Thanks for the comments, Joe. I don't blame you for wondering about the authenticity of the mountains. It's a pretty weird looking place. But I agree with your first impression. Overall the panorama is OK but once you look at it closely you can see the mountain shapes aren't really right.
I decided to rebuild it from scratch and have been taking a crash course in heightfields and heightfield operators. So far I've been skating with Terragen, using it mostly to build maps and large-scale scenes based on DEMs. This is my first real project creating detailed terrain from scratch.
Here's where things stand after a couple of days' work. There's no shading yet, just displacements. This began as a Heightfield Generate node modified with a Heightfield Curve Vertical to create the glaciation effect. Then a bit of erosion added with a Heightfield Erode node. This basic scene was taking up to 30 minutes to load and generate the displacements, so I moved all the heightfield nodes into separate files to generate and export the base terrain and erosion (as a difference erosion field) in two .ter files, combining them in the final scene. What a great feature and timesaver! The working file now loads in a split second, and the erosion can be faded and masked as needed.
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I'm much happier with this because it seems to more closely resemble the original terrain, with its heavily eroded sedimentary rock layers, taluses and debris fields. I can see where some things may need to be tweaked but will wait till it's shaded and properly lit. Thoughts, suggestions and comments welcome.