Just before 6 p.m. Thursday three growling tractor trailer big rigs hauling wide-load bulldozers on flatbed trailers headed east up Highway 120 through the oldest part of downtown Groveland.

The sight may have been a relief to some and unnerving to others. But at least the heavy machines were moving through town and into position to try to keep the biggest, nastiest fire in California out of the overgrown, beetle-killed Stanislaus National Forest, the Highway 120 corridor and the rest of Tuolumne County.

Evacuation advisories remained in effect Thursday for all residents south of Highway 120 to the Tuolumne County line, said personnel with the Tuolumne County Sheriff’s Office. An evacuation warning was in effect for the areas of Jackass Creek Road, Jackass Ridge, Cuneo Road and Priest Coulterville Road. The fire had not crossed into Tuolumne County as of 6:30 p.m.

By 6 p.m. the Detwiler Fire had scorched more than 70,500 acres in Mariposa County since Sunday, said Cal Fire spokesman Steve Kauffman, and it was burning hot and active close to Tuolumne County. Madera-Mariposa-Merced Unit personnel with Cal Fire estimated the blaze was 10 percent contained. The fire had destroyed 99 structures, including 50 homes, and damaged 16. At least 1,500 other structures were threatened. CalFire pushed back the date it expected containment to Aug. 5.

An army of 3,737 personnel were assigned to the fire, including 68 fire crews, 462 engine crews, at least one jetliner converted to drop retardant, 13 helicopters and 56 water tank trucks.

Further east and south on Smith Station Road, Mariposa County sheriff’s volunteers were out in force in Greeley Hill, where dark smoke from the hottest, northeast edges of the Detwiler Fire spilled off ridges within view.

Dogtown and the southwest part of Greeley Hill were under mandatory evacuation orders, said sheriff’s volunteer David Reeder, a resident of the Lake Don Pedro area in Mariposa County.

“We’re here to help out the people of Greeley Hill and the rest of Mariposa County,” Reeder said. “We love this area and we love our people.”

In Coulterville, about four miles south of the Tuolumne County line, people sat outside in deck chairs to watch ridge-climbing flames hurl up black and white smoke columns while pilots in helicopters, spotter planes and a converted jetliner dumped water and red clouds of retardant on the hottest parts of the blaze.

“We’re OK, me and my husband, we’re just here for the firefighters,” said Jennifer Zimmerman, holding her 17-month-old daughter, Emmalynn, while she served free cold drinks and cookies from her business, Cakewalk Heirloom Baking Company on Main Street in Coulterville.

Firefighters from Mariposa County asserted nothing in Coulterville or Greeley Hill had burned, and complained of wild rumors circulating on social media.

South of Coulterville, three more bulldozer drivers put their tracks on pavement and clanked their way toward the nearest fire front uphill from Highway 49.

An hour earlier, Ken Pimlott, director of the California Department of Forestry and Fire Protection, and Dave Teter, Cal Fire deputy director for fire protection, parked their command vehicles on an overlook near Fremont’s Fort to watch the hottest part of the Detwiler Fire resurging to life in the afternoon heat.

Asked for the cause of the Detwiler Fire, Pimlott said he did not know, and someone with Cal Fire was investigating.

“Our focus right now is putting this fire out,” Pimlott said. “It’s the largest in the state with the greatest threat to lives and property. We’re concerned for the communities here, the infrastructure, the local economies.”

The fire was burning hottest out to the northeast across the Merced River watershed toward Coulterville and Greeley Hill, Pimlott said, with the sky to the northeast already painted purple and gray by heavy smoke and distorted afternoon sunlight coming from clear skies to the west.

“It’s moving slowly toward the Stanislaus National Forest, where tree mortality is a concern, toward communities on Highway 120, including Groveland,” Pimlott said.

Weather conditions favored firefighting efforts most of Thursday, compared to the unremitting heat Sunday, Monday, Tuesday and Wednesday. By Thursday morning the Detwiler Fire was already the size of the deadly 2015 Butte Fire in Calaveras County, 110 square miles and growing. That’s more two times larger than the consolidated city and county of San Francisco.

