From the back of the book:

For much of twenty years, John McPhee travelled back and forth across the United States in the company of geologists. His aim was to write a complex work describing a cross-section of North America at about the fortieth parallel and, in so doing, to give an account of the "deep history" of the continent — 4.6 billion years — as well as of the science of geology and the styles of the geologists he travelled with. The breadth of the work led him to complete it in stages, each of which was acclaimed upon publication; and when it was published in full, as "Annals of the Former World", it was recognized as a masterpiece of nonfiction writing: an organic succession of set pieces, flashbacks, biographical sketches, and histories of the human and lithic kind. Like the terrain it covers, "Annals of the Former World" tells a many-layered tale, and the reader may take many paths through it. Profoundly informed, clearly and succinctly written, it is our finest popular survey of geology, and a summation of John McPhee's work so far.