 When the last glacial period or Ice Age arrived , a series of four tremendous valley glaciers filled the upper basin of the Merced River . These glaciers rose in branches upstream of Yosemite Valley , descending from the Merced River headwaters , Tenaya Canyon and Illilouette Creek . Tenaya Canyon was actually eroded even deeper by an arm of the Tuolumne Glacier , which formed the Grand Canyon of the Tuolumne and Hetch Hetchy Valley on the Tuolumne River in the north . Little Yosemite Valley formed as a result of the underlying rock being harder than that below the Giant Staircase , the cliff wall containing Vernal Fall and Nevada Fall . These three branches of each glacier combined to form one large glacier about 7 @,@ 000 feet ( 2 @,@ 100 m ) thick at maximum , stretching 25 miles ( 40 km ) downstream past the mouth of Yosemite Valley , well into Merced Canyon . These glaciers formed the granite cliffs that now constitute landmarks such as Half Dome , El Capitán , and Cloud 's Rest . 
