What are they digging on Bishop Peak? SLO quarry supplies rocks for Big Sur road repair

Highway 1 along the Big Sur Coast is getting a facelift using rocks from the foot of Bishop Peak in San Luis Obispo.

Each day over the past several months, as many as 30 large dump trucks carrying thousands of pounds of stone have made the trek from the outskirts of the city up Highway 1, supplying a construction effort to keep Highway 1 stable.

The rocks are being used to bolster about a half-mile of the highway in the area called Big Sur Lookout, between Ragged Point and Gorda.

They come from a Madonna Ranch quarry that’s so old, it predates whole swaths of the city of San Luis Obispo.

Big Sur Lookout road repair

This map shows the location of the road repair site on Highway 1, where John Madonna Construction is repairing the road base using rocks mined from a quarry at the base of Bishop Peak in San Luis Obispo.

Map created with the assistance of ChatGPT.

Quarry has a long history in SLO

The quarry sits on Madonna Ranch land on the northern side of Foothill Boulevard just west of the city limits. It’s visible on the flanks of Bishop Peak, where piles of gray rock have been unearthed from a hole in the grassland.

Property owner John Madonna said the land surrounding Bishop Peak has a long history of mining, with mines dotting the landscape near Froom Ranch and Los Osos Valley Road.

Madonna said that his efforts to dig up the quarry’s history (pun intended) with the help of the San Luis Obispo Railroad Museum revealed that the Madonna Ranch mining site and several others in the San Luis Obispo area date back as far as 1890, when they were connected by narrow-gauge railroads to Port San Luis.

Once there, some of the stone was used to build out Avila Beach’s breakwater, Madonna said. The operation wasn’t exactly popular around town at the time.

“One of the (Railroad Museum) guys said, ‘There was even NIMBY-ism back then, because they were dynamiting, and the people who lived down down below Bishop’s Peak, they didn’t like sound of the dynamite,’” Madonna said.

Early on, the quarry only produced materials for around 10 years before going unused for a several decades, Madonna said.

The mines around SLO primarily supplied red rock until the Surface Mine Reclamation Act of 1975 forced many to close, leaving the Madonna Ranch mine as one of the last to be grandfathered in and permitted to continue operation, Madonna said.

Though the Madonna family has maintained a mining permit there since its heyday, Madonna said it has been only sporadically used in recent years for infill projects that require loads of rock to support roadbeds, like on Highway 1.

One of those was the Mud Creek Slide that obliterated a stretch of Highway 1 in May 2017 when a massive hillside collapsed, sending so much material tumbling into the sea that it changed the shape of the coastline.

Using material sourced from the Madonna Ranch quarry along with other California sources, John Madonna Construction removed more than 6 million cubic yards of material that swept across the roadway, laying down 15 new acres of coastline for the Highway with infilled stone, Madonna said.

The repair project showed the value of having a nearby source of quality stone, Madonna said.

“By being able to have local quarries, it saves the state a considerable amount of money,” Madonna said.

Similar to his company’s previous projects, the Big Sur Lookout repairs will mix large chunks of stone from the base of Bishop Peak with material from other suppliers across the state to build up the base of Highway 1.

Madonna said the stone dug up from the family’s ranch — a Franciscan melange that shares characteristics with basalt and lava flows — is strong and can be mined in large pieces.

That’s compared to the lighter stones used in structural work at the site in the 1980s, which have been swept away by the ocean over the ensuing years, Madonna said.

He said the work is considered an emergency repair because erosion was starting to eat away at “the prism,” the infilled area on a hillside that supports the roadway — necessitating the latest structural work.

Caltrans public information officer Kevin Drabinski said much of the damage occurred in late December and early January, when wave erosion damaged the supports along Highway 1.

Drabinski said the project cost $51 million and kicked off in February, sourcing rock from Bishop’s Peak, a pair of locations in Cambria, Cayucos’ Negranti Quarry and the Central Valley’s Papich Construction and Academy Quarry.

When will mining wrap up on Foothill?

All told, a stretch of around 3,000 feet along the road is being reinforced at Big Sur Lookout, requiring a total of 260,000 tons of stone, Madonna said.

The Big Sur Lookout repairs are less aggressive than the $54 million of repairs made to Highway 1 at Mud Creek, looking to fortify the highway’s base rather than repair a major landslide.

It’s unknown how much stone will be sourced from Bishop Peak, with construction efforts at Big Sur Lookout likely continuing through November, Madonna said.

He said the quarry will continue to supply the construction site through the end of repairs.

“Everybody tells me how they see (trucks with) rocks coming down the highway from everywhere,” Madonna said. “We’ve got 30 trucks on the road every day from the valley. We contribute just a fraction of what we’ll be using up there, but it’s something.”