Attached is couple of simple heightmaps I have created. These can be used as bottom layers and set the above texture layers to multiply. Most likely you will get the best results if you take pieces of these bumpmaps and combine them to fitt your purpose (instead of using attached heightmaps as is).
As an example here is a link to a set of maps I have created using similar heightmaps.
These look wonderful! Thanks! Can't wait to try them out.