The dimpling......I thought that was the initial start of rough bark forming. No knowledge really....
I have several self-sown field maples I found in my garden, they are very variable. Some have deeply cut lobes, whereas others have very rounded ones, some have a rather smooth bark, others have a kind of cork bark when young.
For instance, I kept this one because when in full leaves, it hides the bins from the street. It's in a mainly sunny place, but I tink it's not the only reason why it has this kind of bark, and more deeply cut lobes :
This other one is in a live hedge, almost all the time in the shade, but I took several root cuttings, some of them I placed in full sun, and they all have the same characteristics : less divided lobes, and a much smoother bark :
I also remember a post from a British member that found one with very deeply cut lobes, maybe he'll read this post and give us the link...