A provocative new hypothesis suggests that Yosemite Valley was carved by an ancient volcano and a disappearing river, both of which have long since eroded away.
Geologists have long debated why Yosemite Valley is so deep, with walls that tower up to 4,000 feet (1,219 meters) above the valley floor. The prevailing explanation is that in the last 10 million years, the Sierra Nevada mountains of California experienced a period of uplift, steepening their slope and causing the rivers to flow more quickly and erode more quickly into the granite around them.
