Yosemite’s Not-So-Solid Rock

Wise men and poets talk of the rock of ages and of things being as solid as a rock--the symbol of indestructible permanence. But anyone who has visited Yosemite Valley in recent weeks knows better. Even the rock ultimately yields to time.

Nineteenth century naturalist John Muir noted that the mighty rock succumbs eventually to the tiny “snowflower,” as billions and billions of flakes form snowfields and then glaciers. That’s what sculpted Yosemite Valley and its soaring rock formations such as Half Dome and El Capitan.

The glaciers have retreated and now the walls are slowly coming apart. Several stupendous rockfalls have occurred in the Glacier Point area near the eastern end of Yosemite Valley in recent years. One person was killed and a dozen were injured near Curry Village in 1996, and two weeks ago rock falling from the west shoulder of Glacier Point killed one climber and injured three.

That normally would be the end of it. The National Park Service would avoid taking any special precautions because rockfall is unpredictable and one fall does not necessarily presage another. But there have been frequent smaller falls from Glacier Point since June 13 and cracks have been expanding with a popping noise. Parts of Curry Village and nearby employee housing have been evacuated because of the danger.

The chances of anyone being struck are small, says UC Davis geologist Eldridge Moores, “but at some time it will come down--all of it.” Rockfall has a variety of causes. A major one is embedded in the granite itself. Fractures or joints occurred as molten material cooled and crystallized into granite under tremendous pressure millions of years ago. Freezing and thawing further force apart the sections of rock. At some point, gravity wins and massive blocks of granite plunge to the valley floor.

Big rockfalls often go virtually unnoticed because they occur in rarely visited areas. It’s easy to tell where falls are most common by observing the giant piles of boulders at the bases of cliffs. Climbers are keenly aware of rockfalls because they can dramatically alter climbing routes. In fact, cracks and fault lines in the rock provide natural ascent routes. Prudent climbers wear hard hats against the occasional small falling rock, but Yosemite granite is considered quite safe in that respect.

Other valley visitors should be aware of the potential hazard but not let it affect their enjoyment of the park. Like flood, fire and blizzard, this is nature at work. We come and witness the artistry in awe.