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.
Like the red dead nettle, 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.
Picture: Suzanne Humphries