In a new symbol of California’s worsening drought, construction crews are putting the finishing touches on a $10 million emergency project to build a massive rock barrier through part of the Delta in Contra Costa County to preserve water supplies for millions of people across the state.

The 800-foot long barrier — the size of San Francisco’s Transamerica Pyramid laid on its side — is essentially a rock wall, 120 feet wide, built in water 35 feet deep.

Its purpose: To block salt water from the Pacific Ocean and San Francisco Bay from flowing too far east and contaminating the huge state and federal pumps near Tracy that send fresh water south to 27 million people — from San Jose to Los Angeles — and to millions of acres of farmland in the Central Valley and beyond.