Wednesday was the last day of the formal sessions at the GSA Cordilleran Section meeting. The talks I attended were the final segment of the "meeting in a meeting" and centered on the hot new tool for sorting out the history of the highly scrambled western margin of North America: detrital zircons. Many bodies of rock have zircons with a mixture of ages, reflecting zircons inherited from previous rocks and (in the case of igneous rocks) zircons they have created. It's conceptually straightforward to use them like barcodes for tracing the history of rocks and the mountain chains, volcanic arcs and plate interactions that make up that history. Detrital zircons are the ones that go downstream with sediments and end up inside younger sedimentary rocks. We can use them to tell what kind of rocks they came from—what the ancient, long-vanished mountains were made of.

So that was fun. In the afternoon, I took a drive in the Sierra foothills. After sunset, I was pleased to see three planets together low in the western sky: Mercury, Venus and Jupiter. They'll be there all week, folks.

Thursday is a field trip into "hard-to-access outcrops of the Mesozoic metasedimentary framework and gabbroids of the Early Cretaceous Sierra Nevada Batholith." In lay language, the field-trip leaders will take us to rocks on private land that document the earliest phases of the great story behind the Sierra Nevada. Some geologists occasionally treat fences the way Woody Guthrie did: there are no "No Trespassing" signs on the other side of them. More specifically, they answer the old philosophical chestnut in the negative: "no tree fell (or trespass occurred) if no one was there to see it." Another approach is the quantum one: an observer can have a virtual existence on both sides of the fence at once. Be that as it may, the ideal is to visit a location with the owner's full knowledge and permission, and it should be a great time.

The heart of the GSA Cordilleran Section meeting is over; the Wednesday session will be just half a day, giving me time for a side trip or two before the post-meeting field trip. It has been fun hearing from and talking with some of our local West Coast experts and the visiting experts who have come to do the same.

The sessions I attended on Monday focused mostly on California, which always interests me because that's where I live. One "meeting within the meeting" is a continuous series of talks about subduction zones—more generally, tectonic convergent zones—and the special rocks they make. California's extensive set of subduction-related rocks has made it a "type area" for this particular scientific problem (I present a tour of some of them on this site). Speakers talked about everything from the behavior of earthquake faults, to the ways that continents have their edges sharpened against other tectonic plates, to the hard-won insights from mapping the highly scrambled rock successions left behind on the margins of convergent zones.

The afternoon session I sought out was about the fossils of the Irvingtonian age (which I mentioned in yesterday's post). Now I feel a bit more familiar with that side of my local geology. And it was fun to see the people I hung out with on Sunday stepping up to give organized presentations to each other.

Today I attended talks on a tectonic problem that a small but dedicated group of researchers is working on: the story of the Gulf of California—that narrow stretch of sea between Baja California and the Mexican continent—how it formed, what series of events affected its history, what light it sheds on the geologic history of its neighboring lands and greater North America. And in the afternoon I caught up with the cutting edge of research into ophiolites: pieces of oceanic plates that are somehow pushed ashore.

These are both intricate problems, but they have different outlooks. The Gulf of California is a blurry picture that is inevitably getting clearer as new data comes in from the field. Ophiolites are a blurry picture that is getting more complicated with more detail. Researchers are still settling on the right concepts for insightful progress, and the right names to label them with. As Yilderim Dilek told the audience, "You can't form ophiolites from one template; they're like different flavors of ice cream." Nevertheless, Eldridge Moores pointed out, all of them always involve some form of subduction—a continent and a fault cutting downward through it—to be preserved. That doesn't always mean the textbook kind of subduction, he pointed out, just some version of the basic geometry. That permits ophiolites to be recognized in the oldest rocks on Earth without forcing us to say, against much evidence, that modern-style plate tectonics existed back then. There's nothing happier than a roomful of baffled scientists.

I'm spending most of the week in Fresno, California, at the Cordilleran Section meeting of the Geological Society of America. Yesterday, in one of the preliminary field trips for the event, I joined a party of 25 to visit the fossil collection from the Fairmead Landfill project a few miles north of town. For 20 years, paleontologists from Fresno have been picking over the ground excavated by the landfill operators. The fossils represent a community of grazers: 60 percent of them are horse bones, followed in abundance by camel, mammoth, and elk. There are no bison bones, which puts this assemblage firmly in the Irvingtonian section of the North American Land Mammal Age sequence. If you've gotten used to the Pleistocene, Miocene and other time terms, the NALMA units are strange and awkward. But they need to be, as I explain in my new article on provincial time terms. When I get back I'll put up some photos from a remarkable afternoon.

If you think you know your way around, think again. A geologic map will transform the way you see your stomping grounds. The familiar features—roads, cities, buildings and all that—are grayed out while the aboriginal land itself shines forth in colors, patterns and lines. Sprinkling this strange scene are symbols and markings with purely geological significance, things you may never have known existed. I'm saying beware: the geologic maps quiz may leave you all at sea. And I think that even the grizzled old pros may miss a trick or two in this challenging test.