El Capitan is composed almost entirely of El Capitan Granite, a pale, coarse-grained granite emplaced approximately 100 mya (million years ago). In addition to El Capitan, this granite forms most of the rock features of the western portions of Yosemite Valley. A separate intrusion of igneous rock, the Taft Granite, forms the uppermost portions of the cliff face.
A third igneous rock, diorite, is present as dark-veined intrusions through both kinds of granite, especially prominent in the area known as the North America Wall.
Along with most of the other rock formations of Yosemite Valley, El Capitan was carved by glacial action. Several periods of glaciation have occurred in the Sierra Nevada, but the Sherwin Glaciation, which lasted from approximately 1.3 mya to 1 mya, is considered to be responsible for the majority of the sculpting. The El Capitan Granite is relatively free of joints, and as a result the glacial ice did not erode the rock face as much as other, more jointed, rocks nearby. Nonetheless, as with most of the rock forming Yosemite’s features, El Capitan’s granite is under enormous internal tension brought on by the compression experienced prior to the erosion which brought it to the surface. These forces contribute to the creation of features such as the massive Texas Flake, a large block of granite slowly detaching from the main rock face about halfway up the side of the cliff.