ABSTRACT The geology, stratigraphy, and paleontology of the Santa Ana Mountains of Southern California span 150 m.y. of subduction and 30 m.y. of transform faulting, producing complex geologic, stratigraphic, and paleontological settings. The mountains are bounded by the Elsinore fault zone on their east side, uplifting the mountains and tilting them westward, where sediments eroded from them were deposited in a variety of marine to terrestrial environments; most of these formations yield fossils so that a rich history of life can be reconstructed. The most recent geologic history includes the continued transform faulting with displacements of many kilometers northwesterly, juxtaposing separate blocks and biotas. The modern sediments are dominated by the Santa Ana River, which flows westerly at the northern end of the Santa Ana Mountains onto the coastal plain of Orange County. It is the primary aquifer supplying significant amounts of water to the residents. Humans have occupied the region for the last 12,000 yr, developing large, sophisticated populations, which, in the most recent years, have impacted the geology significantly. This field-trip guide starts north of the mountains in Ontario, California, and describes the Elsinore fault zone, the east side of the Santa Ana Mountains, and the ascent of the steep eastern side of those mountains. Extensive vistas of the geology to the east of the mountains can be seen from stops along the way. In the mountains themselves, the guide describes the granitoids of the Peninsular Ranges batholith, sedimentary rocks of the Jurassic Bedford Canyon Formation, rocks of the Cretaceous Santiago Peak Volcanics, and overlying sedimentary rocks of Mesozoic and Cenozoic age. At Ronald W. Caspers Wilderness Park, stops show the early Tertiary Silverado and Santiago formations preserving terrestrial environments that rest unconformably on the marine Cretaceous Williams Formation. On the west side of the mountains, stops at Cretaceous to Miocene conglomerates through mudstones reveal abundant marine mollusks, foraminifera, and vertebrate faunas among others, and a wide variety of sedimentary structures. Younger sediments, faults, and river courses occur along the final leg of the trip from the northern Santa Ana Mountains back to Ontario. Humans have interacted with the geology and its resources for possibly the last 12,000 yr, in ancient times utilizing rock resources and in modern times dealing with geological hazards in developmental and infrastructural construction.

Southern California, extending from Point Conception (34 1/2 °N) to the Tijuana estuary (32° N), has a varied and attractive coastline, with a moderate Mediterranean climate and a large human population. Rapid urban development has resulted in coastal modification for shipping, military, industrial, housing and recreational needs. Because of their flat topography and availability of water, the coastal wetlands have been the prime target for these modifications. According to Zedler (1982, p.l), the ‘disturbance has been so pervasive, taken such different forms, and had such different results in each wetland, that sorting out natural and unnatural features is extremely difficult’. Southern California coastal wetlands are small and discrete, confined to narrow river valleys, and separated by coastal hills, mountains, harbors and extensive urban tracts. There are about 30 wetlands, occupying a total area of about 12,500 acres (Fig 1) which represent about 10% of their extent prior to arrival of Europeans. The wetlands occur on intertidal slopes and along the mouth of creeks, and support a variety of salt tolerant plants, called halophytes. Poised at the edge of both land and sea, these habitats receive fresh water, sediments and nutrients from the watershed, and tidal water with its salts, minerals and sands from the sea. At some locations, coastal dunes enclose these wetlands and occasionally, sand bars form, cutting off tidal circulation. This has resulted in decimation of certain plant communities and dependant bird populations.