Castle Rock State Park, a scenic expanse of forests, rocky outcroppings and breathtaking views to the Pacific Ocean that lies between Silicon Valley and the redwood forests of the Santa Cruz Mountains, is getting a 21st century upgrade.

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In one of the largest improvement projects to any Bay Area state park in recent years, work began this week on a grand new entrance to Castle Rock, a 5,200-acre landscape located on Skyline Boulevard at the border of Santa Clara and Santa Cruz counties. The park has been popular for nearly 50 years with hikers and rock climbers.

“This is an amazing park,” said Sara Barth, executive director of the Sempervirens Fund, a Los Altos environmental group spearheading the project. “Steve Jobs used to come here to clear his head during his early days at Apple.”

But the park has suffered from severe funding shortages, a lack of rangers, a cramped gravel parking lot and pit toilets at its current entrance.

“A lot of people enjoy it despite the minimal facilities,” Barth said. “But for others, that’s been a barrier. We are really looking to raise the visibility of the park and make it more accessible to everybody.”

Construction of the new entrance is expected to be finished by next May. The project includes a new parking lot for 90 cars with pavement that allows water to seep through and recharge groundwater, along with a modern restroom with flush toilets, new trails, a picnic area and a rustic ampitheater for up to 60 people to allow talks for schoolchildren about wildlife, local history and other topics.

The project is more than eight years in the making. In 2010, Sempervirens — named for the Latin word for redwood tree, Sequoia sempervirens — paid $1.55 million to purchase a 33-acre Christmas tree farm from the Whalen family. The property sits adjacent to the existing entrance at Castle Rock.

This week, crews with heavy machinery began cutting down the trees, many of which have grown to more than 25 feet high. They will be chopped into mulch and used on the trails.

California State Parks officials say they are excited to see the park’s renewal.

“This park is sort of the gateway for Silicon Valley and the South Bay to access the redwoods,” said Chris Spohrer, superintendent of State Parks’ Santa Cruz district.

“Castle Rock is a jewel of a park,” he said. “And the facilities we have are not commensurate with that. We’d like the public to have a new and wonderful entrance experience.”

Sempervirens Fund raised money and won approval from the Santa Cruz County Board of Supervisors in 2014. So far it has spent $6.7 million on the design, planning, permitting and construction of the project.

Barth said the group is still seeking to raise about $1.3 million to cover the rest of its costs, which were higher than originally planned because of high construction prices in the booming economy. Plans also originally called for construction of a 6,000-square-foot visitor’s center. That part is on hold, however, because it will take more money — and because State Parks says it doesn’t have the money to pay for staff at a visitor’s center.

Spohrer said State Parks remains in talks with Sempervirens. If the organization and the state agency can agree on an endowment or other funding source to cover those costs, he said, a visitor’s center still could happen.

Sempervirens is no stranger to the Santa Cruz Mountains. Without it, many of the area’s ancient redwood forests would have been cut down long ago. In 1900, San Jose photographer Andrew P. Hill, alarmed at how loggers were clear-cutting 2,000-year-old redwoods for fence posts and railroad ties, formed the Sempervirens Club.

The group convinced the state Legislature to purchase 3,800 acres of redwoods near Boulder Creek in 1902 for permanent protection. That established Big Basin Redwoods State Park and began California’s state parks system.

Since changing its name to Sempervirens Fund in the 1960s, the organization has preserved more than 34,000 acres in the Santa Cruz Mountains, including most of the land at Castle Rock, Portola, Butano and Big Basin state parks.

When State Parks has run short of money, Sempervirens has often stepped in. When Gov. Jerry Brown proposed closing 70 state parks in 2011 to balance the state budget, Sempervirens put up the money to keep Castle Rock open. This past spring it donated $15,000 so State Parks could clear downed trees and re-open the Skyline-to-the-Sea Trail, a 31-mile-long hiking trail that Sempervirens volunteers built from 1969 to 1976 with the Boy Scouts, Girl Scouts, Sierra Club and other organizations.

Castle Rock itself was established in 1968 when Dorothy Varian, widow of Silicon Valley pioneer Russell Varian, donated money to buy the first 566 acres. The park, which has 34 miles of hiking trails, tent campsites and unique rock formations that climbers use to train for bigger challenges like Yosemite National Park, celebrates its 50th birthday next summer.

There are still some issues surrounding the new project.

“I have a concern about the increase in the number of cars and the impact of the number of people in the area,” said Miles Standish, a former State Parks ranger who worked at Castle Rock. “Right now if you look at the trail system, it’s pretty heavily impacted.”

“It’s not a problem if the visitation goes up,” said Standish, a direct descendant of the famous Pilgrim. “It’s a problem if you have too many people in one spot.”

Standish said he’d like to see the park close the existing gravel lot after the new one opens, and for Caltrans and other agencies to better limit parking along Skyline Boulevard, which is also known as Highway 35. On busy weekends, dozens of people park there when the lot is full or to avoid paying the $8 entrance fee.

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“People now park on the highway on both sides,” said Santa Cruz County Supervisor Bruce McPherson. “You see mothers pushing their children in strollers down the road on the shoulder. This project will get people off the highway, and it will be much safer.”