redwood sorrel
Oxalis oregana
Oxalidaceae

As the name suggests, I often see massive carpets of shamrock-shaped redwood sorrel under redwoods. The pic above shows leaf litter from coast redwood (Sequoia sempervirens) and tanoak (Lithocarpus densiflorus) which is a beech, not a true oak. Most of the redwood sorrel flowers I've seen in the area are white like shown here, not deep pink like is commonly pictured online.