Origin

• The Sherwin Glaciation is known for forming the Yosemite Valley. This glacier filled the valley to its rim, carved out the basic shape of the valley and gouged out a deep basin. The Sherwin probably lasted 300 thousand years and ended about 1 million years ago.
  
  

History

• The Tahoe glaciation probably peaked about 130,000 years ago while the Tioga, the most recent glaciation in Yosemite, peaked about 20,000 years ago. By the time the Tioga retreated, Lake Yosemite, numerous waterfalls at hanging valleys (e.g. Yosemite Falls, Bridalveil Fall), as well as waterfalls at glacial steps (e.g. Vernal Fall, Nevada Fall) remained.
  
  

Effects

• Because no other glacier has filled the Yosemite Valley since the Sherwin glaciation, no other excavation has been done by a glacier. This is why the walls are not smooth like other glacial valleys (e.g. Hetchy Hetchy Valley).
  
  

Features

• The walls of Yosemite Valley are ragged, with numerous pinnacles and spires ascending upward from them. Leaning Tower, Cathedral Spires, Sentinel Rock and Lost Arrow are the most striking .
  
  

Fun Fact

• Lyell Glacier is the largest glacier in Yosemite, covering approximately 160 acres. Lyell is one of only two glaciers still intact in Yosemite; the other is Maclure Glacier. Both Mounts Lyell and Maclure can be visited by a three-day backpack trip from Tuolumne Meadows.
  
  

Considerations

• The rockfall in July 1996 and the flooding of the Merced River in January 1997 destroyed many human "improvements." This shows that powerful geologic forces are still continuing the evolution of Yosemite Valley.