Well, as I have built terrain before and didn't supply enough critique (due to answering perhaps too quickly), I'll try to compensate.
The way the canyon has been done is having put 3 blocks of foam on top of each other, some roughing at the edges, some texturing with filler, some sand and then painting and drybrushing. Yes, it seems impressive enough, but it's not really that difficult to accomplish and the way it is done is certainly something I'll consider also doing in the future.
What's bothering me a bit is that the edges of the canyon are perhaps a bit too rough to my tastes and perhaps I'd prefer texturing it with a filler material and then using a comb or a similiar instrument. (see reference: http://www.planetside.co.uk/terragen/tgd/images/deep_canyon_v04.jpg
). Also, the top level could benefit from a bit of cork board that's been frayed at the edges and also some sand to the edges. I would also have done the board in a more modular fashion, but this again is more a matter of taste than anything else.
As the board is just a canyon and lacks objectives (which certainly can be added later on), I would suggest adding statues with bluish-gray hues to bring some "life" to the board.