セキ, シャク, コク, いし, いさ, いす, いわ, し, せっく, と
JLPT3 G1 S5 F342
A depiction of a *rock* or *stone* beneath a cliff, suggesting a substantial pile of rocks/stones. *Sterile/barren* is by association (← rocky, barren soil), as is *rigid* (← rigid objects). *Unit of volume* is a borrowed meaning.
[Full Kanji Etymology List]