variability may be large and the distribution of variation probably differs among sand bodies, it is suggested that future studies of paleostreamflow directions may require more detailed sampling than has been performed in the past.