Evolution of Yosemite Valley

500—200 million years ago

The Sierra Nevada region was once submerged beneath a sea. Sediments slowly accumulated on the ocean floor, growing thousands of feet thick and compressing the layers into sedimentary rock.

200—80 million years ago

A slab of the Earth's crust, called a plate, began to slide under the North American continent in a process called subduction. Deep within the earth, tremendous heat and pressure caused the downgoing plate to melt into magma. The magma rose toward the surface, where some of it erupted to form a mountain chain of volcanoes. Much of the magma cooled underground to become granite.

60 to 10 million years ago

After subduction ceased, prolonged erosion stripped away the overlying volcanic rocks and exposed the granite. The land destined to become Yosemite National Park was made up of low mountains with shallow river-cut canyons. Hardwood forest flourished.

10 million years ago

Over the next 5 million years, the Sierra Nevada, California's "backbone," rose. The Sierran block uplifted, tilting westward, increasing the Merced River began to carve a narrow canyon. Redwood forests flourished.

3 million years ago

The Merced River carved its canyon deeper, while its tributaries cut the land more slowly. Forests began to thin as the Ice Age approached.

1—2 million years ago

A series of large glaciers flowed from the crest of the mountain range into the river canyons. Glaciers repeatedly filled the "V"-shaped Yosemite Valley, widening, deepening and carving it into a "U" shape, forming hanging valleys from which waterfalls now cascade.

15,000 years ago

Temperatures warmed and the last glacier retreated from Yosemite Valley. Its terminal moraine (rock debris) dammed the Valley 14,000 to 15,000 years ago, creating a shallow lake. Sediment eventually filled the lake, which became the flat valley floor we see today.

Today

The same process of sedimentation continues at Mirror Lake, which is even smaller today than in this photo. Soon, sediment will completely fill in the lake, creating a meadow. Water and gravity continue to shape Yosemite's landscape. Events like the Middle Brother rockfall of 1987 and the flood of January 1997, were the most dramatic in the recorded history of the park. Be aware that rocks can fall at any time.