For Mother Loders who care about the Highway 49 corridor between Moccasin and Mariposa, it means densely-carpeted, green chaparral slopes are now incinerated white, gray and black, from ridgetops to canyon bottoms thousands of feet below. The next heavy rains are going to wreak havoc and chaos in the Detwiler burn, with untold tons of ash, loosened rock, burned topsoil, tree trunks, limbs and boulders ready to roar downhill.

Dead rabbits, lonesome hawks and scavenging turkey vultures were among the few signs of wildlife along Highway 49 between Coulterville and Bear Valley, and down in Hunters Valley near the point of origin.

Horses and cows left behind in Hunters Valley looked content in unburned corrals and pastures. Their only company were a few utility and tree removal workers, and Battalion Chief Randy Titus of Altaville-Melones Fire District in Calaveras County, who said he was on tactical patrol.

Devil winds kicked up spirals of smoke and dust in places, between properties where burned structures lay prone in ash. Deputies from Kings County were also on patrol in Hunters Valley. At Bear Valley Road and Hunters Valley Road, firefighters from Tuolumne Fire District Engine 733 said they were part of a strike team that included crews from Columbia Fire and Sonora Fire.

The fire was all but out where it started five days before. Back up at Fremont’s Fort on Highway 49, Teter said Thursday offered “more opportunities to increase containment of the fire, thanks to slightly cooler temperatures and slight decreased fire activity.”

The Detwiler Fire is Cal Fire’s number one priority in the state, even though 15 other large fires are burning in California and there have been an average of 50 fire starts a day so far this week, Teter said.

Before noon Thursday, Raul Silvestre, 44, of Stockton, sat with his parents’ two German Shepherds, Blackie and Millie, in shade under an awning attached to a motor home parked in gravel on the roadside where Highway 120 meets Highway 49 at Moccasin.

“My dad and stepmom live up Greeley Hill,” Silvestre said. “We brung all the vehicles down for now, to save time. They’re here on voluntary evacuation.”

Moments later, Raul Silvestre Sr. and Lydia Silvestre drove up in a sedan and embraced their temporary roadside home with upbeat attitudes.

“We’re here,” Silvestre Sr. said. “We’re OK.”

Before 1 p.m. in Coulterville, tourists hoping to reach their reserved resort in Mariposa County, approached a California Highway Patrol officer at the Highway 49 roadblock.

“We’re trying to get to Yosemite,” said Conor Coulter, 27, of County Armagh, Northern Ireland. “We’ve come from San Francisco.”

“We’re booked to stay at Yosemite Bug,” said Patrice Hardy, 27, of County Antrim, Northern Ireland. She was referring to Yosemite Bug Rustic Mountain Resort on Highway 140 in Midpines outside Mariposa.

The people who run Hetch Hetchy Water & Power and their company town of Moccasin were monitoring the Detwiler Fire, Charles Sheehan of the San Francisco Public Utilities Commission said Thursday.

Hetch Hetchy priorities listed in order by Sheehan are the safety of employees and their families, maintaining water deliveries and coordinating with the commission’s Bay Area water resources team, and protection of facilities in Moccasin.

Sheehan said there have been no impacts to water quality so far.

“We are still delivering high quality water from the Hetch Hetchy Reservoir,” Sheehan said. “We are not concerned about debris and ash at Moccasin because we have bypass facilities that we can activate, if necessary. These bypass facilities protect the water from any adverse conditions in the environment.”

As of 11 a.m. Thursday, Moccasin was under an evacuation advisory, Sheehan said.

“We are currently housing two families that lived in the evacuated areas,” he said. “Approximately 100 people live at Moccasin. On a normal workday, there is approximately another 250 working at Moccasin.”

As part of mutual aid agreements, two San Francisco Fire Department strike teams have been deployed and they were fighting the fire in Mariposa County, Sheehan said.

Hetch Hetchy Water & Power is billed as the third-largest municipal utility in California, serving 2.7 million residential, commercial, and industrial customers in the Bay Area and Groveland.

Contact Guy McCarthy at gmccarthy@uniondemocrat.com or (209) 588-4585. Follow him on Twitter @GuyMcCarthy.