Why are many Sierra Nevada valleys U-shaped?

Many valleys in the Sierra Nevada are U-shaped because they were carved by glaciers during the Pleistocene Epoch.1

While rivers typically create V-shaped valleys, these were transformed into U-shaped glacial valleys as massive glaciers moved through them. Because ice has greater viscosity and a larger cross-section than water, it erodes the landscape more uniformly, deepening and widening the valley floor to minimize frictional resistance.21

During the Pleistocene, extensive ice fields and glaciers covered the Sierra Nevada, extending from the Lake Tahoe area to the southern high sierra. These glaciers carved deep troughs into the granite landscape, leaving behind characteristic features such as steep walls, cirques, and hanging valleys—the latter of which often result in waterfalls like Yosemite Falls after the ice melts.12