SARGENT RANCH — As shadows of red-tailed hawks dart across rolling hills of green, cougars creep through hidden canyons of this pristine expanse — long revered as a sanctuary for threatened species such as burrowing owls, red-legged frogs and steelhead salmon.

It’s also a place where investors are now hoping to build a 320-acre sand and gravel mine that over its 30-year lifespan could generate 40 million tons of materials to make cement for Northern California construction projects.