What John McPhee's book successfully delivers is an accessible cross-section of the geology of the golden state at your fingertips, including those, including myself, who wax nostalgic about being a former inhabitant of this geologic wonderland. McPhee explains not only geologic processes but also how geology affected exploration and exploitation of the state's resources. The geology is not dead, for it resonates to this day and to the far future, what with the awesome power yet to be unleashed from California's labyrinthine faults and from the still burgeoning mass of the Cascade volcanoes to the north. Nevertheless, McPhee gives a personal and friendly touch to California's big-time geology.
Most significantly, McPhee is one of the first writers to apply recent developments in the field of plate tectonics to California's geological story. Most exciting, perhaps, is his discussion of the ways in which California itself has been "assembled" from various pieces and processes, all of these the results of the various ways that tectonic plate boundaries can interact. The immense granitic batholith that forms the backbone of the mighty Sierra Nevada, for example, is the result of the Mesozoic melting and recrystalization (as granodiorite)of the now-defunct Farallon Plate as it slid beneath the neighboring North American plate to the east. The currently existing Sierra range itself has been raised to its present height only recently (mostly within the past three million years) through faulting associated with crustal stretching that apparently extends clear across the Great Basin.
Perhaps even more interestingly, the Sierra foothills and several other portions of the state exhibit peculiar rock types and geology as a result of their actually having been shoved and glommed onto the continent in bits and pieces, the result of ancient plate movements that brought "exotic terrances" to the present location of California from great distances away.
More recently (and geological terms, thirty million years is indeed "recent"), the tectonic plate boundary along the length of California has changed from one characterized by subduction to one in which the Pacific Plate is now sliding slowly northwestward past the North American Plate. The principal boundary between these plates is the San Andreas Fault--hence, the major effect of this particular form of plate collision is the frequent earthquakes with which Californians have become so terribly familiar.
The story of how these ever-changing plate interactions have "assembled" California is a fascinating one, and McPhee tells it well. Along the way, he provides more than just geological information. He also includes historical insights into the California Gold Rush (while also explaining in geological terms where the gold originally came from), and provides colorful descriptions of California's history of great earthquakes.
Although McPhee tries his best to make the complex geology of ophiolite sequences, etc., comprehensible to the non-geologist, there's very likely some frustrating reading here for people with no previous background in the intricacies of plate tectonics. Also, the lack of adequate maps and diagrams is a handicap. Were such illustrations provided, some of the plate relationships and historical progrssions of landscape formation would be far easier to envision.
Still, this is a "must read" for anyone interested in gaining an understanding of how California's landscape has been formed over the past several hundred million years.