The antiago Peak Volcanics of Late Jurassic age are the oldest rocks exposed in the Northwest portion of the Rancho Santa Fe quadrangle. There are more than 5000 feet of interbedded flows of andesite, basalt, agglomerate, brecoia, tuff, and felslte exposed along strike normal stream beds. The origin of this andesltic sequence 1s thought to be from strong steam explosions of ultra-volcanian or Peleen volcanism whose source ls thought to be close by. Low grade metamorphism is indicated by the occurrence of epidote, chlorlte, and pyrophylllte. Joints are well developed in these highly 1ndurated and locally sil1c1fied rocks. Two small northwest trending plutons of midle Cretaceous leucogranodiorite have intruded the older basement rocks. The granodiorite is fine gre.ined, contains abundant maflc inclusions and is quarried in one locality as riprap. Flat lying Eocene sandstone unconformably overlies the older meta-volcanic and granitic rocks. The sandstone is of marine origin and has been terraced by wave action. Stream gravels and conglomerates locally overlie the sandstone with unconformity.
