The Lighthouse Fire in Emery County in July started when a boulder fell 800 feet from a cliff face and collided with a larger boulder.

RANGE CREEK, Emery County — A wildfire that scorched more than 800 acres and threatened a number of prehistoric archaeological sites in July was caused when two massive boulders collided, according to state fire investigators.

The Lighthouse Fire began July 18 when a boulder fell 800 feet from a cliff face, bounced several times, collided with a larger boulder and came to "an instantaneous stop," said Jason Curry, spokesman for the Utah Division of Forestry, Fire and State Lands.

"It's kind of a rare one," Curry said Monday, describing the conclusion investigators reached after looking at physical evidence found at the fire's point of origin and interviewing people who were in the area when the blaze began.

The falling boulder was the size of a refrigerator, Curry said. The boulder it collided with was the size of a sport-utility vehicle.

The heat generated by the "pressure friction" that occurred when the two boulders came together ignited the surrounding grass and brush, according to Curry.

"It's like hitting an anvil with a hammer," he said. "It doesn't cause a spark, but it still generates heat."

There were no campfires burning in the area when the fire started, and lightning was ruled out as a factor as well. "That left rock fall as the only cause," he said.

Nearly 400 prehistoric Native American sites have been discovered in Range Creek, including granaries, rock art and cliff dwellings.

University of Utah archaeology students and staff were excavating a Fremont Indian site with a buried residential structure in the canyon this summer. They had just wrapped up a 10-day excavation session on July 18 and were leaving Range Creek when the fire was spotted.

The fire, which was contained July 22, exposed a number of previously undiscovered sites in the canyon, according to Shannon Boomgarden, assistant director of the Range Creek Field School.

"We got right back in there and started surveying, and we were able to record 12 new sites," she said, noting that most of the new discoveries were historic in nature, rather than prehistoric.

"We found a road that we didn't know was there because it had been covered by sage brush," Boomgarden said.

Neither the fire nor the efforts to extinguish it damaged any of the known archaeological sites in the canyon, she said.

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