valley with a stream in it, gorge
嵠 centers on a steep, narrow valley with a stream running through it: a gorge. The given meanings describe the same landscape feature.
嵠 combines 山 (mountain) with 奚, which likely contributes the sound and may relate to a stream or narrow passage. The exact historical development is uncertain.
The left side 山 is a mountain, and the right side 奚 looks like a winding stream cutting through. Picture a mountain split open by a rushing stream, forming a deep gorge.
For ケイ, imagine a kayak (ケイ) navigating the narrow stream at the bottom of the gorge: kayak -> ケイ, and the mountain walls rise on both sides.