gorge, ravine, in the mountains, cape, promontory
岫 centers on a mountain landform: a deep gorge or ravine, or a projecting cape or promontory. Both senses describe a distinct mountain feature, either a cut into the land or a point extending from it.
岫 combines 山 (mountain) with 由, which likely contributes a phonetic element. The exact historical development is uncertain, but the mountain component clearly relates to the geographical meanings.
The mountain 山 on the left and the shape 由 on the right suggest a mountain with a passage or opening: a gorge cutting through, or a cape jutting out like a mountain's finger.
For コウ, imagine a deep gorge echoing with the sound 'kohhh' as wind rushes through: koh -> コウ.
cave; cavern