monoliths with names like El Capitan and Sentinel Rock stand watch like mythical giants of old

It's not known how long they've dominated the landscape, with previous estimates ranging from tens of millions to just 15 million years ago.

A new attempt to clarify the timing of the evolution of the valley has found that previous measures may have been off the mark.

The majority of the sculpting could have taken place as recently as 5 million years ago, according to a research team from the University of California, Berkeley.

"This upland surface that people are familiar with from parts of the Tioga Road and TuolumneMeadows is a very old landscape," says Kurt Cuffey.

There is a question about the deep canyon. Is it young or old? Our big contribution is that it's young. The best guess is for the last 3 to 4 million years, but perhaps as far back as 10 million years for the beginning of the rapid incision.

The Sierra Nevada is a mountain range created by clashing of continental plates 100 million years ago.

Not to mention rock climbers willing to brave life and limb to scale them are some of the people who visit the monoliths in the valley.

The rocks themselves are easy to analyze for signs of aging, but the sculpting of their flanks is something else.

It is known that the Sierra was a high mountain range 100 million years ago. It might have been a chain of volcanos that looked like the mountains of South America.

The question is whether the elevation has been coming down through erosion since that time or if it came down some and then was raised again.

There are lots of indications that both mechanisms are working, steepening slopes and exposing parts of the range to increased weathering and erosion.

Similar phenomena could have been present in the area tens of millions of years ago.

The technique used by the researchers is based on evidence baked into the minerals that make up the rock formations.

The method involves an assessment of the helium-4 isotope, which decays into the helium 3 over time.

The rate of cooling of a distribution of rock samples can be approximated at temperatures above 30 C.

Researchers can use the results of these tests to figure out how long a large rock has been above the warmer parts of the crust.

The researchers used mineral samples from 16 sites in and around the valley to map the history of cooling.

Imagine if a river scratched out a path for what would become a valley over the course of tens of millions of years.

The range was tilted about 5 million years ago by activity beneath the crust.

With the cooling of the climate in the past 2 to 3 million years, glaciers took their icy chisels to the valley floor and its rock walls.

Millions of people visit the park each year. With a single glance, they can see formations as old as the dinosaurs and as recent as the emergence of humankind.

The research was published in a journal.