inor episodes occurred between these two phases (1:303).
The distribution of the different rocks' ages suggests that "major episodes of plutonism were often separated by large intervals of time" (1:303). During the early Cretaceous period, the Jurassic and older sedimentary rocks were intruded by a sequence of magmas called, in the case of Yosemite Valley, the Western Intrusive Suite. These intrusions are the most-widespread and most-recognized features in the valley, as with the El Capitan Granite.
The face of the El Capitan Granite is a massive cliff with a dark irregular shape in its center called the "Map of North America." The granite itself is extremely sheer, with few through-going joints, and the discoloration of the so-called map is caused by "a complex of diorite dike intrusions that invaded the plutons of the earlier Western Intrusive Suite" (1:303). The very thin white lines that can be seen crisscrossing the granite's face as well as the map are later pegmatite dikes, or sheetlike intrusions.
After the Western Intrusive Suite, the next sequence known in the valley is the Minor Intrusive Suite. Geologists can establish the "relative" age (that is, whether one is older or younger than the other) and order of these intrusions by determining which intrusion has cut through the other. The youngest of the Cretaceous intrusions in the Yosemite Valley is the Tuolumne Intrusive Suite. This suite exhibits a certain concentric zonation that resulted from later intrusions making inroads into the cores of earlier plutons, or individual masses of granitic rock (6:128), thus causing what are called "nested" groups. These magmatic intrusions took place over a short period of time, geologically speaking.
For scientists such as geologists, the rocks of the Yosemite Valley can be read like the exquisite illuminated manuscripts of the Middle Ages, whose beauty also holds knowledge for those who know how to read them. T...