Salton Basin formed in the late Miocene/early Pliocene as a result of the collision between the East Pacific Rise and continental North America. In 1987, Charles Winker and Susan Kidwell developed a tectonic model to explain the development of Salton Basin. Their model implies that the site of deposition of the Pleistocene Borrego Formation was adjacent to the Colorado River delta, and that this site has been subsequently displaced to its present location in the San Felipe Hills, a distance of approximately 95 kilometers. Unfortunately, Winker and Kidwell did not provide rigorous petrologic data in support of their model. As an example, if the Winker-Kidwell model is correct, then detritus in sandstones in the Borrego Formation were derived from the Colorado River. This statement implies that sandstones in the Borrego Formation should be petrologically similar to Holocene Colorado River sand recently studied by G.H. Girty and A. Armitage. If, however, sandstones in the Borrego Formation are petrologically similar to Holocene sand derived from the mountains adjoining Salton Basin, Kidwell model would be proven false. The composition of the latter sands has been recently established by C. Nuffer. The petrology and provenance of sandstones in the Borrego Formation are established for the first time in this study using a precise, quantitative petrologic method. Data from this study, in conjunction with data from Nuffer, Girty and Armitage, serve as the first independent test of the Winker-Kidwell model. Data presented in this study show that poorly lithified sandstones in the Pleistocene Borrego Formation have an average composition of 61.6% ± 4.6% Q, 26.4% ± 3.9% F, and 11.9% ± 4.3% L, and are compatible with the idea that sandstones in the Borrego Formation are composed of a mixture of approximately three-quarters Colorado River sand and one-quarter sand derived from the mountains adjoining Salton Basin. Thus, the presence of Colorado River sand in the Borrego Formation supports the tectonic model of Winker and Kidwell.