California Blackberry
I wasn't going to post this at first because I thought it was a Himalayan Blackberry which I already have plenty of shots of. But then I noticed that the buds on these have red thorns. I think the flowers are a bit smaller too. The stamens on some of the other of these flowers are a bit darker, like the HImalayans.
This, this, and this all seem to be related and at first I thought they were all poison oak. =)
(Jepson)