The hidden beads are turned into cacti by embroidering whipped spiders wheels on top of them.
This stitch can be found here.
I started with a jade colored thread, it was the best a could find. I thought I knew how to do that stitch, but after a few laps I realised there were no ribs. On the picture you can see what I did wrong. I worked from left to right, instead of from right to left.
Now we've got ribs. The thread I used is Appleton crewel wool. I suddenly realised I had another drawer full of threads. This color is much better than the jade for a cactus.
I exaggerated when I talked about mountains and hills. The cacti are much too big in proportion to the "mountains". The mountains are reduced to heaps of sand or boulders.