steep and sheer
If you mean 'precipice' as in hazard or danger, then safety, care, preservation are antonyms. If you take 'precipice' to mean a steep decline, then incline or rise are antonyms. If 'precipice' means hill, then ditch, canyon, valley and crevasse are antonyms.
A valley can become a canyon by rivers and rocks.
Valley
A canyon is a deep narrow valley.
A deep valley could be called a canyon, such as the Grand Canyon.
It is a Canyon as in the Grand Canyon.
Valley is not shown on the map of California.
a valley is the space between two hills while a river forms a canyon
A 'hanging valley' is a valley which is cut across by a river flowing in a deeper valley.
Valley or canyon
It is a Canyon as in the Grand Canyon.
Death valley(eastern california),Grand canyon(Arizona state of US)