My QTH is located in a deep canyon. The walls are about 1,000 ft high...
How far away are the walls? The vertical angle to the top is what really
matters rather than the vertical height. If the wall is 1 mile away then
the vertical angle is arctan(1000/5280) = 11 degrees, and you should
be able to work plenty of DX on 20m at that angle.
Out the ends of the canyon you often have an even lower horizon.
On 80m for relatively local contacts, high angles of radiation are quite
effective: you should have good signals out to hundreds of miles from
a relatively low dipole when the ionosphere is cooperative.