In the final short book of Annals of the Former World, John McPhee examines the craton, the flat land that lies in the central Midwest of the continental United States. If you have read my reviews of the other books, you might remember that McPhee wrote each one about a separate geologic area near I-80, along which he traveled with different geologists telling the story of the formation of the country. Each of those four books was published separately, but Crossing the Craton was added when the complete volume was published, perhaps for completeness. (I think it was published separately at a later time.)
Because there are few outcroppings in the Midwest, little can be seen of the rock underlying this area, a thin veneer over the basement rock that comprises 90% of geologic time. McPhee explains that until very recently this basement, or Precambrian, rock was neglected in geology texts. Because Precambrian rock by definition has no carbon in it from living things, carbon dating was not available. Nothing was known about the rock. For a long time it was thought to have been there since the creation of the earth, but that idea has been found to be incorrect.
Just in the last 40 years or so, new kinds of dating methods and other technological advances have allowed geologists more insight into what is going on beneath the surface in these older rocks. Gravity maps have revealed a huge tectonic rift, for example, that runs from eastern Nebraska through Iowa, Minnesota, and Wisconsin and under Lake Superior, where it joins one rift shooting north into Canada and another running right through Michigan. This three-pronged rift is similar to the one that runs down the Red Sea to meet the rift in the Gulf of Aden and the East African Rift, only that one is much younger.
In this book McPhee explains how the Canadian Shield and the central portion of North America were mostly likely created. He also looks at recent technologies such as zircon dating and aeromagnetic mapping, and speculates on the discoveries about the basement rock that could emerge in the future.
Although this is the shortest book in the volume, more the length of an essay, its emphasis on technology makes the subject matter of lesser interest to me than that of the previous books.