9.6: Ice Shapes the Landscape

How Glaciers Form

Glaciers form in regions where the accumulation of snow exceeds its melting over long periods, allowing snow to build up and compact into ice. In the Sierra Nevada, this process began during the Pleistocene Epoch, when colder global temperatures enabled snow to persist year-round at higher elevations. As snow accumulates, it compacts under its own weight, causing the individual snowflakes to recrystallize into firn, a granular type of ice. Over time, the firn becomes increasingly dense as the air trapped within it is squeezed out, eventually forming solid glacial ice. This ice, under the pressure of its own weight, begins to deform and flow.

The Sierra Nevada's climate and topography were ideal for glacier formation during the Pleistocene. High elevations provided cold temperatures necessary for sustaining snow and ice, while the steep topography encouraged snow to accumulate in mountain valleys and basins. This process was especially evident in areas like Yosemite National Park, where glaciers formed in high alpine valleys and carved out many of the park’s iconic features, such as the deep U-shaped Yosemite Valley and the steep cliffs of Half Dome and El Capitan. The Kings Canyon region also saw significant glacier formation, with glaciers carving out the park’s deep canyons and rugged peaks​.

As glaciers advanced and retreated with fluctuations in climate, they left behind a record of their activity in the form of glacial deposits, including till and outwash. These deposits, made up of unsorted material ranging from fine silt to large boulders, provide geologists with clues about the extent and movement of ancient glaciers. In the Sierra Nevada, these deposits are found throughout the range, marking the boundaries of former glaciers and helping to reconstruct the region's glacial history.

How Glaciers Move

Glaciers move under their own weight, a process that is driven by gravity and facilitated by the unique physical properties of glacial ice. The movement of a glacier occurs through two main mechanisms: internal deformation and basal sliding. Internal deformation happens when the ice within the glacier flows plastically, meaning it deforms without fracturing. This flow is driven by the immense pressure exerted by the overlying ice, which causes the ice crystals to rearrange and slide past each other, allowing the glacier to move slowly downhill. This process is particularly important in cold, dry climates where the base of the glacier remains frozen to the bedrock.

Basal sliding, on the other hand, involves the entire glacier sliding over the bedrock beneath it, facilitated by a layer of meltwater that acts as a lubricant. This meltwater is produced by pressure melting at the base of the glacier or by geothermal heat. In temperate glaciers, such as those that once covered much of the Sierra Nevada, basal sliding is a significant contributor to glacier movement as exhibited in Figure 9.6.1. The presence of meltwater not only speeds up the glacier’s movement but also enhances its ability to erode the landscape, carving deep valleys and fjords.

Figure \(\PageIndex{1}\): Anatomy of a glacier by the National Park Service is licensed under public domain. Access a detailed description.

The glaciers that once covered Yosemite and Kings Canyon National Parks moved in much the same way. For example, the Maclure Glacier in Yosemite, still active today, moves primarily through internal deformation, though it also exhibits some basal sliding during warmer months when meltwater is present. This movement, while slow, is powerful enough to have carved the deep valleys and polished granite domes that are hallmarks of Yosemite’s landscape. Similarly, the glaciers that once filled Kings Canyon were responsible for shaping its steep-walled valleys and creating features such as hanging valleys and cirques.

Erosional and Depositional Features of Glaciers

Glaciers are powerful agents of erosion and deposition, capable of reshaping entire landscapes as they move. The process of glacial erosion involves two primary mechanisms: plucking and abrasion. Plucking occurs when a glacier moves over fractured bedrock, causing pieces of rock to break off and become embedded in the ice. As the glacier continues to move, these rocks are transported along with the ice, grinding against the bedrock and deepening valleys. This grinding action, known as abrasion, polishes the bedrock surface and creates striations—linear grooves that indicate the direction of the glacier's movement (Figure 9.6.2).

Figure \(\PageIndex{2}\): Glacial striations and polish near Lambert Dome, Tuolumne Meadows by Daniel Mayer licensed under CC BY-NC 4.0. Access a detailed description.

In the Sierra Nevada, the erosional power of glaciers is evident in the many U-shaped valleys, cirques, and arêtes that characterize the region (Figure 9.6.3). Yosemite Valley is perhaps the most famous example of a U-shaped valley (Figure 9.6.4), formed as glaciers carved through the softer rock, leaving behind steep walls and a flat valley floor. Similarly, cirques—amphitheater-like hollows found at the head of glacial valleys—are common throughout the Sierra Nevada. These features, along with the sharp, knife-like ridges known as arêtes, are remnants of the glaciers that once dominated the region during the Pleistocene.

Figure \(\PageIndex{3}\): Geologic history and formation of Yosemite National Park by National Park Service and Eric Knight is licensed under public domain. Access a detailed description.

Figure \(\PageIndex{4}\): Yosemite from Tunnel View by Cole Heap is licensed under CC BY-NC 4.0. Access a detailed description.

Glacial deposition, on the other hand, occurs when glaciers lose their ability to carry debris, often due to melting as they retreat. As the ice melts, it deposits the material it has transported, creating a variety of landforms. Moraines, which are accumulations of debris deposited at the edges of glaciers, are common in the Sierra Nevada. Terminal moraines mark the furthest advance of a glacier, while lateral moraines form along the sides. In Kings Canyon National Park, for example, the El Capitan Moraine is a prominent feature that illustrates the extent of glaciation during the Tioga glaciation. Additionally, glacial outwash plains—flat areas formed by sediments carried away from the glacier by meltwater—are found throughout the region, further shaping the landscape​.

