By Mary Slosson

SACRAMENTO, California (Reuters) - Portions of a popular visitor campsite tucked below the sheer granite walls of Yosemite National Park will close permanently because they are at risk of being hit by large falling rocks, the National Park Service said on Thursday.

Roughly 20 guest tent cabins in Curry Village, a large cabin camping area at the foot of the Glacier Point cliffs, will shut down, along with two dormitories that house about 30 private concession employees, park spokesman Scott Gediman said.

The closures come four years after two rock falls damaged the site, affecting about a dozen tent cabins in October 2008. There were no deaths or injuries but the Park Service said it caused the most infrastructure damage of any rock fall in the history of the park.

The closures were announced after the park completed a new rock fall risk study.

Rock falls pose a risk to the California park's nearly 4 million annual visitors, roughly 70 percent of whom congregate in Yosemite Valley, the report said.

Because of Yosemite's steep, tall valley walls and relatively narrow valley floor, there is no area there that is absolutely free of risk from "extremely large rock falls," according to the report.

Those same features are what make landmarks like the towering Yosemite Falls, the Half Dome and the popular rock-climbing destination El Capitan so breathtaking.

"Rock fall is a part of the ongoing evolution of Yosemite Valley," Gediman said.

Between 1857 and 2011, there were 15 fatalities and at least 85 injuries from rock falls and other shifts on the slopes, according to the report.

(Reporting and writing by Mary Slosson; Editing by Steve Gorman)