round stone, monument
碣 centers on a stone object that stands upright and carries an inscription: a round stone marker or monument.
碣 combines 石 (stone) with 曷, which likely contributes the sound. The character refers to a stone marker or monument.
The left side is a stone 石, and the right side 曷 looks like a standing marker with a sun 日 and a curved base. Picture a round stone monument standing under the sun.
For ケツ, imagine a stone monument shaped like a giant kettle: kettle -> ケツ, and the round stone kettle stands as a monument.
stone monument; gravestone; tombstone