California Was Formed by Geologic Immigrants

DRAMATIC is an adjective people often use to describe California's scenery, and for good reason: It is.

Driving north along the coast, you see the grass- and oak-covered slopes of the coastal ranges rising up out of the Pacific Ocean. Going north inland from Los Angeles to Bakersfield, Interstate 5 reaches a final crest at Tejon Pass, a gap in the Tehachapi Mountains, and plunges into the Great Central Valley.

As the highway begins its descent, the mountains break to either side and run north as far as the eye can see, embracing the flat expanse of valley that stretches out ahead.

Driving along the twists and turns of the Angeles Crest Highway in the San Gabriel Mountains, you can glimpse eastern slopes of forest pine giving way to the buff-colored desert floor to the east. From that same vantage point and to the north, the Tehachapi Mountains rise from the desert and stretch northward until they meld with the snowcapped Sierras. The eastern slopes of the Sierras stand like granite ramparts above the floor of the Owens Valley.

These dramatic vistas reflect the impact - literally - of hundreds of millions of years of restless motion of the earth's crust, a history that, with the help of tectonicist Eldridge Moores, John McPhee sketches in his book "Assembling California."

"For an extremely large percentage of the history of the world," McPhee writes, "there was no California.... The continent ended far to the east, the continental shelf as well. Where California has come to be, there was only blue sea.... Then, a piece at a time - according to present theory - parts began to assemble. An island arc here, a piece of continent there - a Japan at a time, a New Zealand, a Madagascar - came crunching in upon the continent and have thus far adhered."

"Assembling California" completes a four-part series that began with McPhee's "Basin and Range" in 1980. Traveling west along Interstate 80, McPhee and his geologist-guides use roadcuts as windows into the geological history of North America. In this case, the guide is Dr. Moores of the University of California at Davis, one of the key figures in the development of the theory of plate tectonics.

Over a 15-year period, the two researchers traveled I-80 from Reno to San Francisco, examined similar geological formations in Cyprus and Macedonia, and in the process set the outlines of a story that unfolds in this book much as California did: in fascinating but distinct chunks.

Along the way, McPhee highlights some of the noteworthy convergences of geologic and human history: the California gold rush and the geological genesis of the mother lode; the San Francisco earthquake of 1906; the Loma Prieta earthquake of 1989, a punch from deep in the earth that raised sections of the fault trace along the San Andreas by 1.5 feet.

Given the state's tectonic evolution, it is easy to see how that process has come to stand as a metaphor for California's demographic and cultural evolution, with waves of immigrants coming in both from east and west.

Moores himself, as McPhee profiles him, represents on an individual level that demographic evolution. He was born to a family that owned a small gold, lead, and zinc mine in Crown King, Ariz. In 1965, at age 16, he moved to California to enroll at Caltech, where he majored in geology.

Of all of the state's features, the Great Central Valley seems to impress McPhee the most. He points out that with two possible exceptions (in Chile and Pakistan), the valley "has no counterpart on this planet."

As McPhee relates it, between 208 million and 144 million years ago the valley formed deep beneath what has become known as the Farallon Ocean. The Farallon plate, an immense segment of the crust beneath the Farallon Ocean, plunged back toward the earth's mantle at a deep-sea trench located west of present-day Davis and in doing so depressed a large undersea region.

The depression filled with sediment and over the millennia rose to the surface in a process that also would form the Sierra Nevadas to the east and to the west the Coast Ranges. Its origins give the valley the distinction of being filled with sediment that came not from its surrounding mountains via its two major rivers, the San Joaquin and Sacramento, but from the depths of the sea. Its geologic evolution has left the valley with nine of the 10 major soil types in the world, accounting for its richly di verse agricultural industry.

"Assembling California" has its share of geological jargon. And in some places, particularly where McPhee describes various rock types, their locations in a road cut, and Moores's excitement over what he finds, one wants to see pictures or diagrams of the formations. But this is not a geology text, but a well-told story.

Much has been written on how geography has helped shape civilization's development. In an accessible, engaging way, McPhee's book sheds light on the geologic forces that shaped - and will continue to shape - that geography.