A big rock that has been lumbering across Southern California for the past week and a half is scheduled to arrive at the Los Angeles County Museum of Art before dawn Saturday, cheered on by what has become an audience of tens of thousands. There it will become the centerpiece of acclaimed earth artist Michael Heizer\'s latest creation, \"Levitated Mass.\" The 340-ton boulder, accompanied by an entourage of about 100 people, left a dusty quarry in Riverside on Feb. 28, chauffeured toward its destination by a specially built carrier as long as a football field . As it made a long, circuitous journey toward the museum that was aimed at avoiding narrow streets, low-slung bridges and pesky utility lines, it gained a following of rock star proportions. At one stop a man proposed to his girlfriend in front of the rock. Later, when it arrived in Long Beach, that city threw a block party that attracted thousands of revelers. There were a couple of small bumps along the way, however. Because of its size, the rock could only be moved late at night and in the early morning, stopping each day at prearranged locations. Two days into its journey it had to pull up two miles short of its destination when a transmission in the engine of the vehicle pulling it became balky. It was parked partially in the roadway of a highway just down the street from a freeway entrance in Diamond Bar, giving passing motorists an exceptionally good view of it. It got back on schedule the following day, but as it navigated its way through South Los Angeles earlier this week, movers discovered two unaccounted for palm trees blocking its path. They cut them down and proceeded on, promising the community they would eventually return to replace them. At the museum, the rock is to be placed over a 465-foot-long trench, where Heizer has promised that visitors who walk underneath will experience the illusion that it is floating above them. The artist, noted for mammoth-scale works in which the earth itself becomes his palette, is perhaps best known for \"City,\" a Mount Rushmore-sized creation he has been building near his home in the Nevada desert for decades. He has kept most people from seeing it, but photos that have surfaced show a number of huge, pyramid-like buildings, some as high as 80 feet, stretching across more than a mile of desert terrain. He came up with the idea for \"Levitated Mass\" more than 40 years ago, then spent decades searching for the right rock to pull it together. He finally found one six years ago in the two-story high, 340-ton megalith he located in Riverside, 60 miles from the museum. Because of the rock\'s size it took museum officials months just to work out an acceptable route to take it on. They finally settled on a roundabout journey that carried it through 22 Southern California cities. The project, anticipated to cost as much as $10 million, is being funded by well-heeled museum donors. With the rock finally in place, museum officials hope it will be ready to be unveiled sometime this summer.