Contorted tree - what is it?

I've been fascinated with this tree in the neighborhood for a while now. I thought I'd better photograph it before it fills in with greenery and we can't see the interesting trunk. I have no idea what it is - do you?

In this photo you can also see the clear line between blue sky and encroaching clouds. I was struck by that whilst on my scoot and almost photographed/blipped that == so here we get a two-fer!

I hope you're enjoying your April Fool's Day. Have you fallen for any pranks yet?