I am surprised that pencil drawings scan so well. I've always found that the contrast and brightness get all messed up when I try to scan a pencil drawing.
I'll give two examples of rugged mountains, one by me and the other by Max:
Mine:
http://www.cartographersguild.com/al...chmentid=62473
Max's:
http://www.cartographersguild.com/al...chmentid=68838
I make no claim that these are the best examples, but they are the first to come to mind. Clearly they have very different styles (personally, I like mine more; no disrespect intended to Max) and methods: I believe Max almost always works with a graphics tablet, whereas I, like you, use pen and pencil.
I spent a little while looking at your mountains from the second most recent drawing (like I said, the caldera looks great as a non-rugged formation, in my opinion). A few things come to mind:
--the sides of your mountains look fairly straight, like an upside down V-shape. In both mine and Max's, the sides of the mountain curve inwards, so the slope is shallow at the bottom, and steeper as you go up (shading has a bit of a role here, too);
--your mountains appear as single ridges, whereas mine and his have numerous spurs and outcrops coming from them, and valleys between. I think, from this observation, that your mountains suit a more local-scale map (hence the caldera being perfect) whereas the ones I did are more realistic for something on a larger scale.
--the shadows are more mixed on mine and Max's than they are on yours: on yours, all the shadows seem to be on one side of the mountains, whereas on mine, there are shadows and highlights on both sides, marking areas where lumps of rock on the lit side are casting shadows, and where spurs on the shadowed side are catching glints of light (mine actually isn't quite as clear on this regard as I would like it to be; Max's might be a better example of this).
Hope that's helpful.
THW