History of Red Hill
Red Hill has a long history. It was identified on Spanish maps as Serrita de las Ranas (translation: Hill of the Frogs). The original route for El Camino Real ran close to the hill and the because of the frogs and red color the hill was used as a land mark during travels by the Spanish and Mexican explorers traveling up and down the coast.
The frogs thrieved because of a swampy bog that connected the hill and the back bay in Newport Beach. What became of the frogs? The bog disappeared with the development of the city of Tustin. The final end to the frog population resulted from the lining of the drainage ditches with concrete in the 1950s.
The red color results from the mineral cinnabar (mercuric sulfide, HgS, the major source of mercury). Mercury was mined here up into the 1930s. The mines are closed off and the appearance of the hill is preserved as a California historical landmark.
References: Tustin Tintypes, A Pictorial History of Tustin, published by Tustin Area Historical Society, 1974.