Glaciers on Granite: The Geologic Story of YosemiteYosemite preserves a fascinating geologic story hundreds of millions of years in the making. Powerful forces have sculpted this landscape—sometimes gradually, sometimes instantaneously—into the spectacular features seen today.
Yosemite’s Enduring BedrockThe bedrock foundation of Yosemite was forged underneath volcanoes millions of years ago. The Ancient Sierra NevadaEarth was a different place 100 million years ago. Dinosaurs roamed the land, and the climate was much hotter. A chain of volcanoes ran the length of California, erupting lava and ash. Under these volcanoes, Yosemite’s granite bedrock began to form.The volcanoes were fed by rising magma (molten rock) that originated about 60 miles (100 km) below the surface in the subduction zone, where one of the Earth’s tectonic plates (an oceanic plate) dove beneath the western edge of North America. 
Slow cooling under great pressure produced large, tightly interlocking crystals of quartz, feldspar, hornblende, and other minerals. The resulting rock, called granite, has a sparkly salt-and-pepper appearance. Nearly all the rock you can see from here is a type of granite, including the walls of this building. Yosemite’s towering cliffs testify to the great strength of granite. 
The Mountains RiseAs the Sierra Nevada mountains uplifted, erosion removed the overlying rocks to reveal the granite core of the range. If Yosemite’s granite formed deep under a chain of volcanoes, why is it now exposed on the surface? Erosion! Over the course of 100 million years, rivers and glaciers stripped away miles of overlying rock, gradually wearing down the ancient Sierra Nevada. In the last 10 million years, faulting along the eastern side of the Sierra Nevada created the asymmetric shape of the mountain range, which has a long, gentle western slope and a short, steep eastern slope. The mountains rose and tilted westward, a process that continues during occasional earthquakes along the east-side faults. Ice Blankets the LandscapeOver the past few million years, fluctuations in the Earth’s climate caused a series of ice ages that transformed Yosemite. During colder periods, glaciers formed along the crest of the Sierra Nevada and descended the range’s flanks. Two glaciers converged in Yosemite Valley to form one massive glacier thousands of feet thick. What Are Glaciers?Glaciers are masses of ice that flow due to gravity, both downhill and from thicker to thinner regions. In very cold environments, snow may persist on the landscape year-round. Over centuries, new snow buries the old and compresses it into ice. Ice also forms by the refreezing of meltwater. Glacier ice is solid but slowly moves like honey flowing down a tilted plate. It also flows by sliding across bedrock made slippery by meltwater. How Glaciers Erode the Underlying RockA glacier’s weight pushes unevenly on the bedrock. Pressure differences break off bedrock fragments, which the ice carries away like a conveyor belt. Sliding ice drags embedded rock fragments across the ground surface. This scratches and abrades the bedrock, leaving behind parallel grooves called striations and smooth, shiny surfaces of glacial polish. When the glacier retreats, it leaves behind solitary boulders termed erratics and piles of rock debris called moraines. 
Image credit: NPS Photo The Formation of Yosemite ValleyOver the last 10 million years, rivers carved narrow canyons. Later, repeated glacial advances scoured the canyons, eroding them deeper and wider. Today, waterfalls plunge from sheer cliffs above a flat-floored valley.
Pre-Glacial River CanyonAs the Sierra Nevada rose, rivers and streams cut downward into the bedrock. This action dissected the landscape into narrow V-shaped canyons bordered by rolling granite hills above.
Major Glaciations Enlarge the CanyonAs the climate cooled, glaciers formed along the Sierra Nevada crest and flowed down into the canyons. Glacial erosion transformed the narrow V-shaped canyons into broad U-shaped valleys. The largest glaciers filled Yosemite Valley almost to the top of Half Dome and gouged down far below the modern valley floor.
The Most Recent GlaciationThe most recent glacier occupied Yosemite Valley about 20,000 years ago, filling the valley approximately halfway with ice. As the climate warmed about 15,000 years ago, the glacier melted back and eventually disappeared. Tributary streams that could not keep pace with glacial erosion were left hanging far above the valley floor, forming tall waterfalls.
Post-Glacial ValleyToday’s Yosemite Valley is a deep bedrock basin with steep walls carved by multiple glaciations. The basin is partially gilled with as much as 2,000 feet (600 m) of sediment left behind by the receding ice, creating a flat valley floor. Rockfall debris piles up beneath the cliffs, forming cone-shaped deposits of boulders called talus.Sculpted from StoneYosemite’s granite bedrock is split by two distinct types of fractures that give shape to today’s iconic landforms. Regional JointsA network of fractures, or joints, crisscrosses the Sierra Nevada and extends deep into the bedrock. Water collects in these joints, concentrating erosion there. The network of joints becomes a framework around which the landscape evolves. Regional joints define the overall shape of Yosemite Valley and many of its features, such as the vertical face of Half Dome and the cliffs that form Vernal and Nevada Falls. 
Exfoliation Joints and DomesSmaller, curving fractures called exfoliation joints lie in between the regional joints. These fractures produce slabs of rock like layers of an onion. Shedding of these layers gradually transforms sharp ridges into rounded domes. Many of Yosemite’s famous domes are visible from here, including North Dome, Basket Dome, Mount Starr King, and Half Dome. 
Allen Glazner What Happened to the Other Half of Half Dome?Half Dome is unique because it looks like it’s been cleaved in half. In fact, more than half of the dome remains. Falling Rocks and Rising WatersAfter glaciers retreated from Yosemite Valley, powerful geologic forces emerged to further sculpt the landscape. RockfallsYosemite Valley’s cliffs experience many rockfalls each year. Earthquakes, rain, freeze-thaw cycles, and heat from the sun all loosen slabs from the cliffs. Rockfalls continue to shape Yosemite’s landscape. They can also be dangerous, leading geologists to study their risk. 
Photo: Brian Degenhardt FloodsWinter rainstorms and rapidly melting snow can cause rivers to rise, flooding the flat floor of Yosemite Valley. Floods nourish meadows with water and sediment but can damage roads and buildings. 
A View Like No Other“True appreciation of landscape comes only when one is alive to both its beauty and its meaning.” – Fritiof Fryxell, GeologistThis bare-earth map reveals details of the land surface that might ordinarily be obscured by vegetation, providing a unique view of the geologic processes that shape Yosemite. To learn more about Yosemite’s geology, and to see what this view might have looked like during the most recent glaciation, visit the Glacier Point Geology Hut at Glacier Point. 
B. Glacial moraines are distinctive ridges of rocky debris left by melting glaciers. C. Talus slopes, made up of rockfall boulders, line the bases of Yosemite’s cliffs. 
A National Park FirstWelcome to the first trailside museum in the National Park System.Architect Herbert Maier designed this Rustic-style observatory in 1925. For more than 100 years it has hosted the park’s geology exhibits and provided a unique viewpoint for visitors to observe Yosemite’s geological landscape. The Yosemite Museum Association (today, Yosemite Conservancy) funded the original construction of the Geology Hut. In recognition of the hut’s centennial, Yosemite Conservancy donors funded a major rehabilitation of this iconic landmark. Yosemite Conservancy provides millions of dollars annually to support projects and programs that preserve Yosemite National Park and that enrich the visitor experience. To learn more about Yosemite Conservancy, visit yosemite.org. 
|
Last updated: May 28, 2026