Geological Conditions Affecting Dams in San Gabriel Canyon

In view of the different plans being considered for the building of high dams in San Gabriel Canyon a statement of the geological conditions as they affect impounding structures in that drainageway may be pertinent and of interest. The writer's examination of the canyon reveals that the geological conditions from its mouth to the forks are quite similar, differing only in minor details. The dominant rocks are gneiss and a variety of coarse-grained intrusive igneous rocks of the general nature of granites and diorites. Dikes of both acid and basic rocks are common. Quartz and calcite veins occur occasionally.