In 1972, the fate of the San Geronimo Valley was on the table with a countywide plan that would have transformed West Marin. The proposal sought to accommodate 20,000 new residents, malls and a heliport, and all but transform Sir Francis Drake Boulevard into a four-lane freeway. With two supervisors for the plan and two against, newly elected District Four Supervisor Gary Giacomini, a progressive backed by the ad hoc San Geronimo Valley Planning Group, tipped the balance. Five years later, supervisors approved the planning group’s own community plan, which set the standard for protecting the natural resources, unique culture and community values of Woodacre, San Geronimo, Forest Knolls and Lagunitas. On Saturday, Oct. 8, the planning group is celebrating its 50th year as the oldest environmentally focused nonprofit in West Marin. The event will also mark the grand opening of a community library at the Wilderness Way center. Jean and Paul Berensmeier founded the nonprofit Wilderness Way in 1999 and will be among the residents, authors and poets who will share stories and art inspired by the valley at Saturday’s event. Members of the Coast Miwok Tribal Council of Marin will perform opening and closing ceremonies and discuss the Indigenous history of the Nicasio and San Geronimo Valleys. Councilman Joe Sanchez describes how over 80,000 acres were returned to the Coast Miwok, then taken again just before the Civil War. “Though there is a dark past, our tribe has been an integral part of this land for thousands of years,” he said. Nancy Binzen, a member of the planning group’s steering committee, said residents can look forward to “meeting Coast Miwok people, and getting a sense of the passion, creativity and the fight this group has endured—it’s all to keep the valley the special place that it is.” Ms. Berensmeier co-founded the planning group in 1972 and served on the county Parks and Open Space Commission for two decades, where she helped acquire lands that became the valley’s many open space preserves. In 1981, the now dissolved Hendricks & Horne Development Co. acquired 1,600 acres on the southern side of the valley with the goal of creating a gated community called Skye Ranch. The plans would have undermined the community plan by building homes along land previously labeled as undevelopable, something Ms. Berensmeier never anticipated. When Hendricks & Horne went belly up and development fell through, the planning group and open space commission helped facilitate the purchase of the land in 1991, creating what would become the Gary Giacomini Preserve. Ms. Berensmeier said she is looking forward to the opportunity to bring the community together for a celebration of its history and resilience. The San Geronimo Valley Planning Group’s 50th Anniversary celebration takes place from 3 to 5 p.m. in the Wilderness Way building, across from the gym on Lagunitas School Road.