A backward glance
So far we have seen that the Precambrian basement exposed in the Teton Range contains a complex array of rocks of diverse origins and various ages. Before passing on to the younger rocks, reference to our yardstick may help to place the Precambrian events in their proper perspective.
In all of Precambrian time, which encompasses more than 85 percent of the history of the earth (31 of the 36 inches of our yardstick), only two events are dated in the Teton Range: the intrusion of granite and pegmatite about 2.5 billion years ago, and the emplacement of the black dikes about 1.3 billion years ago. These dates are indicated by heavy arrows on the time scale ([fig. 30]). The ancient gneisses and schists were formed sometime before 2.5 billion years ago, and probably are no older than 3.5 billion years, the age of the oldest rocks dated anywhere in the world.