Zircons--bits of zirconium and silicate that form in granite--are durable minerals. It’s very difficult to break them down, says geologist Nancy Riggs of Northern Arizona University at Flagstaff. If a zircon ends up in a river, it just goes and goes. As Riggs and her colleagues reported this past year, the zircon’s steadfastness is what allowed them to map the course of a river that flowed through the western United States hundreds of millions of years ago--when North America was still part of the supercontinent of Pangaea, and the ocean began in Nevada.
Riggs collected zircons from a sandstone deposit in northwestern Texas. Through isotopic dating (she measured what portion of two different uranium isotopes had decayed to lead in the crystals), she determined that some of the zircons had first crystallized 525 to 515 million years ago. That allowed her to trace the crystals--and the river that eroded them out of the parent rock--to a nearby granite formation of that age called the Amarillo-Wichita Uplift. Although zircons of that particular age happen to be rare, they have also been found at a site in western Nevada. The Nevada zircons, Riggs figures, must have come from the same Texas mountain range and been carried west by the river, which she calls the Chinle.
The Chinle had its source in the highlands of Texas, Riggs says, and then from Amarillo we think it flowed west as one large river system from northern New Mexico to eastern Nevada, until it reached the ocean in what is now the middle of Nevada. Eventually the Texas highlands eroded down into lowlands, and Pangaea drifted away from the equator into the drier midlatitudes. By the time North America separated from the other continents around 200 million years ago, the mighty Chinle was drying up.