From the state plaque:
Known as Santa Rosa de Las
Lajas (Flat Rocks), this site was used on March 8, 1774
by the Anza Exploring Expedition, opening the land route
from Sonora, Mexico, to Alta California. On December 11
to 15, 1775, the three divisions of Anza's colonizing
expedition used this first good watering spot beyond the
Colorado River on the way from Sonora to San Francisco.
Plaque placed by the State Department of Parks and Recreation in cooperation with the U.S. Department of the Interior and Squibob Chapter E Clampus Vitus, April 24, 1993.