The Los Angeles Basin in southern California is filled with locally derived Cenozoic rocks probably exceeding 25,000 feet in maximum thickness. Marginal facies of some of these sediments are exposed in and near the foothills of the San Gabriel Mountains from west of Glendora to the vicinity of Pomona, Los Angeles County, where they either rest on or are interbedded with volcanic rocks. These last, the Glendora volcanics, consist of massive and autobrecciated basalt, calcic andesite, andesite, dacite(?), and rhyolitic lavas, and an approximately equal volume of interbedded tuffs and tuff breccias. Among both flows and pyroclastics andesitic compositions predominate. Similar rocks encountered in wells in the eastern half of the Basin are considered part of the same series.
The later members of the Glendora volcanics are interbedded with middle Miocene marine sediments; the earlier representatives are probably not older than Miocene and may increase in age toward the center of the Basin. Maximum known thickness occurs in wells and exceeds 3500 feet.
The exposed Glendora volcanics here studied constitute but a small, much-faulted and eroded remnant of a volcanic field whose original extent was probably at least 35 miles from north to south and almost as much from east to west. Local evidence shows that part of the northeastern shore of the middle Miocene sea passed through the mapped area. Considerable volumes of andesitic tuff breccia containing, on the landward side, numerous large blocks with what are interpreted as radial cooling cracks are believed to be the deposits of several eruptions of nuée ardente type. The fact that within an 8- by 9-mile rectangle more than 20 varieties of volcanic rock are exposed in less than 8 square miles of outcrops seems to indicate a considerable number of small vents or a few remarkably versatile sources. The scene may have resembled Vesuvius and the Phlegraean Fields bordering the Bay of Naples.
Fourteen new chemical analyses of members of the volcanic series emphasize their general andesitic character and notably low K2O and ferromagnesian constituents.