The earthy embankment is, in this case, sawdust mixed with PVA. (On the extreme outside of chance, 'elmers'). This makes for a comparatively heavy terrain peice. The blue bits are aquarium gravel that I bought on a whim 'cause someone said it was good for scenery.
Now; the advantages of aquarium gravel are that it is only gravel; unlike real gravel which includes all manner of natural and man made detritus. And it is regularly sized as well. So all good so far. The disadvantage is that the blue colour leeches in PVA and poster paint (which would seem to indicate that its water soluble colouring on aquarium gravel....) This is not really a problem, as I just paint over it again. But it makes me worry for the fish.