Looking for guidance

I suggest that you abandon this idea unless you are planning a garden railroad.  You may be able to build a strong table but can the floor of your layout handle the weight?  Another consideration is that most model railroads undergo many changes.  Do you always want to move 300 pounds of rock to make changes?   Another consideration is that natural rock will bring dirt into your layout room.  You will probably have to use real mortar to hold the rocks in place.  This is a messy process and adds a lot of weight.    

I suggest that you use one of the standard scenery building techniques that have been published in the model press for and indoor railroad.   I think you will be much happier in the long run.

Use the rocks in the garden.   See the G&O Story for O gauge outside.

https://ogrforum.com/t.../the-gandamp-o-story

NH Joe

Hi Tom, no offence to Joe, But I think you have a great idea. If it were me I would use foam board to shape your Mountain, then use Liquid nail for hobby projects. It doesn't melt the foam board and holds like vice grips. Then all you would have to do if needed if fill between rocks. If your worried about dirt being brought into your room, just wash the rocks first. It would help hold them in place anyways.

You can also make molds from the actual rocks, and cast light weight plaster rock formations.

Don

I could make moulds easily enough, but I could never duplicate the marvelous colors and patterns.

Any chance you could post a picture of the pile of rocks?  Much depends on their shapes and individual sizes. Working with real stone can produce excellent results, but it can be tricky. You have to avoid ending up with a mountain that looks like a bunch of stones glued together. It is also often a challenge to handle the transitions between "real" and "simulated" stone. I have had the best luck using flat, thin sheets of stone to simulate exposed rock escarpments.

As for glueing the stones together, I would start with a rough armature of plaster over strips of cardboard or chicken wire. Then use hot glue to piece the stones together. For filling in between the cracks papier mâché is a good choice. There are several commercial compounds, such as Celluclay that give a good texture. The challenge is painting to match the colors, which is a little challenging with the colors of the Southwest.

Many of the large exposed surfaces here are real stone:

The rest is a mix of painted plaster and Celluclay.  BTW: Don't be afraid to dry brush a bit of paint onto the stone to help with the transitions.