Then, if you can stay high enough to cross, great. If not, turning back is now only a 90 degree turn rather than a steep-banked 180 right in the midst of all the sink and turbulence. Many mountain flying accidents occur when pilots elect to fly up a narrow valley hoping to cross the pass at the top. If you must fly up a narrow valley, always crowd up tight against one side. Not only do you need room to turn around, you need room to make that turn while flying through turbulence, while loosing altitude, and possibly while being blown way off your intended route.
Narrow valleys are notorious collectors of sink and turbulence.
Probably a great deal tighter than you think. The valley wall close beside you is not much of a hazard. If you lose altitude, you can just turn away from it. The tighter you glue yourself to the valley wall, the more room you have left on the other side to turn around. The sunny side of the valley may have convective lift while the shady side is sinking.