Un val is a small vallée. Une vallée is typically several kilometers wide; un val is a couple of hundred meters wide, tops.
The "Trésor de la langue française informatisé" (which hasn't been updated since 1994) says val is deprecated, but it's common in classic literary novels, together with un vallon, a near synonym.
The "Trésor de la langue française informatisé" (which hasn't been updated since 1994) says val is deprecated, but it's common in classic literary novels, together with un vallon, a near synonym.