First of all, NEVER make the whole map detail brushes--you need to seal off your map from the void!
However, if your canyon is a terrain or a large number of brushes in general, making it detail (inside a structural hull) is a very good idea.
There are three good ways to select lots of stuff:
1) Hold ALT and drag a box with your mouse.
2) Make a brush and click the "Select Inside" or "Select Touching" button.
3) Select any brushes (if any) that you don't want in the final selection, and then hit I. (Invert selection)