retaining wall

I want to have different levels of substrate in my tank and it makes sense that after time the substrate would flatten without something holding it back. At first, I thought I would pour a concrete wall and sink some lava rocks into it but since I cant come up with a suitable mold for pouring, I am having seconds thoughts about it.

So, does anyone have any ideas on how I can go about building up a retaining wall that won't come crashing down over time?

TIA