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SAMPLE Q&A SECTIONS: What is the North America Wall on El Cap? El Capitan's granite walls rise over 2,000 feet above the valley floor in Yosemite National Park. Within El Cap's east wall, there is a dark body of rock shaped like the North American continent. Route and feature names like the North America Wall, the Pacific Ocean Wall, South Seas, and New Jersey Turnpike come from this startling feature. So how did it form? The dark rock is called diorite and it intruded El Capitan Granite as magma after El Capitan had already formed. Diorite, like granite, is an intrusive igneous rock, but it has a higher percentage of dark minerals than granite. These minerals, usually biotite or hornblende, owe their dark colors (black to dark brown and green) to an abundance of iron and magnesium. There are actually two different sets of diorite intrusions on El Capitan's east face. The oldest set can be seen at the base of the wall. Here, dikes and wedge-shaped pods of diorite are surrounded by a light colored rock that has a foliation, or an alignment of minerals into planes. This light colored rock is El Capitan Granite that became heated and squished when the diorite magma came through. The second set of intrusions forms two complex dike bodies: a western body that is shaped like North America and an eastern body that surrounds a circular area of white El Capitan Granite - the Great Circle of the Zodiac wall. The shapes of these bodies are due to the path the diorite magma took as it invaded the granite, and also the interactions that occurred between the magma and granite during emplacement. Many geologists think that the diorite partially melted the surrounding granite as it intruded, forming hybrid rocks and complex shapes. The western body's resemblance to the North American continent is just a neat coincidence. The oldest set of diorite dikes intruded the El Capitan Granite early in its history, when the granite was still cooling, about 102 million years ago. The younger set of dikes is related to intrusion of the Taft Granite that makes up Glacier Point Apron, about 96 million years ago. How do boulders form? There are three primary ways boulders and boulder fields are created: (1) by mass wasting of unstable slopes, (2) by transport via glaciers or rivers, and (3) by in-place weathering of rock formations. The first, mass wasting of unstable slopes, is just a fancy way of saying rock fall. Cliff bands and steep mountain faces are typically cracked, the cracks usually resulting from unloading pressure as rocks move from deep in the Earth to the surface of the Earth. Water and wind exploit these cracks, forming stacks of blocks where competent walls once stood. The blocks eventually succumb to gravity and tumble into talus fields where the largest of them become boulders. The sandstone bouldering area at Big Bend, near Moab, Utah, is an example of this process. Here, large angular blocks of red-colored sandstone fell from the cliffs that border the Colorado River. Why the boulders are particularly concentrated at Big Bend is a matter of speculation: perhaps the topography there allows for the accumulation of large blocks; perhaps the spacing of cracks in the cliffs near Big Bend is wide enough that the blocks that fall are larger than in other places. Another way boulders form is by the erosive force of glaciers. As glaciers move slowly down hill, they pluck chunks of stone from the surrounding landscape. These stones can range in size from small pebbles to house-size boulders and they can be plowed along the front and sides of the glacier (the terminal and lateral moraines), or they can move within the ice sheet itself. When the glaciers retreat, the stones are left behind, sometimes ending up hundreds of miles from where they originated. Most of the boulders of New England and in the European Alps were formed this way. The swirly patterned boulders of Pawtuckaway and Lincoln Woods in New England are chunks of gneiss that originated in Canada. The third primary way boulders form is by in-place weathering of large bodies of rock, commonly granite plutons. The boulders and rock formations of Vedauwoo, Wyoming are an example of this process. At Vedauwoo, the 1.4-billion-year-old Sherman Granite is eroded preferentially along fractures. The way the fractures are oriented-many parallel to the ground, with some perpendicular-results in segmentation of the granite. Over geologic time, the segments become separated and their edges become rounded - and boulders are formed.