What kind of snake is this?

Judging by the dark head, size and location, I think it's a juvenile/young adult black rat snake - Pantherophis obsoleta obsoleta. Young black rat snakes and yellow rat snakes - Pantherophis obsoleta quadrivitatta are often mistaken as gray rat snakes - Pantherophis obsoleta spiloides. Most of the sites I have seen state that grays are only found in Southern Georgia. All the larger juveniles and adult gray rat snakes I have found in Northern Florida, around Tallahassee, had much lighter heads.

That is, I you want to confirm the exact subspecies of Pantherophis obsoleta.

http://srelherp.uga.edu/snakes/elaobs.htm

It's not venomous, you can tell because the head is an oval shape rather than a triangle. I think it may be some kind of rat snake.

Yes, it is a harmless rat snake- a gray rat or gray rat/black rat intergrade.

However, head shape is not a good indicator of whether or not a snake is venomous.

Its either a rat snake or a kingsnake, both are harmless. You should look at pictures online to be able to tell the difference. I wish I had it.