When John McPhee’s four-book geologic tour of the United States was collected in 1998 and published as the Annals of the Former World, McPhee was forced to confront a serious omission. McPhee’s geology tour ran along the 40th parallel, on route 80; the first two books covered America east of Chicago, and third and fourth, west of Nebraska. The Great Plains were tactfully omitted, because they had no geology to tour: tThe perfectly flat plains are perfectly flat because no rock intrudes to break up the monotony. The rock is there, it’s just buried, “tens and hundred and even thousands of feet down, but seldom does it outcrop, and, where it does, it is such an event that it is likely to have been named a state park.” But a coast to coast tour of the United States cannot skip Illinois, Iowa and Nebraska, and McPhee’s sense of duty compelled him to augment his four books with a fifth, Crossing the Craton. It tells the story of America’s deep rock basement, which underlies the waves of grain.
There’s interesting stuff down there, the North American craton: the continental nucleus, made up of precambrian rock—”precambrian” being the period which covers the first four billion of the Earth’s four-and-a-half billion year history. Precambrian rock is literally and metaphorically foundational. Nevertheless, says McPhee, Geological textbooks have generally given precambrian rock “one short chapter. First there was the basement, and on that grew the world.”
