Geology is fascinating to me and we touch on it a bit in our industry (landscape construction). We actually worked on a geologists house once and it was a cool experience. He explained that some of the mossy boulders weren't actually true granite but mostly mica.

He also informed us we shouldn't be putting barrier fabric at the bottom of our weeper pit because it will create a biofilm and slow the water from seeping into the surrounding earth (we put fabric on the walls of the pit to keep the soil from intruding into the drainage stone)