Yosemite Valley is a geological wonder, revered globally for its sheer cliffs and expansive, flat floor. Its dramatic appearance, with granite walls soaring thousands of feet above the meandering Merced River, resulted from a slow, forceful transformation. While water initially cut the valley, repeated invasions of massive ice sheets during the Pleistocene Epoch—the Ice Ages—sculpted the landscape into its distinctive U-shape. This monumental carving process took place over hundreds of thousands of years.
Yosemite Before the Glaciers Arrived
Long before the first glaciers descended, the landscape of Yosemite Valley was a deep, steep-sided river canyon. The Merced River carved this initial valley over millions of years, creating a typical V-shaped profile. The river’s erosive power was amplified by the gradual uplift and westward tilting of the Sierra Nevada mountain block, which accelerated the water’s speed and cutting ability.
The bedrock was the Sierra Nevada batholith, a massive body of igneous rock primarily composed of granite. This hard, crystalline rock, exposed by uplift and erosion, was already fractured by a network of joints and fissures. The pre-glacial river carved its canyon as much as 3,000 feet deep, but the resulting valley was narrow and confined, providing the V-shaped starting point for the subsequent ice sheets.
The Mechanics of Glacial Erosion
The transformation from a V-shape to a glacial U-shape was achieved through the immense weight and slow movement of the ice. Glaciers formed high in the Sierra Nevada and moved downslope, confined by the river channel. These thick tongues of ice filled the valley, reaching several thousand feet in depth and exerting enormous downward pressure on the valley floor and sides.
Glacial erosion relies on abrasion and plucking. Glacial abrasion occurs as the ice drags embedded rock fragments, or till, across the bedrock surface. This grinding action polishes the rock, leaving behind parallel scratches called striations, and produces a fine silt known as rock flour. Abrasion smoothed the less resistant sections of the valley.
Glacial plucking, or quarrying, created the near-vertical walls. Meltwater seeped into joints and cracks in the granite, re-freezing and adhering to the base of the moving glacier. As the massive body of ice flowed onward, it pulled away large, joint-bounded blocks of rock. This process exploited natural weaknesses in the granite to deepen and widen the canyon.
Iconic Landforms Shaped by Ice
The repeated cycles of glaciation, particularly the Sherwin and Tahoe glaciations, carved the valley into a classic U-shaped glacial trough with steep walls and a broad floor. The shapes of Yosemite’s famous granite formations resulted from glacial sculpting interacting with the granite’s internal structure. Monoliths like El Capitan and Half Dome resisted the full force of the ice because their granite had relatively few internal fractures, making them less susceptible to plucking.
El Capitan’s sheer, 3,000-foot vertical face was created as the glacier flowed past, plucking rock along a prominent, near-vertical joint set. The rock mass was too large and cohesive to be quarried away entirely. Half Dome has one rounded side and one sheer drop; the glacier scraped along its base, but a major vertical fracture allowed the ice to quarry out the large mass of rock that formed its straight face.
Other features resulted from differential erosion between the main valley and its tributaries. The main Merced Glacier cut the valley floor significantly deeper than the smaller glaciers in the side canyons. When the ice retreated, these tributary valleys were left “hanging” high above the main valley floor, giving rise to features like Yosemite Falls and Bridalveil Fall, which plunge over the newly exposed, glacially steepened cliffs.
The Evolution of the Valley Floor
The final stage of the valley’s formation began when the last major glacier retreated around 15,000 years ago. As the ice melted, it left behind a massive terminal moraine—a ridge of rock debris—near the valley’s western end. This moraine acted as a natural dam, impounding the meltwater to create a large, deep body of water known as ancient Lake Yosemite.
This lake persisted for thousands of years, gradually collecting vast amounts of sediment washed in by the Merced River and other streams. The fine-grained material, including glacial till and the pulverized rock flour created by abrasion, settled to the bottom of the lake. Over time, the lake basin filled with this outwash material, accumulating to depths of up to 2,000 feet in some places.
This extensive infilling process created the flat, expansive valley floor visible today, which is now home to meadows and forests. The Merced River now flows across this thick layer of sediments, having shifted from its ancient canyon-cutting role to meandering across the broad, glacially-engineered plain. The modern landscape is thus a fusion of the initial river erosion, the profound glacial carving, and the final stage of sedimentary infilling.