A puddle of rich gold fills a dip in a meadow; a blanket of gold covers a hillside. Hundreds upon hundreds of little yellow daisies crowd out all other color. This is California goldfields (Lasthenia californica), an aptly named and spectacular spring bloom.
According the Jepson database, the plants can grow over a foot tall, but usually they are much smaller–generally just a few inches high. They look like minute sunflowers, with spreading rays around a mounded central disc of tiny flowers. Indigenous tribes would make a flour of the diminutive seeds, or eat them dried.
There are several other species of goldfields–which tend to look quite similar, and frankly are tough for beginners to key out. In Marin, one tip for a quick & easy way to narrow down what species you might be looking at is to check and see if CNPS has a plant list for the area (Rock Springs, in this case).