All you have to do is gather it yourself. It's amusing to think how much hassle I've gone through in the past to make things look like rocks. A quick trip to the Free Model Railroad Scenery Material Store and I have nearly enough of this Inorganic All Natural Rock Product to put on the layout. To think I used to assemble and blend plaster when now I can assemble and blend these fine scenic "rocks" which look just like "rocks".
Coincidenally, I just got finished re-doing this canyon scene on my layout that looks very similar to your prototype pic. I used consecutive washes of craft acrylics thinned with 70% isopropyl alcohol on a couple of Hydrocal rock mold castings. I used washes of Tan, Raw Sienna, Burnt Sienna, Raw Umber and Burnt Umber, finishing with a spray of India Ink/alcohol wash to bring out the detail:
I too collect natural rocks, dirts, sands, and decomposed granite for use on my modeling projects too. Where I live, we also have sagebrush, which roots make very convincing oak tree trunks.
Nice one! I did the same many years ago, and basically have a lifetime supply of location-accurate material! Two key notes: bake it on a disposable cookie sheet in the oven for 250° for an hour or so, to kill bugs, bacteria, etc. Also, run a strong magnet through it to grab iron and steel particles. It's amazing how much you'll find, and this stuff has a tendency to migrate into model locomotive motors (via their own magnets) and can wreak havoc inside them.
Wondered when you'd find this discussion. I assume you can tell exactly where I am gathering my pressure treated silt rock simulators from. I have a picture I will not post here that I can show you if you have any doubt. It was taken inside Tunnel #2. Also, now we are both safe from the Dirt Hurt prototype crowd that would criticize your non prototype rock on your layout.
Kind of an amazing selection of rock colors up there. I need to gather some more. I expect yours is safe if you got it at Rainbow cut.
I actually got it at Crescent, in the cut near the parking spot. The rock strata in Rainbow Cut is so neat to explore.
Was at an N Scale Weekend in Ohio years ago, a Lady was selling her late Husband's estate. He had worked for a PA coal company. Got a case of COAL SAMPLES in plastic bottles. This was probably 10 years ago and I'm still using them. You NEVER know where you'll come upon scenery items: Keep yer eyes OPEN.