This is more of a Photoshop question than it is a Unreal one. You want to ensure that the left part of the texture connects to the right. This way you can place multiple walls together - instead of one giant one which could consume more texture memory.
make 2 landscapes and rotate one 90 degrees for being the wall. you can add static meshes for the snow/rocks at the seam and also use the landscape tools to create holes in them for the tunnel.