http://en.wikipedia.org/wiki/Wyoming_craton
First time I've seen the use of SI prefixes for years. I think Ga means Giga Annum. Mike Payne Local preservation of 3.6-3.0 Ga gneisses and widespread isotopic evidence for crust of this age incorporated into younger plutons indicates that the Wyoming craton was, and still is, a 100,000 km2 middle Archean craton that was modified by late Archean magmatism and tectonism and Proterozoic extension and rifting.[2] The Wyoming, Superior and Hearne cratons were once sections of separate continents, but today they are all welded together. The collision of these cratons began before ca. 1.77 Ga, with post-tectonic magmatism at ca. 1.715 Ga (the Harney Peak granite). This tectonic-magmatic interval is 50-60 million years younger than that reported for the Hearne-Superior collision of the Trans-Hudson orogeny in Canada.
