Petrographic modal analysis of the medium sand fraction (0.35 to 0.50 mm in diameter) of 274 samples principally from the shore zone of Lake Tahoe indicates that source-rock composition and mechanical-energy level at the depositional site explain approximately 46 and 32 percent of the total sample variance, respectively. A strong mechanical-energy signal may persist within a restricted grain-size interval. Q-mode factor analysis and stepwise discriminant function analysis were used to define six petrofacies, which reflect source-rock composition and relative mechanical-energy levels. Mean values for total quartz, total feldspar, and total rock fragments for petrofacies 1, 2, 3, 4, and 6 fall in the lithic arkose field of McBride (1963), whereas the mean for petrofacies 5 falls in the feldspathic litharenite field. The areal distribution of these six petrofacies may be used to identify nine shore-zone divisions, one of which is due to a major beach-nourishment project completed near Tahoe City in the early 1900s. Given the 1-km sampling interval employed, the areal positions of seven of the nine divisional boundaries showed no change from September 1978 to October 1988. This compositional stability reflects the consistency of the upland drainage systems and that of the wind-derived current systems operative in Lake Tahoe. The importance of source-rock composition, relative mechanical-energy level, and the apparent temporal consistency of the areal distribution of the shore-zone petrofacies should prove useful in constraining interpretations of older siliciclastic shoreline deposits associated with large lakes.