I didn't do it!
The concrete block is mine, I use it to hold down the tarp on my fire wood, but how it fell on the snake I do not know. I have not seen any other (living) snakes around here. The mentioning of snakes is also rather rude, as Native Americans see them as harbingers of bad luck.
As far as I can deduce as an amateur, this is a Striped Whipsnake and not poisonous at all for humans, pity. They are supposed to be very fast, but apparently not fast enough for falling rocks. Dunno what I’ll do with it yet…