A mat of green, heart-shaped leaves march along the side of a steep hillside, punctuated here and there with bright yellow blossoms that seem to glow in the shade of the forest. These are redwood violets (Viola sempervirens). These little flower grow from a creeping stem that sends out rooting stolons, which then grow a new rosette of leaves.
There are a couple of other species of yellow violets in the Bay Area so be sure to check a key before deciding which you’re looking at. Only one other species has heart-shaped (cordate) leaves, but it can easily be distinguished from redwood violet because the stems are upright (not creeping) and the leaves are located on the stem, right below the flower.