Call me pedantic, but I would argue that the Grand Canyon is composed entirely of air.

Chalky] Possibly - but then you'd have to apply that to practically every place or geological formation. Would that definition help you to get Everest, for instance?

Rosie] I may have misunderstood what I Googled - I'm no scientist. But this is from the GC National Park's website: The Canyon’s mile-high walls display a largely undisturbed cross section of the Earth’s crust extending back some two billion years.
from http://www.nps.gov/grca/pphtml/subnaturalfeatures14.html
and lots of sites talk about the GC being made by upheavals in the earth's crust, so that was the basis of my reaction. I'm sorry if it was misleading, and you're right about the self-indulgent bit - sorry again.

Kim] Well, that was why I was unsure about the answers to "solid" and "object". But it can't be all air - what about the floor or the sides of the canyon?

move] Is there just one of it?

[Kim} No. Not a geographical feature. [Irouleguy] There are more than one of these.

(Irouléguy) Talk of the earth's crust made me think the answer was something to do with the material of the earth well below the surface whereas the Grand Canyon (and Cheddar Gorge) were gouged out by running water, i.e. surface erosion. Not to worry. :-)

[Rosie] No.