Most of the conglomerates of the middle Miocene Topanga formation and the upper Miocene Puente formation in the Puente and San Jose Hills are in the form of thick lenses lying close to the present northeast and northwest margins of the hills. The pebbles, cobbles, and boulders are fragments of plutonic, volcanic, and metamorphic rocks, in part identical with types now present in the mountain masses north and east of the hills. Some rocks, abundant in the mountains, have not been found in the conglomerates, and some of the conglomerate rock types are not known to be exposed now in any possible source region. Nevertheless, it seems probable that the basin which included the area of the Puente and San Jose Hills, received débris from the north and east during middle and upper Miocene time and that the transgressing shore line moved slowly inland, crossing the present northeast margin of the hills in early and middle Puente time.

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