Mountains weather directional, meaning rain comes mostly from one direction to press against the mountains and create gullies, so there is a rain shadows on "inner mountains" also there is weathering by gully freezing, so the thin ridges between gullies get freeze cracked to rubble. That rubble again builds up, almost filling the lower slopes of a gully ocassionally.

Finally there are invisible gullies, layers of rock where the water sips into the rock and caves, leaving all things below gully free.