I realize you may want a long straight track, yet the possibility for a snaking curvy bendy track would be very photogenic IMHO.
As I showed in my thread, it's possible to use sedimentary stone. If you collect larger pieces you can pry them apart to make a thin layer of stone with a very rocky look. you just need a rock hammer, or a cold chisel. The debris can be used on the bottom of your slope as natural talus. I would be tempted to have a sloped surface at the bottom to attach talus rocks to.
Maybe create shapes with cut wood and then just use Liquid Nails to glue the stone surface onto it? Is there a specific locale you are trying to make it look like? Of course with an outdoor layout, you could also make troughs for soil that you can grow moss in for grass if it's shady in that area.
Rocks have natural cleavage based on their kind. The shale like mud rocks break up nicely to form jagged edges.
There's some really inspirational videos available online where people create rocks and rock type formations using cement based products but don't confuse any of those with what I'm about to produce on my layout!!
I'm just looking for a backdrop and at this moment in time I'm not planning on any intricate carving, but we'll just have to see where the mood takes me. I never quite know what I'm doing until I happen to be doing it.
Well here goes with my initial effort at creating a rock backdrop along 'Buttlebank' which is the straight elevated section that runs along the bottom of my garden.
I've decided to create a lightweight former out of chicken wire and then cover it initially with a layer of cotton, or some other thin material, soaked in a cement mixture. It should be a similar method to the way I created the embankment up in the attic using plaster bandage which I quite enjoyed. Once the 'cement bandage' has dried I'll then use a light cement mix just to add additional shape and definition. At least that's my thinking at the moment.
I've started off by securing a quadruple layer of of chicken wire between the timber uprights that are secured to the concrete fence posts.
And then I've begun fastening crumpled chicken wire to that to suggest the rock face itself.
I've done all I'm going to do for today but the top needs more shaping to reduce the flatness along it. I also need to create tunnels at either end where the track curves towards the shed, as in the above photo....
....and where it leads onto Stack Gill viaduct as in the photo below.
I would like the tunnels to resemble the one at Loch nan Uamh Viaduct where it's cut into the rock face. I'll have to see how I get on as with me plans can change very quickly
There'll be a low rock formation along the front of the boards, low enough to enable me to film trains passing along without the view being obscured. I had also thought about adding a passing loop here but I'm still undecided on that one.
Just bought two and tried them on 36" radius curve with a Lima 156. Plenty of room there. In fact they'd probably work with 4th radius setrack, if not 3rd. It'll do for my purposes anyway.
I'll build the tunnel over winter but having them is helpful for planning. The remainder of this year is about gardening, which I best get on with.