Cornus flowers to be sure. You can see the cross-like structure of the inflorescence in the picture - I'm too lazy to look up the botanical name for that right now. ;)

I just took some pix of my C. racemosa below where you can see that. While the flowers are right, I don't think the foliage is. C. racemosa has a smaller, more narrow leaf. I'd suggest the OP's is a C. amomum. The flowers are much the same but the leaves are larger and broader and I think I see a purplish older twig in the first of the cornus pix above. See: http://dendro.cnre.vt.edu/dendrology/syllabus/factsheet.cfm?ID=28

FYI, you never find one grey dogwood that is 15' tall, as the OP sees. One, that is too tall. But more importantly, this is a clonal plant that makes dense, low hedges. Excellent roosts for birds as the hedge it makes is too thick for predators to navigate easily. But a royal pain in the butt if it gets a foothold where you don't want it.

Cornus racemosa

Narrow young leaves

Flower head structure