I thought I'd share the very easy method I have used to create stone walls......
I use the thin and low quality cardboard that you get as packaging for shirts produced in the far east. This card is shiny white one side and a stone gray fibrous finish on the other.
Cut a strip of width just over twice the height you want the wall to be. It doesn't matter if the width drifts...the wall will end up varying in height slightly! Unlike what I did...which shows in one of the photos below....if you mark the card for cutting, do it on the white side! Fortunately, most of the line you can see will be obstructed by undergrowth!
Now an important bit.....carefully hand crease the strip down its length and fold in half, crimping the fold just using finger and thumb. Don't be tempted to press the crease flat on a flat surface, or with a ruler, etc. You want the crease finish to be non-uniform! Apply a bit of glue inside the creased card and squeeze closed so you have what you can now see as a stone wall with an undulating top. Here's a couple of close-ups to illustrate what you should now have: On the left is the creased card; on the right it is glued down.
Once dry, the wall is ready to fix down on your layout. Using lots of pins, fix the wall into position, ensuring it sits down on the ground surface. Use pins inserted at an angle to hold it down, as you can see in the photo below.
Using a small paint brush, put a line of neat PVA glue along the bottom of both sides of the wall. Here's a close-up of where I got to on a couple of walls on The Bridge:
Leave the PVA to dry overnight and then carefully pull the pins out. Here's the end product....a stone coloured stone wall:
I've used a similar fixing technique for fixing wood panelled fencing, but use a single thickness of plain thin white card that has been pre-painted the right shade of brown.