November 2025 Meeting of the Chapter

As my home, it has always puzzled me by how unstudied Orange County really has been. Sure there are geologic maps, and we can see the geomorphology out there, but do we really understand it? And why not? Hence my decades (career) long effort to generate that understanding and communicate it to others as best I can. This was never a solitary effort, there are too many to name who have helped in many ways: trench scraping, field trips, discussions, suggestions, and sometimes laughter. To them I am deeply appreciative, but any errant conclusions within this talk are not their fault.

Orange County California is home to over 3 million people making it the sixth densest county in the U.S. From its high point atop Santiago Peak it is 5,689 ft (1,734 m) down to the coastline at Laguna Beach, a distance just shy of 100,000 feet (30,000 m). The San Juaquin Hills at (height) and Puente Hills at (height) bound the west and northern county. But at 3 Ma, none of this expensive real estate yet existed; Orange County was part of the Pacific Ocean. This talk will try to explain how this came to be, by starting the clock at 3 Ma and ticking our way up to today.

The Santa Ana Mountains are an indenter (think hydraulic piston) driving northwesterly at ~6 mm/yr by the Elsinore fault. As they close the basin, the entire Cretaceous – Pliocene sedimentary section is folded, faulted and piled onto the front of the indenter. Meanwhile the Puente Hills thrust forms in response to north-south compression against the San Gabriel Mountains. As the compression tightens by 1 Ma, other structures begin to emerge as transpressional folds and faults; the San Joaquin Hills and the Peralta Hills, while the Whittier fault accommodates about 3 mm/yr of right-lateral strain as the basin slides out to the west.

Today, the indenter is in full train-wreck mode as it completes the collision with the Puente Hills in Santa Ana Canyon. We can see this expressed in both the geology and geomorphology of the Canyon. As that collision has tightened, the uplift rate of the Puente Hills has tripled to ~3 mm/yr today. Today we see extensive landsliding in the Canyon area due to that jump in uplift rates for already crushed and seismically weakened Puente Formation rocks, as well as hundreds of small faults, fractures and folds any of which could be candidates to accommodate (minor) deformation in a future earthquake. Orange County is a happening place.

Eldon Gath is President and Senior Consultant of Earth Consultants International since founding the company in 1997. As a consultant he has worked on projects in Turkey, Panama, Costa Rica, Taiwan, Japan, Mexico, Papua New Guinea and hundreds in California. He is also a Past-President (1987) of the South Coast Geological Society (SCGS), an Honorary Member (2012) and currently serves as the SCGS Board Treasurer. He was President (1996-1997) of the Association of Environmental and Engineering Geologists (AEG) and U.S. National Group Leader (2014-2018) and North American Vice-President (2017-2018) of the International Association for Engineering Geology and the Environment (IAEG). In 2014-2015 he was the AEG/Geological Society of America’s (GSA) Richard H. Jahns Distinguished Lecturer in Applied Geology. Along the way he has acquired other notable distinctions such as outstanding paper awards (2010 Burwell from GSA & 2012 Holdredge from AEG), outstanding presentation awards (1995 Aki from S.C. Academy of Sciences and 2008 Hanson from American Association of Petroleum Geologists (AAPG)), Johnston Service award from AEG (2008), GSA Fellow (2011), and others from the National Academy of Sciences, American Geological Institute, and the European Geosciences Union. To date, he has 56 published papers, co-edited 7 SCGS Field Trip Guidebooks, has presented 61 times at professional conferences, and 230+ times at schools, professional societies, and community meetings.