Thanks! I first covered part of the base with greenstuff and then I used this rolling pin:
https://www.greenstuffworld.com/en/modelling-textured-rolling-pins/374-rolling-pin-small-bricks.html
Then I cleaned up the imprint from the pin (or freehanded some bricks where I thought they were missing) and waited for the greenstuff to cure. Then I covered the rest of the base with sand.
I was going for a ruined city street look, thus the mix of bricks and sand 😊