In my reading, there were two principal scientific ideas. First, McPhee lets the geologist question the pervasive acceptance of plate tectonics, that is, how it is now the first explanation that geologists seek to advance, which may mean that they do not seek alternative explanations when appropriate. More specifically, the geologist accepts the theory for oceanic plates, but not the land/continental versions. She chafes against the preference of many young geologists to create micro-plates for every new unexplained phenomenon, a kind of reductionism that may be similar to that used by proponents of "heavenly spheres" to explain the motions of the planets prior to Kepler and Newton. Second, McPhee goes over the notion of glacial ice flows and what they explain about the current landscapes. As I was quite ignorent of these theories except in the crudest outline, I learned a lot from this. What I cannot do is evaluate whether, after 20 years, this book is outdated, which it almost certainly is.
Beyond those 2 issues, the reader also gets to know how geologists work and think, which was equally fascinating and pleasurable for me. THere are long passages on a technique that the geologist developed - using the teeth of long-disappeared marine worms to date and evaluate the conditions of the sediments in which they appear - that are clearly explained. Nonetheless, the level of the reasoning and vocabulary can at times be technical and was sometimes beyond my level: those "teeth" above are called conodonts, which I happened to know about from a Gould essay; otherwise, I would have found use of that word confusing, as I did many others that are explained perhaps once. THat made the book quite dense and necessary to re-read in certain sections, which is not a criticism so much as an indication of the experience the reader should expect.
Warmly recommended.