Crews have made significant progress clearing a major rockslide that closed one of the primary routes into Yosemite National Park last week, reducing fears of a long-term disruption just as warmer weather is starting to increase the number of visitors to the park.

Hundreds of tons of rocks came crashing down last Monday night across both lanes of Highway 140, a scenic winding route along the Merced River between the town of Mariposa and Yosemite’s Arch Rock entrance.

Now at least one lane of the highway — which is one of the two main routes into Yosemite for most Northern Californians — is expected to be open by the end of this week, said Brian Hooker, a Caltrans spokesman.

“Crews are working 24-7,” he said. “They are making good progress.”

The slide closed about two miles of Highway 140 in both directions between Yosemite Cedar Lodge and Briceburg, 10 miles west of El Portal.

On Thursday, Caltrans geologists investigated the area and flew drones to take 3-D images of the steep cliffs above the road. They were trying to find loose boulders and other material that might still collapse onto the roadway.

Friday and Saturday, using that information, crews from a specialty contractor, Neil’s Controlled Blasting, based in Placer County, used an aerial lift to move workers up to 130 feet high on the sheer cliffs. Some workers rappelled on the cliff with ropes.

They fitted large industrial pillows, like air bags on a car, behind the suspended loose rocks, and inflated them with compressors, Hooker said, causing the unstable geology to fall down on the roadway below.

“For safety reasons, they had to remove those materials before we could get the main contractor working on the debris on the road,” he said.

On Sunday, that contractor, Teichert Construction, based in Sacramento, began using trucks, front loaders and other equipment to start hauling away the main pile blocking both lanes. That work should take several days, Hooker said.

“There is damage to the roadway,” he added. “There are going to be repairs needed before we can put the traveling public back there. But things are moving along at a good pace.”

Until the road is open, Bay Area residents have one main route into the park — Highway 120 through Groveland. A longer way from the south is also possible, on Highway 41 through Oakhurst.

For daily updates, visitors should check Caltrans site before heading to the park: Quickmap.dot.ca.gov.

Hooker said engineers on the job believe that the cold weather in recent weeks, which dropped snow levels lower than normal, froze water that had accumulated in cracks on the cliffs. That ice expanded, causing the rockslide, he said.

Local tourism leaders and residents of Mariposa County were happy Monday to hear that the big slide won’t close the road for weeks or months, as some in the past have.

“It’s fantastic news,” said Jonathan Farrington, executive director of the Yosemite Mariposa County Tourism Bureau. “Caltrans is very professional working through situations like this and understanding the importance of all the major routes into Yosemite National Park.”

Highway 140 is one of the most colorful highways in California.

In the first few decades after President Abraham Lincoln set aside Yosemite Valley for preservation in 1864, visitors who wanted to see the park’s waterfalls, granite cliffs and other breathtaking wonders took treacherous two-day horse wagon rides, then trips in early automobiles, across steep, unpaved mountain grades on the Big Oak Flat and Wawona roads to Yosemite Valley and other sites. The routes were impassable in snow.

Then in 1907, the journey became much more comfortable when the Yosemite Valley Railroad opened. Investors built tracks along the Merced River, across the river from where Highway 140 is today. Visitors could enjoy a three-and-a-half-hour journey by rail up the Merced River Canyon from Merced to El Portal, just outside what is now Yosemite’s Arch Rock Entrance.

Everything changed in 1926 when Highway 140 was built along the other side of the Merced River. The road, nicknamed “The All Weather Highway,” allowed motorists to easily drive to Yosemite at all times of the year.

The railroad’s passenger revenue fell 38% the following year. And although it still had some high times — President Franklin Roosevelt rode it to the park in 1938 — it failed to make up for the losses and by 1945 closed.

The tracks were sold for scrap. The dusty railroad bed can still be seen by motorists driving along Highway 140 today.

Other slides have come off the steep cliffs. In 2006, more than 300,000 tons of rocks slid off Ferguson Ridge, closing Highway 140 for years. Caltrans built a temporary bridge and detour across the river which is still used today. The agency has removed the enormous pile of rocks, and plans to build a rock shed — basically covering the road with a strong roof — for 675 feet starting later this year.

Farrington noted that another rock slide closed Highway 120 into Yosemite National Park last year. In fact, many of Yosemite’s spectacular rock features, including Yosemite Valley itself, were carved out by ice, more specifically glaciers, over millions of years.

“These types of events do happen,” he said. “It’s a geologically active region. Mother Nature is a busy gal.”