This is ground ivy (Glechoma hederacea), a little blue flower so common as to be almost invisible. It grows all over the park and flowers at any time of the year.
Its flowers are zygomorphic, with a single line of symmetry. Its five petals form a hood and two wings and and the lower two petals fuse to form a keel; the keel is the landing platform for foraging insects and is marked with patterns that point to the nectar.
Ground ivy is gynodecious, which is uncommon: the plants are either female with only a pistil, or hermaphrodite with stamens and pistil. All of these pictures are of hermaphrodite flowers with four stamens and a forked pistil above them.
Each flower can make four seeds but the plant can clone itself easily; the stems bend down to the ground and root where they touch. There are circumstances, not clearly understood yet, where ground ivy finds cloning the easier option; big patches of ground ivy in an area may be a single genetic unit.
Pictures: Suzanne Humphries