Hi all, I'm going to respectfully disagree with Walnut I.D. and go for Ash.
If you zoom in (as I just have) on the third pic you can see the distinctive black buds. In the same pic the younger branches have the distinctive smooth greenish grey colour - also indicative of a young Ash branch. The lack of "keys" is not an issue as although some ash have flowers with both male and female parts, others have only male or female flowers. To confuse matters further, some ash have male flowers on particular branches but female flowers on others, and then come the next year a branch that had formerly produced female flowers can then produce males! It is the female parts that produce the "keys" or seeds.
I'm also inclined to take into account the comment that there are lots of these trees, which although by no means full proof evidence that it's not walnut - as there are areas where walnuts grow in abundance - there are far more areas where ash is abundant.