Rocking Samples

This week I have been experimenting with trying to create coastal rocks. The method I have used so far is by hotwiring the shape of the rock from blue foam and then with my fingernails digging out random pits in it. I then covered the foam in a layer of PVA glue to seal it and used plaster to cover it over. Once the plaster had nearly set I tried out different ways of creating surface texture. These included using a stipling motion with a hard bristle paint brush and dabbing over the plaster with my fingers. I then painted them up with different layers of paint.
I am fairly pleased with the results so far, however I need to spend more time on them and make more samples to ensure they look authentic on the model.
Need to carry out further research into the geology and types of rock of the region I'm looking at to give me a reference point.
http://www.jsce.or.jp/committee/2004sumatra/chapter_3.pdf (Indonesian Geology)
http://www.ucmp.berkeley.edu/tertiary/pli/bodjong.html (Geology of Western Java)

The type of rock making up West Java region is Quaternary Volcanic rock, similar to the types from the links below.
http://k41.pbase.com/g3/57/395157/2/67059786.j975OvXe.jpg