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Video 9.6.1: Glaciation and Hanging Valley Formation

Video 9.6.1: Animation showing glaciers and how hanging valleys form by InterAct Consortium. Used with permission. Access a written description.

 

Sierra Nevada Glaciations

The Sierra Nevada has experienced multiple glaciations, particularly during the Quaternary Period, with each glaciation leaving a distinct mark on the landscape. The Tahoe, Tenaya, and Tioga glaciations are the most notable, with the Tioga glaciation being the most recent and best-preserved. The Tahoe glaciation, which occurred around 70,000 years ago, was the most extensive, covering large portions of the Sierra Nevada with thick ice sheets. These glaciers carved deep valleys, shaped mountains, and left behind a landscape that was dramatically altered by the power of ice.

Following the Tahoe glaciation, the Tenaya and Tioga glaciations occurred, with the Tioga glaciation reaching its maximum extent around 20,000 years ago. The Tioga glaciation, though less extensive than the Tahoe, was responsible for many of the glacial features we see today in Yosemite and Kings Canyon National Parks. During this time, glaciers extended far down the valleys, carving the iconic U-shaped valleys, hanging valleys, and cirques that are now synonymous with the Sierra Nevada’s glacial landscape. The retreat of these glaciers left behind a wealth of moraines, erratics, and other depositional features that continue to shape the region’s topography. Figure 9.6.5 shows the extent of late glaciation and direction of ice flow in Yosemite National Park and further emphasizes the vast geomorpological features created by glacial movement.

Figure \(\PageIndex{5}\): Glacial extent and direction of ice flow in Yosemite National Park by National Park Service is licensed under public domain. Access a detailed description.

The study of these glaciations provides valuable insights into the climatic conditions of the past and the processes that have shaped the Sierra Nevada. By analyzing glacial deposits and the geomorphology of the landscape, geologists can reconstruct the extent and timing of these glaciations, offering a window into the region’s dynamic geological history. The glacial history of the Sierra Nevada also has implications for understanding the potential impacts of future climate change, as the region’s glaciers continue to retreat in response to warming temperatures​

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Glaciers Today

Today, the glaciers of the Sierra Nevada are mere remnants of their former selves, a testament to the region's cooler, glacial past. The most notable of these remnants are the Lyell and Maclure Glaciers in Yosemite National Park, the southernmost glaciers in North America. These glaciers, although significantly smaller than they were during the Pleistocene, continue to provide important insights into the processes of glacial movement and climate change. The Maclure Glacier, for example, still exhibits some movement, albeit much slower than in the past, while the Lyell Glacier has ceased moving altogether, indicating that it may no longer be considered an active glacier (Figure 9.6.6).

Figure \(\PageIndex{6}\): Repeat photography of the Lyell Glacier showing nearly 140 years of ice loss by the United States Geological Survey and National Park Service is licensed under public domain. Access a detailed description.

The retreat of these glaciers is indicative of broader trends in global climate change. As temperatures rise, glaciers worldwide are shrinking, and the Sierra Nevada is no exception. The reduction in glacial ice has implications not only for the landscape but also for water resources in the region, as glaciers contribute to streamflow during the dry summer months. The loss of these glaciers would mark the end of an era in the Sierra Nevada’s geological history, but they continue to be closely monitored by scientists studying the impacts of climate change.

Video 9.6.2: Retreat of Lyell and Maclure Glaciers

Video 9.6.2: Animation showing the retreat of the Maclure and Lyell glaciers from 1883 to 2016 by the United States Geological Survey and National Park Service is licensed under public domain.

Despite their retreat, the glaciers of the Sierra Nevada remain a critical part of the region’s natural heritage. They are not only key indicators of environmental change but also serve as living laboratories where scientists can study the ongoing interactions between climate, ice, and the landscape. As the climate continues to warm, these glaciers offer a poignant reminder of the powerful forces that have shaped, and continue to shape, the Sierra Nevada​.

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References

1. Bateman, P. C. (1992). Plutonism in the central part of the Sierra Nevada Batholith, California. U.S. Geological Survey Professional Paper 1483.
2. Calkins, F.C. (1930). The granitic rocks of the Yosemite Region, in Matthes, F.E. Geologic history of the Yosemite Valley. U.S. Geological Survey Professional Paper 160, 120-129. Kessler, M.A., Anderson, R.S., and Stock, G.M. (2006). Modeling topographic and climatic control of east-west asymmetry in Sierra Nevada glacier length during the Last Glacial Maximum. Journal of Geophysical Research – Earth Surface, v. 111, F02002.
3. Hildreth, W., & Fierstein, J. (2016). Eruptive history of Mammoth Mountain and its mafic periphery, California. U.S. Geological Survey Professional Paper 1812.
4. Huber, N. K. (1981). Amount and timing of Late Cenozoic uplift and tilt of the central Sierra Nevada, California—evidence from the upper San Joaquin River basin. U.S. Geological Survey Professional Paper 1197.
5. Lindgren, W. (1911). The Tertiary Gravels of the Sierra Nevada of California. U.S. Geological Survey Professional Paper 73.
6. Moore, J.G. and Mack, G.S. (2008). Map showing limits of Tahoe Glaciation in Sequoia and Kings Canyon National Parks. Scientific Investigations Map 2945, https:pubs.usgs.gov/sim/2945/
7. Slemmons, D. B., & Brock, V. E. (1975). *Late Ceno
8. Wakabayshi, J. and Sawyer, T. L. (2001). Stream incision, tectonics, uplift, and evolution of topography of the Sierra Nevada California. Journal of Geology, v. 109, 539-562.