pope77, I dont have any experience with impression mats and hopefully someone will answer you because i was planning to draw lines forming diamonds on a cake soon. But I did make a cake tht was a fish pond. I got the idea from a user on here who called her a wishing well. She said she used rolled butter cream to make her rocks for the side. THat seemed to difficult so I iced the cake with buttercream and used chocolate covered nuts, cremes and fruits. They come in assorted packages for rocks. Looked pretty similar but so much easier than shaping a bunch of different sized and colored rocks. They already came in different sizes and had slight color variations. I thought rocks were similar to bricks. Maybe you could use an alternative, that is unless you're set on fondant in which case I hope someone answer for both of our sakes.