Any tips on how to make an "indent"?

I’m trying to model part of a cobblestone bridge, like the one in the picture. I can just use a displace modifier for the smaller cobblestone, but I’m wondering if there’s any trick where I can model the indents in the bigger flagstones on the bridge, rather than just raw modeling, which would take a while on my model.

EDIT: Here’s a screenshot of my model so far. I want to have indents on all of the inner edges on the upper circular loop, like in the picture.

If i understand well what you need, here’s an example with a very simple and fast method but that should work nice

Have your model having this kind of topology (looks like your is good for that) at the rounded part
http://i.imgur.com/Tr2WP6s.jpg

In Face selection mode, select those faces
http://i.imgur.com/BBMVCns.jpg

Press ALT+E and select “Individual Faces”
http://i.imgur.com/zIOAOP5.jpg

Then extrude a bit
http://i.imgur.com/sU2Vw9W.jpg

Still in Face selection mode, change the Pivot to Individual Origin
http://i.imgur.com/PGOG64k.jpg

Press S and scale only a bit
http://i.imgur.com/DKssi74.jpg
http://i.imgur.com/FGhEj7q.jpg

That should work. Thanks for the help!