I found this large patch of Rose Clover (Trifolium hirtum) at Russian Ridge Open Space Preserve. It was a natural subject for a photograph, especially when I saw all of the bees gathering amongst its flowers. This set contains 3 photos - the lead, which gives you a relatively decent closeup shot, plus two wider shots showing the clover in situ.

Each of the shots shows how densely packed this patch of clover was with flower heads. Each flower head, in turn, consisted of dozens of tiny, pink flowers only a few millimeters in length. If you examine the flower heads closely, you'll see that they resemble tiny pea flowers - a good clue that Rose Clover (and clovers genrally) belongs the the Pea Family, Fabaceae.

In some cases, the Clover's flower heads have gone to seed, a fact indicated by the dropping of all their flowers' pink corollas, revealing calyces that are developing into actual seed pods. Note that neither the calyx tubes nor the corolla's banner have inflated into balloon-like proportions to form the seed pods, as is the case with some species of clover. The seed pods also remain erect, rather than drooping down as they mature. Both of these facts that will be useful in helping identify this clover to species.

Below is my keying to species, a la Jepson's eFlora:

1) Involucre absent or inconspicuous. In the field, I verified that the involucre was absent for this clover, although it did have a pair of stipules subtending the inflorescence that resembled a vestigial involucre.)
2) Three leaflets to each palmately compound leaf. (Confirmed in the field. This can also be seen in Shot 3.)
3) Inflorescence comprised entirely of unmodified, fertile flowers with actual petals.
4) Calyx not inflated in fruit, so inflorescence does not become a woolly, spheric or star-shaped ball. (Most obvious in Shot 2.)
5) Stipules generally not green. Verified in the field; you can also see this if you look at the stipule on the stem of the flower head that the bee is on in the lead shot.)
6) Leaflets less than 4 cm long. (Leaflets measured in the field were 1.6 cm long or shorter.)
7) Calyx tube is hairy. (Verified this in the field. This trait can also be seen somewhat where the clover flowers have become pollinated, causing their corollas to drop.)
8) Inflorescences consist of >> 5 flowers.
9) Flower pedicels < 2mm, flowers not reflexed. (In the field, there hardly seems to be a pedicel on individual flowers at all. Also, in Shot 2, even flowers that have been pollinated and dropped their corolla tubes remain erect. They might become more reflexed as their seeds mature; however, they weren't far enough along in fruit development to tell.)
9) Inflorescence immediately above a reduced leaf. (Verified in this in the field. You can also see this with some of the flower heads in this shot.)
10) Banner not inflated in fruit. (More obvious in those flower heads that are going to seed.)
11) Flowers not becoming reflexed after opening. (Obvious in all of these shots.)
12) Inflorescence single above subtending leaves. (Verified this in the field.)
13) Corolla uniformly pink in color. (Obvious in all of these shots.)
14) Calyx lobes densely plumose. (Verified this in the field.)
15) Inflorescence spheric, 1.5 to 2 cm in diameter. (Measured inflorescence to be 1.5 cm in the field.)
16) Calyx lobes >> corolla tube, densely plumose. (Verified this in the field. You can also see this in this shot, whre the bristly tips of the calyx tube are project out past the lower lip of the flowers - i.e., well beyond the tube of the corolla.)

Photographed near the Alpine Road trailhead of the Ridge Trail at Russian Ridge Open Space Preserve in San Mateo County, CA.