That looks nice! Mind if I link to this post from my big 'ARM' tut?

Also a couple of tips that might help, play with them an include them if you wish...

1) with the magic want (fuzzy select) tool in gimp, click and hold the mouse button down. moving the mouse up or down will change the threshold slider and the screen will update to show you the selection! (I only found this out last week, and it is really handy. It also works with the select by colour tool)

2) another option to the mathmap noise filter is the felimage noise plugin which is what I use to avoid the artifacts the built in noise filter has. It also makes a nice forest bumpmap, btw.

3) I recently discovered a good way to draw uniform mountain ranges. instead of what you made (the big white bugs), draw the ridge lines in white using the 3 pixel hard brush, then use the Filters->Generic->Dialate to grow the white area, and crtl-F to get it 1/2 the size you want your mountain, then blur by the 2x the same amount:
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In any case, great addition to the tutorials here!

-Rob A>

P.S. I added a few tags to your post.