The 4S Ranch project involved over 2.6 square miles for the construction of a new residential community between Escondido and Black Mountain along the west side of Interstate 15 in unincorporated San Diego County. Draft and final Environmental Impact Reports (EIR) were prepared under CEQA requirements, outlining the need for a Mitigation, Monitoring, and Reporting Program (MMRP) to reduce potential impacts to fossil resources. BFSA met the challenge to satisfy this task, which involved the unraveling of the complex geologic relationships of interfingering marine and non-marine stratigraphy, collecting very small to extraordinarily large fossils, and the management of multiple field crews, as well as the recruitment of top scientists and specialists to analyze the specimens. The detailed geology of this area of the county, while broadly known, was poorly understood before the project was initiated. After eight years of monitoring, a wealth of Eocene floras and faunas were revealed, including the fossilized remains of leaves from a variety of plants and trees, bivalve and gastropod mollusks, fish and rays, crocodiles, soft and hard-shelled turtles, snakes, wading birds, lemur-like primates, rodents, carnivores, tapirs, brontotheres, rhinoceros, and other primitive hoofed mammals. A log of petrified wood 48 feet long was found, as well as new species of freshwater algae and land snail. This project remains one of the most scientifically important projects undertaken by BFSA, as the value of the collection underscores interpretations of the region’s climatic and biotic record 45 million years ago.