stonewalls

One word--Milliput.

It's an epoxy 2-part putty.

Or, you can use Playdough, but I prefer Milliput, although it can be a bit pricey... 

Plaststruct makes styrene sheets of various scales in field-stone, cobblestone, brick and other patterns. Since you don't get a whole lot for your money, I make latex molds from the sheets and cast replicas out of plaster or water putty. This way you can make large amounts rather cheaply and as often as you want.

thanks for the info good idea.

Try the foam blocks that florists use for flower arranging,You can cut them easilly into strips and scribe stones or bricks really easilly as its quite soft.

Here's a thought.... Use stones.   I gathered these from an empty lot a couple blocks from my house.