How does the text explain the formation of urinary gravel (sharkara)?

Detailed answer

Verse 3 compares it to mud forming by itself at the bottom of a new pot of clear water. Verse 4 adds that strong vata breaks the dried kapha into two, three or many pieces, which lodge in the urinary passage and cause severe pain in the waist, groin, bladder and penis; a footnote says a stone broken up by vata is called sharkara.