Yosemite National Park Geology

Yosemite has a simple geologic history compared to many other mountain parks. Most all of the rock that you see in Yosemite is part of the Sierra Nevada Batholith, a large body of magma that intruded, crystallized, and lay buried as granitic material beneath a thick layer of metamorphic rock.

Geologists have estimated that the batholith was about six miles below the surface and it took about 100 million years for it to form. Inside Yosemite, dozens of plutons of different ages and composition were emplaced. Most of the rock in Yosemite was thrust up between 80 and 120 million years ago. Some were intruded between 150 and 210 million years ago.

The entire Yosemite region rose and has been extensively eroded.

Overtime, almost all of the overlying metamorphic rock was eroded away and the batholithic granite remained.

The latest period of uplifting occurred around 10 million years ago which created the Sierra Nevada range, which has a distinct westward tilt, steep east face, and gentle western slopes.

With the increase of the gradient the erosive power of the westward flowing rivers and streams increased. The Merced River cut a narrow canyon over 3,000 feet deep. By the time of the Pleistocene ice age, the top of the Sierra Nevada had reached elevations of 14,000 feet.

Yosemite bears the mark of at least four major periods of glaciation. The most recent period ended around 10,000 years ago. The earliest period of glaciation was about one million years before that.

Alpine glaciers formed along the Sierra Nevada range and joined into larger glaciers as they moved down slope. The ice in Yosemite might have been 4,000 feet thick during the peak of the glaciation. The power of the glaciers and boulders widened the valleys, deepend the floors, and made the walls steep. Almost anywhere you look in Yosemite Park you will see the effects of glaciation.

Yosemite Valley

One of the world's best known glacially carved canyon is Yosemite Valley which is framed by El Capitan, Half Dome, and Yosemite Falls. The Yosemite Valley is geologically very young at around 30,000 years.

 About 50 million years ago, the Yosemite Valley was a wide trough with the Merced River meandering among rolling hills of hardwood forests. As the Sierra Mountains rose and tilted westward, the Merced accelerated and dug deeply, creating a narrow canyon that is 3,000 feet deep.

Then, about one million years ago, a series of Pleistocene glaciers covered the land, sometimes filling the valley. The ice cut the narrow, zig-zagging canyon into a straighter, broader, and deeper canyon. The bottom of the trough filled with glacial rock and sediment. The last valley glacier melted around 10,000 years ago. This resulted in a moraine that dammed the Merced River and created a shallow lake. The lake eventually filled in to form marshy bottoms and meadows.